Coffee Processing in Peru

Much of what we've learned in Peru has been the seed for us to do further research and expand on. Outside of the anecdotal stories and learning from peers, we've listed a bunch of resources we've found useful and used for this post which can be found at the bottom. If you're interested in pre harvesting, and everything that can be done to improve flavour profiles before the cherry is picked, check out Coffee Agroforestry in Peru.

For everything after the cherry is picked (post harvest processing) we will explain below the options we're aware of.

Popular Coffee Processing Methods in Peru

Peru has been traditionally known for washed coffees, however, as the specialty coffee world becomes smaller and access to information is more available, the desire from coffee producers to experiment with new processes is becoming apparent with the rise of naturals and honeys across the country.

With this explosion of new and novel coffees coming to the international market, Peruvian coffee has developed leaps and bounds and the potential for more coffee producers to undertake cutting edge techniques like anaerobic and extended fermentations is vast.

Overview of Coffee Processing

Coffee processing, for us, includes everything from once the cherry is picked from the tree to when it’s fermenting in a bucket, peeled or not, to drying on beds and being milled and placed in polypropylene bags/boxes, ready for export, in storage.

In recent years, Peruvian coffee producers have been experimenting and innovating with processes across the country and yet, this is a minority in the grand scheme of things. Many, who work in specialty coffee are still yet to try novel approaches with proven results. 

Quality control of the green bean is essential for bringing out the best flavours possible. There are many stages in a coffee processing journey that warrant attention and detail. Each stage plays a significant role in the flavour development of the coffee but also the longevity of freshness.

Currently, a lot of focus is put onto the coffee fermentation but this post is to show that every stage is important and some, like drying, is just, if not, more important some could argue, than how many hours a coffee was fermented for.

Green coffee processing encompasses several stages:

1. Harvesting - Selecting ripe cherries for processing.
2. Fermentation - Breaking down sugars in the mucilage to influence flavour profiles
3. Drying - Stabilising moisture content to preserve and enhance bean quality.
4. Storage - Maintaining optimal conditions to prevent defects before roasting.

Coffee processing is the bridge between the raw cherry and the final cup, with every step influencing the sensory profile, quality, and market value of the bean.

The above 4 steps are so simple yet have so much complexity intertwined and connected between them. It's crucial to be able to understand each one in order to replicate a winning result or learn from a defect.

Primary Coffee Processing Methods in Peru

Washed Process: Precision and Clean Flavours

What Peruvian coffee is typically known for. Clean, classic, washed coffees.

Depending on what region of Peru you have a washed coffee from will determine the levels of acidity, which then bring out the juicy and bright acidity.

1. De-pulping - The outer skin is removed and exposes the mucilage coated beans.
2. Fermentation - Natural microorganisms break down the mucilage over a period of time.
3. Washing - Beans are washed to remove any residual mucilage and remaining sugars.
4. Drying - Mucilage covered green beans are spread out and left to dry for a period of time.

Natural Process: Fruity and Full-Bodied Profiles

The new process on the block and in some regions, the way they’ve done it forever.

Natural coffees are becoming more and more popular and many have experimented with this process for the first time this year. The flavours are full-bodied, boozy almost with intensity.

1. No pulping - The cherries are harvested and kept intact.
2. Fermentation - Some choose to ferment anaerobic at this stage or go straight to drying.
3. Drying - Whole cherries are placed on the drying method of choice for a period of time.

Honey Process: A Balance of Sweetness and Acidity

Regions in Peru such as Junin and San Martin are known for the honey process because they have (in parts) lower altitude levels and have been looking for ways to improve cup score, honey process being one of them.

This hybrid method combines elements of the washed and natural processes, offering a balance between sweetness, body, and acidity. There are a few variations of a honey process, yellow, red and black.

Yellow Honey: Minimal mucilage left, resulting in lighter, cleaner flavours
Red Honey: Moderate mucilage retained, producing a balance of sweetness and complexity
Black Honey: Maximum mucilage left, yielding intense sweetness and a heavier body

1. De-pulping - The yellow, red, and black method is chosen and how much skin to pulp.
2. Fermentation - Different levels of skin and mucilage create different flavours.
3. Washing - Some choose not to wash depending on their chosen method.
4. Drying - Beans are placed in drying methods of choice for a period of time.

Comparison Table: Washed, Natural, and Honey Coffee Processing Methods

 

 

Coffee Fermentation: Digging Deeper

Fermentation is a critical stage in coffee processing, a make or break really, which will end up determining the coffee quality and flavour profile.

At its core, this process is driven by intricate biochemical interactions where microorganisms play a key role. Yeasts and bacteria work together to break down the sugars in the coffee cherry’s mucilage. This breakdown produces organic acids and alcohols, which contribute to the bright acidity, clean flavours, and aromatic complexity.

In addition to microbial activity from the yeasts and bacteria, enzymatic processes also come into play, which are responsible for breaking down proteins and carbohydrates within the mucilage. These enzymatic actions help refine the clarity and enhance its balance.

Washed Coffee Fermentation

Fermentation in washed coffee is a carefully controlled process aimed at removing the mucilage layer surrounding the beans while enhancing their clean and crisp flavour profile. Yeasts and lactic acid bacteria dominate the microbial ecosystem, converting sugars into organic acids and alcohols that contribute to vibrant acidity and cup clarity. 

This controlled fermentation produces beans with low residual sugar content, reducing caramelisation risks during roasting and ensuring the coffee’s natural brightness is preserved. Although the process is water-intensive, sustainable practices such as water recycling and semi-washed methods can help minimise its environmental impact. In Peru, the Vetiver plant is used quite a lot in order to recycle the waste water from washed coffees. 

The biggest challenge with washed coffees is making sure that it doesn’t over-ferment. When this happens the sour, vinegar-like flavours become present in the cup because of the excessive acetic acid production. There are many ways that this can be managed, mainly by controlling the pH levels and temperature.

Natural Coffee Fermentation

The natural coffee fermentation is quite different because of how the whole coffee cherry and pulp interact. The prolonged contact between the coffee bean and the fruit sugars creates a rich microbial ecosystem which allows the fermentation to occur around the bean. Yeasts play a significant role in this ecosystem, producing esters that assist in creating the sought after fruity and wine-like flavours. As the cherries dry, sugar concentration increases, creating an environment favourable for more microbial activity. 

This type of fermentation also presents its own set of challenges. Uneven drying is one and without careful monitoring and turning, this can lead into mould growth or over-fermentation because part of the whole cherry dry at different temperatures, resulting in sour or earthy defects. When doing natural fermentation, raised beds inside a solar dryer with different levels of elevation is ideal to allow uniform drying and airflow to dry uniformly. In Peru, there are a few farms from our experience with mechanical dryers, therefore, maintaining the drying process manually is employed and requires constant monitoring of the humidity levels and temperature to make sure the coffee dries at the right speed.

Honey Coffee Fermentation

The honey fermentation creates a microbial environment that is augmented depending on how much mucilage is left on the cherry and which process out of red, white, and black is chosen. Yeasts such as Saccharomyces cerevisiae and lactic acid bacteria actively ferment the remaining sugars from the mucilage. These microorganisms produce acids and esters that are designed to enhance both sweetness and acidity, leading to a coffee with a full body and a variety of flavour profiles.

The increased options during a honey fermentation offers producers a lot of flexibility in achieving different flavour outcomes that tap into different coffee senses. The lighter, fruitier notes can be achieved but also deeper, more syrupy profiles. 

One of the challenges with this type of fermentation is how the beans dry. With more mucilage, the beans dry more slowly than washed or natural coffees fermentation and this creates the need to have key variables in place as well as regular monitoring in order to prevent spoilage.

Secondary Processing Methods

Anaerobic Fermentations

To add more complexity into the mix of coffee fermentation and into the cup, anaerobic fermentation is a novel and innovative method being used with most of the coffee producers we work with in Peru.

Anaerobic fermentation environments are oxygen free and therefore coffees that are termed anaerobic are done so in controlled, oxygen free environments.

As the complexity of the microbial ecosystem increases when they are devoid of oxygen, so does the way of managing this process in order to achieve the desired results of improved aroma, mouthfeel, balanced acidity, sweetness and augmenting key flavours intrinsically from the varietal.

9 Key Management Variables for Coffee Fermenations

1. Brix Level

First up, the Brix levels. Think of this as the sweetness metre and how much sugar is within the coffee cherries. It’s the sugars that keep the microbes in the cherries busy eating and being fed so they can create all of the wonderful aromatic compounds and organic acids. It’s in this process that the complex flavour profiles we associate with anaerobic coffees can emerge. Brix levels, ideally between 18-24°C are measured at different stages, from harvesting to periodically during the fermentation, every 12 hours for example.

Measuring Brix levels is a fundamental and cost-effective practice for many coffee producers in Peru. By tracking the sugar content in the mucilage at various stages of fermentation, producers can gauge the progress of fermentation and make informed adjustments. High initial Brix levels, for example, indicate high sugar content, which, if managed correctly, can lead to vibrant and complex flavours. Different coffee varietals, such as Geisha and Catimor, may exhibit unique Brix level changes, directly influencing their final flavour profiles. Understanding and monitoring Brix levels allows producers to optimise fermentation duration and conditions, enhancing the coffee’s taste, aroma, and mouthfeel.

2. Temperature

Temperature is the most critical factor in fermentation because it directly influences the activity and growth of microorganisms responsible for fermentation. If the temperature is too high, you risk killing or over stimulating the microbes, leading to off flavours or even halting the fermentation process altogether. If it's too low, fermentation may stall because the microbes run out of energy, leading to incomplete or uneven fermentation. Keeping the temperature within the optimal range (18-25°C) ensures a stable and consistent process. 

3. Time

Time is another crucial factor. From those we work with in Peru, fermentations can last anywhere from 8 to 240 hours. The scope between choosing 8 or 240 hours depends entirely on the desired outcome, experience, expertise, and risk tolerance of the coffee producers. We err on the side of caution when it comes to time because in short, the longer it is, the more management required, and also the more likely it can spoil. 

However, the length of fermentation directly affects the coffee’s flavour profile. Too short a fermentation time might not allow sufficient flavour development, while too long could lead to over-fermentation. By carefully monitoring time, along with other variables like temperature and pH, you can ensure that the fermentation is completed at the optimal moment for flavour and quality.

4. pH Level

The pH level measures the acidity. High scoring coffees without acidity is unheard of and certain parts of Peru are more blessed than others with having higher acidity levels, however, all this can be reversed into rancid and sour notes without knowing the key pH ranges for the coffee fermentation.

Measuring the pH level at the start of the fermentation process is ideal and they can be around 5.0 and 6.5 with the idea that, in time, in the right temperature conditions, and with the right amount of sugars, it’ll drop to 3.5 and 4.5.

With more experimental fermentation it’s worthwhile measuring the pH level every 12 hours at least so that managing the controllable variables is being put into place. It’s also worthwhile doing this to record the inputs that hopefully lead to a desirable outcome in order to repeat it.

pH levels are crucial for maintaining the right environment for microbial activity. A pH between 4.0 and 5.0 helps balance the production of organic acids like lactic, malic, citric, and acetic acids. Monitoring pH ensures that the fermentation is progressing well and that the flavours are developing correctly. A pH below 3.5 can disrupt metabolic pathways.

What Are Metabolic Pathways?

Metabolic pathways are like the coffee beans’ internal instructions for how to use energy and build important molecules. When the pH is too low, these pathways can become unbalanced, leading to something called oxidative stress. This is when the cells are damaged because they can’t handle the stress, which also lowers the levels of antioxidants, the molecules that normally protect the fats (lipids) in the coffee beans from damage.

Why Does This Matter?

When oxidative stress occurs, the fats in the coffee beans can break down in a process called lipid peroxidation (degradation of fats by oxygen). This breakdown damages the cell membranes, which are made of fats. As a result, these membranes become more permeable, meaning they allow more water to move in and out of the cells. This increased water movement raises the water activity and moisture content in the green coffee beans. High moisture and water activity levels can reduce the stability of the coffee’s flavours and create an environment where unwanted microorganisms can grow, further degrading the coffee’s quality.

4.1 Applying the pH Levels

1. Start of the fermentation - typically around 5.0 - 6.5, indicating a less acidic environment.
2. Flavour development - as it drops to 3.5 - 4.5 peak microbial activity is producing lactic acid and acetic acid.
3. Microbial activity - depending on the level of acidity will determine which microorganism can thrive or decline.
4. Preventing defects - below 3.5 and microbes start releasing sour, rancid, and extreme tangy notes. For the super funky coffees this may be the desired results, however, for most, this level is too low and runs the risks of spoilage.
   
   - Yeasts: Active in a wide pH range but decline below 4.0.
   - Lactic Acid Bacteria (LAB): Thrive at slightly acidic levels (4.0–5.5).
   - Acetic Acid Bacteria (AAB): Become inactive below 4.0.

5. Oxygen Levels

Controlling oxygen levels is essential because it determines whether the environment remains anaerobic (without oxygen) or, in some cases, completely anoxic (0% oxygen). Oxygen presence can significantly alter the types of microbes that thrive and the metabolites they produce. Managing oxygen levels helps in achieving specific flavour profiles and preventing unwanted aerobic microbial growth that could spoil the fermentation.

6. Visual and Sensory Cues

Visual and sensory cues provide real-time feedback on the fermentation process. Experienced coffee producers use these cues to assess the progress of fermentation, such as the look and feel of the coffee by seeing the translucency of the mucilage and the general aroma. These observations can guide decisions to adjust or end the fermentation process, ensuring the best possible outcome.

7. Microbial Inoculation

In some cases, specific microbes are introduced to steer the fermentation in a desired direction. While not always necessary, inoculation can help achieve consistent results and enhance certain flavour attributes. Understanding how these introduced microbes interact with the native microflora (naturally occurring microorganisms like bacteria and yeast) can improve the predictability and quality of the coffee. This is one of the most advanced areas of coffee fermentation due to the amount of available inoculants.

8. Fermentation Vessel

The choice of fermentation vessel impacts temperature control, microbial activity, and ease of cleaning. Different materials (e.g., stainless steel, plastic, wood) can influence the flavour development and overall efficiency of the fermentation process. Selecting the right vessel helps maintain a controlled environment and can affect the final taste of the coffee. Wood is porous and has the potential to develop flavours while plastic and stainless steel are the preferred options. In Peru, plastic tubs (timbos) are very popular.

9. Moisture Content

Moisture content is crucial for the long-term storage and quality of green coffee beans. If moisture levels are too high after fermentation, say +12% (but in reality +11% due to the potential growth post drying), there’s a risk of mould growth and flavour degradation. Monitoring moisture content during the fermentation process ensures that you have the information to be able to control the longevity of the coffee quality.

Temperature and humidity are key variables in both aerobic and anaerobic fermentations. Coffee producers in Peru effectively manage these factors using practical methods such as natural shade, wet jute sacks, and affordable hygrometers (humidity metres). Proper control of temperature and humidity not only optimises flavour development but also prevents the growth of spoilage organisms, which thrive in poorly managed environments.

Bringing It All Together

As you can see there are many variables, some controllable, some are not for the coffee producers. The fermentation depends on balancing key variables like sugar content (18–24°Bx), temperature (18–25°C), and pH (3.5–4.5). Microbial activity transforms sugars into acids and aromas, while oxygen control, time, and sensory cues refine flavours, preserve them, and stop them from going in the opposite direction. Moisture management and vessel choice ensure quality, and protect the longevity of the coffee quality and something that we must be considered when exporting coffee.

How The Microbes Work

During any fermentation but specifically anaerobic coffee fermentations, the microbes are hard at work. The lactic acid bacteria created during the microbes eating away at the sugars lead to them churning out lactic acid, which in balanced proportions provides coffee with a smooth, creamy feel. Yeasts are also present and bubbling away, producing ethanol and CO2 as byproducts of their metabolism. Ethanol is a precursor to the formation of esters, which contribute fruity and floral notes to the coffee.

Additionally, the fermentation process can lead to the production of succinic acid, which adds a mild, slightly salty taste that can enhance the complexity. Acetic acid bacteria also join the party, adding a touch of brightness; however, this acid can quickly overpower the flavours and turn sour if not controlled.

As you can see, with anaerobic fermentation, there are many variables that fluctuate and are not easy to predict; however, it is opening up a whole new world of flavour possibilities.

Overview of the Microbiology Involved in Anaerobic Processes:

The primary microorganisms involved are:

• Lactic Acid Bacteria (LAB) - Convert sugars to lactic acid, contributing to a smooth mouthfeel.
• Yeasts - Produce ethanol and CO2, enhancing flavour complexity and leading to the formation of esters.
• Acetic Acid Bacteria (AAB) - In small quantities, add brightness and complexity.

These microorganisms produce various flavour compounds:

• Lactic Acid - Enhances body and creaminess.
• Acetic Acid - Adds brightness in low concentrations.
• Ethanol - Precursor for ester formation.
• Esters - Contribute fruity and floral notes.
• Succinic Acid - Adds a mild, slightly salty taste.

Anoxic Processing

Anoxic processing is an advanced method that involves creating an environment entirely devoid of oxygen during fermentation. This approach favours strictly anaerobic microorganisms, resulting in the development of a unique microbial ecosystem and the production of distinctive metabolites. These include organic acids such as lactic, malic, and citric acids, as well as aromatic compounds that are not typically found in traditional coffee processing.

To achieve true anoxic conditions, it is essential to use oxygen absorbers. These absorbers are materials or devices designed to eliminate any residual oxygen in the fermentation environment, ensuring that oxygen levels remain at 0%. This creates the ideal conditions for the desired microbial activity, allowing for the production of unique flavours and compounds that define anoxic processing.

The Precision of Terminology: Anoxic vs. Anaerobic

The term 'anoxic' is preferred by microbiologists and purists due to its precision in describing an environment completely free of oxygen. Although 'anaerobic fermentation' is commonly used in the context of coffee processing, it is somewhat redundant, as all fermentation processes are anaerobic by nature because the metabolic activities of microorganisms occur in the absence of oxygen.

'Anoxic' specifically refers to the environmental condition where oxygen is entirely absent, making it a more accurate term for this advanced fermentation technique.

However, for broader understanding and communication within the coffee industry, the term 'anaerobic' is often used, even though it does not fully capture the exact nature of anoxic processing. We prefer to use the term anaerobic to explain the wider concept and therefore will use this throughout the post when referring to oxygen free environment for coffee processing.

Double Fermentations (Washed & Honey Process)

Double fermentations have emerged as exciting techniques that offer a playground for flavour development. The double fermentation has been gaining traction in Peru since being made famous by Dwight Aguilar Masias, two times Cup of Excellence Winner. 

Two Distinct Fermentation Stages:

The first stage typically occurs with the cherry intact, where the fermentation kicks off using the native microflora living on the surface of the cherries and used to start the fermentation process. These microorganisms are already present in the environment where the coffee is grown and harvested, making them the first contributors to the flavour development.

The second stage involves the coffee cherries being pulped and placed in fermentation tanks of choice and this is where a new microbial community will take shape, often dominated by lactic acid bacteria. 

This two-stage process allows for a more complex flavour development. 

The initial fermentation in the cherry promotes the formation of fruity esters, while the second stage enhances acidity and produces additional aromatic compounds. Both stages require an idea of how long this fermentation will be, similar to any other coffee processing method, the time will be determined by the Brix level, pH level and temperature.

Extended Fermentations

This method is like slow-cooking for coffee, allowing flavours to develop over a much longer period - we’ve seen anywhere from 48 to a whopping 240 hours! As the fermentation progresses, the yeasts that dominated the early stages begin to give way to bacteria, particularly lactic acid bacteria. These microbes thrive in the increasingly acidic environment, with pH levels potentially dropping as low as 3.5-4.0. 

As the fermentation progresses, complex organic acids begin to form. Lactic acid, with its smooth, creamy notes, starts to build up. You might also detect hints of malic acid, reminiscent of green apples, or citric acid, adding a zesty brightness. In some cases, the elusive succinic acid might make an appearance, bringing a subtle umami quality to the cup. 

Alongside the acids developed, an extended fermentation allows time for the formation of aromatic compounds like esters, aldehydes, and ketones. These are the molecules that give coffee its captivating pull - think floral notes, fruity aromas, or even hints of spice. The result of this prolonged fermentation can be truly remarkable. Coffee is not too far off the complexities of wine where experimental and precision fermentations are well developed and this is one of the reasons why coffee can unfold on your palate like a fine wine, with layers of flavour such as bursts of tropical fruit, floral freshness or aftertastes of dark chocolate.

In the end, both double and extended fermentation offer Peruvian coffee producers the tools to craft truly distinctive cups. These methods allow for a level of flavour manipulation that was once thought impossible, opening up new frontiers in the world of specialty coffee.

Double & Extended Anaerobic Coffee Example

Whole cherries being placed in a container, open air, closed, in GrainPro bags and/or submerged in water. Depending on the process will determine at this stage whether it is anaerobic or not. During this time, the natural yeasts and bacteria present on the cherry skin and within the fruit begin to break down the sugars in the cherry pulp.

Many producers we work with like to do this stage in anaerobic and for washed coffees, anywhere between 24 - 48 hours. For those that are doing anaerobic naturals and not pulping, this can be stretched out to 72 - 96 hours. The honey process tends to be a mix depending on which honey process.

This anaerobic environment encourages the development of unique flavour compounds and can lead to more intense, fruity notes in the final cup. The cherry's intact structure during this stage allows for a slow, controlled fermentation, with the skin acting as a natural barrier.

The second stage involved pulping. If it’s a natural process then it’ll be taken straight to the drying beds with no pulping and if it’s a honey it'll be pulped but depending on which honey and how much skin will be left. Either way, it’ll have its second stage of fermentation. For a washed coffee, the aim is to remove all of the cherry skin and leave the mucilage intact.

This stage is also popular for anaerobic fermentation too, either in sealed plastic tubs or in GrainPro bags submerged in water or not. This stage we find many producers tend to keep the same duration as the first stage or increase it by 12-24 hours. Popular durations for this stage range between 24-48 hours.

Tertiary Processing Methods

Anaerobic Slow Dry (ASD)

The anaerobic slow dry (ASD) process combines anaerobic fermentation with a controlled, prolonged drying phase lasting between 2 to 6 weeks. This method is becoming popular in Peru, and we've sourced several coffees processed this way. The drying period is much longer than the fermentation period, yet the focus on fermentation often overshadows the critical drying stages. Uniform, slow drying is responsible for intense flavour clarity and the development of complexity in coffee.

Choosing The Anaerobic Fermentation

Whether fermenting whole cherries, partially pulped beans for honey processing, or fully pulped beans for washed processing, the first step in ASD is anaerobic fermentation. This occurs in sealed environments like tanks, bio-reactors, or GrainPro bags, where oxygen is limited, favoring beneficial microbes like yeasts and lactic acid bacteria.

Choosing The Drying Method

After fermentation, the coffee (in cherry, semi-pulped, or pulped form) must be dried in a controlled environment. Key variables, temperature, humidity, and airflow must be carefully monitored and adjusted to achieve the desired drying time and prevent spoilage.

The Slow Drying Dynamics

Moisture content and water activity are key variables that are needed to be stabilised and contained in order for the green coffee to stay fresh, vibrant, and prevent rapid aging. The slow drying preserves the flavours by minimising the lipid peroxidation while also allowing certain enzymatic activities to persist longer which contributes to the flavour development after fermentation.

Enzymes are biological catalysts crucial for the chemical and sensory transformation of coffee during fermentation and drying. The gradual removal of moisture during slow drying allows these enzymes to remain active longer, enhancing flavour precursors and bean stability. 

The 6 Key Enzymes Involved

1. Pectinases - break down pectin which helps degrade the remaining mucilage and clean up any microbial hotspots.
2. Amylases - break down starches into simple sugars like glucose and maltose allowing sugars to be available during the drying and creating more aromatic compounds at this stage.
3. Lipases - releases volatile fatty acids that are precursors for aromatic compounds like aldehydes and ketones.
4. Proteases - break down proteins into amino acids and peptides which become precursors for the Mailard reactions during roasting and support the development of savoury and umami notes.
5. Polyphenol Oxidases (PPO) - modifies the way chlorogenic acids are oxidised and this influences acidity and bitterness.
6. Invertases - breakdown of sucrose into glucose and fructose helping to develop fruity and sweet flavour profiles.

The Risks of Enzymatic Activity

While slow drying promotes enzymatic benefits, improper management of drying conditions (too much moisture or high humidity) can lead to:

• Over-fermentation, resulting in sour or overly funky notes.
• Increased risk of spoilage organisms, such as moulds, which could degrade flavour quality.
• Rancidity from excessive lipid breakdown

Making It Work

To mitigate risks, it's essential to use controlled drying equipment. Solar dryers with adjustable raised African beds allow coffee to dry uniformly across the outer and inner layers at different temperatures. Utilising humidity meters helps determine when to move coffee to higher or lower beds to maintain optimal drying conditions.

Carbonic Maceration

Carbonic maceration is a technique adapted from winemaking which involves fermenting whole coffee cherries in a sealed, carbon dioxide-rich environment. Whole cherries are preferred because the skin can trap the CO2 within the cherry and promote a slower release of the sugars which enhances complex flavour development due to the slower release of esters and organic acids, creating the tropical and wine-like flavours.

In order to make the environment flush with CO2, there are a few ways to do this.

1. Inoculation
   
   Injecting CO2 into a sealed tank, either plastic, metal or a more advanced, bio-reactor. This process will flush out all the oxygen and create a complex oxygen free environment for the cherries to ferment in the desired way.
   
   
2. Natural Generation
   
   Allowing the natural microbes, such as yeasts and bacteria which are present on the coffee cherries to metabolise the available sugars, results in the production of CO₂ as a byproduct. As the fermentation progresses in the chosen container, the CO₂ naturally reduces the oxygen levels, creating an environment where the microbes switch to anaerobic respiration, further producing CO₂ as part of the process.
   
   

The result of having CO₂ fermenting the coffee is a slower, more gradual breakdown of sugars, allowing for a balanced interaction of yeasts, lactic acid bacteria, and other microorganisms. These microbes work harmoniously in the controlled environment to produce aromatic flavour compounds, resulting in a coffee with enhanced complexity, consistency, and distinct sensory characteristics.

This process is highly advanced due to the precision required in equipment and management. Steel tanks or bioreactors are ideal (also high cost), however, they do enable precise control of an anaerobic environment which optimises the microbial activity of the sugar breakdown and improves flavour development. Using GrainPro bags with naturally generated CO₂ introduces greater variability in oxygen levels, which can start to allow acetic acid bacteria to dominate and potentially degrade flavours by producing excessive acetic acid.

Cold Maceration (Frozen Cherries)

Cold maceration is a technique that at some point in the fermentation process, the cherries are frozen. This could be straight after being picked or after fermenting it in cherry. For the 24/25 harvest we sourced a coffee using this method for the first time and Gregorio Espinoza from Finca Voller froze the cherries straight after harvesting for 120 hours and then dried the coffee on African beds, similar to a natural process coffee, drying the whole cherry.

How It Works

1. Freezing the cherries halts all microbial activity by putting the yeast and bacteria into a dormant state due to the extreme cold. This leads to the water inside the cherry cells to crystallise, helping the release of sugars, acids and other flavour compounds more available later on in the fermentation.
   
   
2. A controlled fermentation and optimal environment has to be in place once the cherries are no longer frozen so that the yeasts and lactic acid bacteria can resume their activity of breaking down and metabolising the sugars and acids available.
   
   
3. The freezing and controlled thawing process aims to improve the extraction of intracellular compounds in the coffee cherry such as sugars, acids, polyphenols, and aromatic precursors. These compounds become more bioavailable to microbes, allowing a deeper microbial activity that develops organic acids and aromatic ester, leading to a more complex flavour profile.
   
   

Challenges and Requirements

Cold maceration demands significant investment in freezing equipment and precise environmental control during thawing and fermentation. Without careful management, the process risks microbial imbalance, excessive acetic acid production, causing sourness. This method is best suited for experienced producers who have explored advanced fermentations and are prepared to manage its complexities.

Co-Fermented / Infused Fermenations

Co-fermented or infused coffee processing is a method exported to the world from Colombia. In Peru, we’ve encountered one example from a coffee producer from the Junin region who was co-fermenting with fruit, a guava, native to the region being used. We predict that this process will grow in popularity with producers because it’s more accessible and requires less upfront investment in machinery, however, it’s not without the risks of spoilage and over fermentation.

The goal of co-fermentation is to push the boundaries of traditional coffee flavour profiles. Fruits or botanicals can be added at various stages: during fermentation in cherry, post-pulping in tanks, or even during drying. Each stage influences how added sugars, acids, and compounds interact with the coffee’s microbial ecosystem. For example, adding fruit during fermentation directly alters microbial metabolism, while additions during drying primarily enhance chemical reactions.

Sugar, Acid, Fat

Adding fruit increases available sugars, boosting yeast activity and extending their dominance in fermentation. Lactic acid bacteria (LAB) metabolises sugars and acids, but the presence of external acids (tartaric from grape or ascorbic from citrus fruits) influences pH, potentially enhancing fruity notes or slowing down/blocking the acetic acid bacteria (AAB) activity. AAB tends to create sour and vinegary notes when too active in a fermentation so certain fruits restricting this can allow the yeasts and LAB to shape the flavours.

Fruit enzymes like lipases break down lipids (fats), which start to release aromatic aldehydes and ketones, amplifying the complexity of the coffee but this has to be monitored closely because excessive lipid breakdown can tilt the balance into undesirable flavours.

Overpowering of Fruit

In many instances with fruit / co-fermented coffees, the fruit used is present with its aroma in all parts from the green bean to ground coffee to the final taste in the cup. Certain fruits are more overpowering than others and it can be easy for the strong fruit flavour to dominate the coffee's natural profile so if that's not the aim careful management is needed.

Lactic Fermentation

Lactic fermentation involves promoting the growth of lactic acid bacteria during processing. This method often involves inoculating the coffee with specific strains of Lactobacillus or creating conditions that favour their growth. The production of lactic acid can be monitored by tracking pH levels during fermentation. Typically, a gradual decrease in pH as opposed to a sharp drop in pH will give indicators as to whether or not fermentation develops complexity or sourness.

Lactic acid bacteria (LAB) generally prefer moderate temperatures and high sugar in the mucilage to feed from. The ideal range for LAB growth in coffee fermentation is between 20-30°C. LAB produces lactic acid and various flavour compounds, leading to coffee with a distinctive tangy acidity and often creamy body.  Regular cupping and pH monitoring are essential to manage lactic acid levels and ensure a balanced flavour profile.

Semi-Anaerobic Processing

Semi-anaerobic coffee processing is a fermentation method that combines elements of both anaerobic and aerobic conditions. This hybrid approach allows for controlled exposure to oxygen, which can lead to the development of unique and complex flavour profiles.

During this type of process, the coffee is exposed to oxygen however, at reduced levels but not entirely eliminated. This controlled exposure to oxygen allows for the activity of both aerobic (acetic acid bacteria) and anaerobic microorganisms (lactic acid bacteria/certain yeasts).

To implement this process, the coffee cherries whole and/or once pulped, can be submerged in water, sealed in tanks, containers or bags but not airtight so that some oxygen is available, usually less than 10% in most environments.

Semi-anaerobic processing, from a microbiological perspective is known as microaerobic and refers to environments with significantly reduced oxygen levels but not entirely anaerobic. The term microaerobic fermentation accurately describes this process, however, as similar to our view on anoxic, we prefer to use the term semi-anaerobic.

Thermal Shock

Thermal shock is an advanced coffee processing technique gaining popularity in Colombia, Costa Rica, and now Peru, where Gregorio Espinoza of Finca Voller has applied it to his Maragogipe varietal for the 2024/25 harvest. This method involves rapidly exposing coffee cherries to extreme temperature changes, first hot water immersion (50–70°C), followed by immediate cooling in cold water (<15°C).

What is it?

The idea is that the cellular integrity (strength of the skin, pulp and mucilage) of the coffee cherry can be manipulated to release intracellural compounds such as sugars, organic acids (citric, malic, and chlorogenic acids), polyphenols and aromatic precursors ( what give the coffee complexity, sweetness, and balance).

At elevated temperatures, microbial activity slows or stalls because enzymes lose their shape and this disrupts the metabolism of yeasts and lactic acid bacteria. However, the sudden cooling and stabilisation create a rebounding effect. The microbes reawaken in a nutrient rich environment where newly available sugars and acids fuel rapid fermentation, enhancing the probability of increased flavour development.

The Chemical Reactions

The introduction of extreme heat and the hot immersion can enhance the caramelisation of sugars, creating toffee like sweetness and complexity. The heat also triggers very early stage Maillard reactions which also helps with sweetness and the development of flavour precursors which go on to be present later in the process. Some of those chemical reactions that are developed during the extreme temperature are esters, aldehydes, and ketones, all are key compounds that contribute fruity, floral, and sweet aromas to the coffee.

A key group of organic acids, Chlorogenic acids (CGAs) are abundant in green coffee beans and tend to be quite stable while in the green bean form, however, when exposed to extreme heat, they can be released into the forms of esters that develop caffeic acid and quinic acid that open the door for the fermentation to have a more balanced acidity.

The Temperature Changes

1. The cherries begin with a hot water immersion (50-70°C) followed by a rapid cooling in cold water (>15°C).

2. The rapid temperature change disrupts the cherry skin, pulp, and mucilage, making them more permeable and easier to access.

3. This shock creates the organic acids such as citric, malic, and chlorogenic acids alongside the other intracellular compounds to be released.

4. The temperature shock, similar to the cold maceration, stalls the microbes of yeasts and lactic acid bacteria which causes a metabolic slowdown.

5. As temperatures stabilise, the microbial activity rebounds rapidly and is fueled with all the newly available sugars and compounds released due to the stress of the extreme temperatures.

6. The thermal stress can also alter the behaviour of chlorogenic acids, breaking them down into caffeic acid and quinic acid, which are involved with the bitterness and acidity of coffee

The Key Differences

The excessive and rapid temperatures change the speed and development of the microbial environment. 

The cherries become restricted of oxygen in these extreme heat environments and this leads to a dynamic fermentation where the stimulation of certain yeasts and bacteria are accelerated, thus changing the flavour profile. 

A higher concentration of sugars is the desirable outcome and this is one of the reasons this process is becoming popular compared to other processes which have a slower release and a lower amount of concentration of sugars.

The Equipment and Cost

Similar to advanced anaerobic fermentations, bioreactors or controlled temperature vats are ideal here. Other tanks could work but it’ll be harder to manage and risks of spoilage increase due to the number of volatile variables at play.

In order to create the hot water it’ll require heating. There are not many farms in our experience that have hot water readily available for this type of endeavour and so the cost of energy is important to consider here. 

Preventing Fermentation Defects

1. Phenolic Taints

Phenolic taints cause bitter and medicinal flavours in coffee, often due to over-fermentation or contamination by specific microorganisms like Brettanomyces yeast. These organisms produce phenolic compounds during fermentation, leading to undesirable tastes. Preventing this involves introducing beneficial microbes to outcompete the phenol producing ones and closely monitoring pH levels. A rapid drop in pH can signal potential contamination.

2. Acetic Acid Sourness

Acetic acid sourness gives coffee a sharp, vinegar-like taste, typically caused by the overproduction of acetic acid bacteria (AAB) during fermentation. This defect often occurs when fermentation is not properly controlled, especially if oxygen is present. Preventing this involves maintaining anaerobic conditions, using CO2 to displace oxygen, keeping temperatures cool, and regularly checking pH levels to prevent excessive acetic acid formation.

3. Over-Fermentation

Over-fermentation occurs when coffee ferments for too long, leading to excessive sourness, alcohol, or rotting fruit notes. This happens when microbial activity continues beyond the optimal fermentation period. Monitoring Brix levels to assess sugar fermentation and conducting sensory analysis can help identify and prevent over-fermentation.

4. Fermenty or Winey Flavours

Fermenty or winey flavours are characterised by an overripe or alcoholic taste, usually resulting from extended fermentation times or an imbalance in the microbial population. When fermentation goes on too long or the microbial community is not properly managed, these flavours can develop. Microbial balance is key to preventing this defect so this is where the more advanced technique of introducing specific microbial strains that promote desirable fermentation outcomes can help maintain the balance and prevent the dominance of microbes that cause fermenty or winey flavours.

The Role of Water Activity and Moisture Content

By understanding and managing water activity and moisture content, smallholder coffee producers throughout Peru can significantly improve the quality and longevity of their coffee. Regular monitoring, controlled drying, and proper storage practices are essential to prevent defects and ensure the coffee retains its desirable flavour profile and structural integrity from farm to cup. Implementing these practices helps maintain the balance between moisture content and water activity, crucial for producing high-quality coffee.

Water Activity (a_w)

Water activity measures the free water in a substance available for microbial growth, enzymatic reactions, and chemical stability. It is expressed as a ratio, ranging from 0 (completely dry) to 1 (pure water).

Maintaining low water activity in green coffee beans is crucial for preventing microbial growth. Optimal water activity levels help preserve the coffee’s flavour, aroma, and overall quality during storage and transport.

Most bacteria, moulds, and yeasts thrive at water activity levels above 0.60. When water activity is high, microorganisms can utilise the available water for their metabolic processes, leading to rapid growth. High water activity promotes the growth of spoilage organisms such as moulds and bacteria. It can also accelerate enzymatic reactions that degrade coffee quality because the breakdown of proteins and fats in coffee beans can result in bitter, rancid, or musty flavours.

Moisture Content

Moisture content is the total amount of water contained in the coffee beans, expressed as a percentage. Proper moisture content in coffee beans is essential to prevent mould growth and ensure safe storage. Optimal moisture content also impacts the coffee's roasting process and final flavour profile. Ideal ranges are between 10-12%.

When moisture content exceeds 12%, the beans become more susceptible to mould growth. Moulds can penetrate the beans cellular structure, using the moisture and nutrients for growth. Mould growth leads to visible spoilage and can age coffee rapidly while also creating health risks from fungi metabolising the organic compounds in coffee beans, leading to the production of secondary metabolites that contribute to off flavours and potential health risks.

Risks when Water Activity and Moisture Content is High

Reactive Oxygen Species (ROS) and Lipid Peroxidation

High moisture content and water activity can create conditions that facilitate oxidative stress. ROS are chemically reactive molecules containing oxygen, such as peroxides and superoxides. ROS can damage cellular structures, including lipids, proteins, and DNA, leading to cellular instability and degradation of coffee quality.

Lipid peroxidation refers to the oxidative degradation of lipids (fat cells). In the presence of ROS, the lipids in coffee beans undergo peroxidation, forming lipid peroxides. Lipid peroxidation results in rancidity and the formation of stale or cardboard-like notes. This process also reduces the shelf life of the coffee and is the reason why water activity and moisture content are a key variable when predicting the longevity of the coffee beans.

Preventing Spoilage and Off Flavours

Maintain water activity below 0.60 to inhibit microbial growth. Optimal storage conditions with controlled humidity can help achieve this. Using water activity metres to regularly monitor the levels in stored coffee beans. Many in Peru do not have access to water activity metres because they are high cost, therefore, moisture content has to be relied on most of the time.

Addressing ROS and Lipid Peroxidation

Introducing antioxidants during the drying and storage stages is an extreme case that can help neutralise ROS, protecting the lipids from peroxidation, however, this is a very last resort because it can also impact the fermentation and at that point it's a trade off between saving flavours and preserving coffee shelf life. Storing coffee beans in a cool, dry place can also reduce oxidative stress and preserve quality after drying.

Water Activity and Moisture Content during Fermentation

Different coffee processes affect water activity and moisture content in different ways. Classic washed, naturals and honeys have their own tried and tested parameters, however, with new processing techniques being explored with different time durations, the door is wide open to increased water and moisture content in the beans so it’s important to understand how it may play a role.

Importance of Managing Moisture Content with Experimental Processing

When undertaking an anaerobic fermentation, this method typically retains higher moisture levels compared to its aerobic counterpart. While this extra moisture helps preserve the cellular structure of the beans and reduce oxidative stress, it also requires careful management to avoid spoilage and mould growth.

Below are list of acids that are created during anaerobic fermentations that can cause problems if not managed properly:

1. Phenolic Taints - Bitterness, medicinal or antiseptic notes
2. Acetic Acid - Sharp, vinegar-like taste
3. Lactic Acid - Sour, tangy, creamy but when produced too much overly sour
4. Butyric Acid - Rancid, cheesy, or vomit like odour
5. Propionic Acid - Pungent, slightly sweet, and sour

4 Reasons Why Extended Fermentation Can Lead to Higher Moisture Content

Extended fermentation involves keeping the coffee beans in contact with water and other elements of the fermentation environment for a longer period. Here's why this can lead to higher moisture content:

1. Prolonged Exposure to Moisture
   
   During extended fermentations, the beans are immersed in a liquid environment (whether it's the natural juices from the cherries or added water) for an extended duration. This prolonged exposure allows the beans to absorb more water from their surroundings. The longer the beans stay in this moist environment, the more water they can take up, leading to higher overall moisture content.
   
   
2. Continuous Metabolic Activity
   
   The metabolic activities of microorganisms (such as yeasts and bacteria) continue throughout the extended fermentation process. These microorganisms produce various byproducts, including organic acids, alcohols, and carbon dioxide. The production of these compounds can alter the osmotic balance, potentially drawing more water into the beans as the environment becomes more saturated with microbial byproducts.
   
   
3. Absorption Through the Bean Structure
   
   Coffee beans are naturally hygroscopic, meaning they tend to absorb and retain moisture from their environment. During extended fermentation, the structure of the bean allows it to continue absorbing moisture from the liquid it's submerged in. The bean's cellular walls and internal structures can hold onto this water, increasing the overall moisture content.
   
   
4. Increased Surface Contact
   
   Extended fermentation often involves more movement and agitation of the beans within the fermentation tanks. This increases the surface area contact between the beans and the liquid, allowing for greater absorption of moisture. The beans are not only in contact with the liquid but are also subjected to mixing, which can enhance the absorption rate.

The Resulting Impact

The combination of prolonged exposure, continuous metabolic activity, and increased surface contact means that beans undergoing extended fermentation can end up with a higher moisture content than those fermented for shorter periods. While this can help in developing deeper and more complex flavours, it also warrants careful management post-fermentation to ensure proper drying and storage, preventing spoilage and mould growth.

By understanding these factors, coffee producers can take steps to monitor and control moisture content during and after extended fermentation, ensuring the beans remain of high quality and free from defects.

Water Activity and Moisture Content during Drying

When we talk about drying coffee beans, slow and controlled drying is the gold standard, but why?

Drying in this way ensures that moisture is reduced evenly throughout the beans, helping to maintain their structural integrity. If drying happens too quickly, the outer layers may dry faster than the inner layers, leading to "case hardening". This creates a barrier that traps moisture inside, increasing the risk of spoilage.

During drying, the moisture within the beans moves from the inside to the surface and then evaporates. This process needs to be gradual to prevent cellular damage and ensure that the beans dry uniformly. Rapid drying can cause the cellular walls to collapse or crack, which not only affects the bean's structure but also its ability to preserve flavours and aromas.

Aiming for a final moisture content of 10-12% is crucial, but we prefer to work with 10-11% due to the potential of the beans, even once dry, absorbing any moisture in the environment, during transportation an increase of 1% is possible. This range is a sweet spot that balances preventing microbial growth and preserving bean quality. If the moisture content is too high, it creates an environment where moulds and bacteria can thrive. On the other hand, too low moisture content can make the beans too brittle and susceptible to damage during handling and roasting.

Within this optimal moisture range, the biochemical stability of the beans is maintained. Enzymatic activities that can lead to spoilage are minimised, and the structural integrity of the beans is preserved, ensuring that the desirable flavours and compounds are locked in.

Targeting water activity below 0.60 is essential to slow down and black mould and bacterial growth. Water activity measures the availability of free water in the beans, which microorganisms need for their metabolic processes. When water activity is kept below 0.60, it becomes difficult for these spoilage organisms to grow and multiply.

By reducing water activity, you're essentially depriving microorganisms of the water they need to thrive. This slows down their metabolic activities, such as the production of acids and other compounds. Levels below 0.6 are considered safe for long term storage, ensuring that the beans remain stable and free from microbial contamination over time.

The Inner Working of Coffee Drying

Enzymatic Activities

Enzymes within the coffee beans play a significant role during the drying phase. These enzymes can break down proteins, fats, and carbohydrates, impacting the flavour and aroma of the beans. Controlled drying slows down these enzymatic reactions, preserving the desired compounds while preventing the formation of undesirable ones.

Oxidation and Lipid Peroxidation

During drying, exposure to air can lead to oxidation, particularly of the lipids in the coffee beans. Controlled drying helps minimise oxidation, reducing the risk of lipid peroxidation. Lipid peroxidation leads to the formation of rancid flavours, as the fats in the beans break down into aldehydes and ketones.

Microbial Inhibition

By carefully managing moisture content and water activity, the growth of spoilage microorganisms is inhibited. Moulds and bacteria rely on available water for their metabolic processes. When the water activity is kept low, their growth is stunted, preventing the production of off flavours and toxins.

3 Practical Tips for Optimal Drying

1. Monitor Regularly

Use moisture metres and water activity metres to regularly check the moisture content and water activity levels during drying. This helps ensure that the beans are drying evenly and reaching the desired moisture range.

2. Controlled Environment

Dry the beans in a controlled environment where temperature and humidity are regulated. This helps achieve consistent drying rates and prevents environmental factors from causing fluctuations in moisture levels.

3. Proper Airflow

Ensure proper airflow around the beans during drying. Good ventilation helps remove moisture from the surface of the beans efficiently, promoting even drying.

Tools for Monitoring and Controlling Water Activity and Moisture Content

Effectively managing water activity and moisture content in coffee beans is crucial for maintaining their quality and preventing spoilage. Using the right tools ensures that coffee producers can keep these levels within optimal ranges, preserving the beans' flavour, aroma, and overall integrity.

1. Moisture Metres

Moisture metres are essential tools for monitoring the water content in coffee beans, expressed as a percentage. These devices are available in various forms, from simple handheld models to advanced versions with precise measuring capabilities.

2. Water Activity Metres

Water activity meters, on the other hand, measure the free water available in coffee beans, which is crucial for microbial growth and chemical reactions. These devices range from basic models to sophisticated digital ones with data logging features.

3. Hygrometres

Hygrometres are used to measure the relative humidity in the storage environment. Available in both analog and digital formats, some hygrometers also include built-in thermometers for dual-purpose readings. By monitoring environmental humidity, producers can maintain consistent moisture content and water activity levels in the beans, preventing fluctuations that could lead to spoilage or quality degradation.

4. Desiccants

Desiccants are moisture-absorbing materials that help reduce excess humidity in storage spaces. Common types include silica gel, clay, and calcium chloride. Desiccants can be placed in storage containers or used as part of desiccant-lined packaging to keep water activity levels low, protecting the beans from mould and microbial growth during storage.

5. Hermetic Storage Bags

Hermetic storage bags provide a robust solution for long term coffee storage by creating a barrier against moisture and air exchange with the environment. Made from materials designed to block external influences, these bags preserve the bean's quality by maintaining a stable internal atmosphere. Using hermetic storage such as GrainPro or Ecotact bags with a jute sack are popular methods in Peru and with all the producers we work with.

Final Thoughts

As you can see there are quite a few critical factors when undergoing a coffee fermentation! 

The risks are there but also the gains. From our experience, those coffee producers who have experimented and found what works for them is the way they go, every year. A few experiments with new and novel processes but the biggest challenge for this to scale across Peru with smallholder coffee producers is how it can fit into their workflow and who pays for extra investment.

If the cup scores improve, which, invariably they will, then there are premiums to be gained and an incentive. However, if buyers refuse because the fermentation went wrong then it's a risk for the coffee producers and many are risk averse.

We hope that this article can be useful for roasters and coffee producers in Peru looking to learn more about the inner workings, benefits, and risks of advanced coffee processing.

References & Further Reading

Books

1. Illy, E., & Viani, R. (2005). Espresso Coffee: The Science of Quality (2nd ed.). Elsevier Science.
2. Hoffmann, J. (2018). The World Atlas of Coffee: From Beans to Brewing – Coffees Explored, Explained, and Enjoyed (2nd ed.). Mitchell Beazley.
3. Thurston, R. W., Morris, J., & Steiman, S. (2013). Coffee: A Comprehensive Guide to the Bean, the Beverage, and the Industry. Rowman & Littlefield.
4. Borem, F. M., Borém, R. A. T., & Schenker, S. (2014). Coffee: Quality Management and Postharvest Handling. Springer Science.
5. Pendergrast, M. (2010). Uncommon Grounds: The History of Coffee and How It Transformed Our World (Revised ed.). Basic Books.

Journal Articles

1. Martins, L. T., Silva, J. M., & Costa, M. H. (2022). Fermentation-Driven Aroma Profile Changes in Coffee: Insights from Multi-Omics Analysis. Journal of Agricultural and Food Chemistry
2. Williams, C. B., Perez, H. R., & Tan, M. L. (2021). The Role of Fermentation Conditions on Coffee Bean Cell Wall Integrity and Surface Characteristics. Colloids and Surfaces A: Physicochemical and Engineering Aspects.
3. Hendricks, A. M., Chen, D. C., & Rao, P. S. (2020). Probing the Dynamics of Flavor Precursors in Coffee Fermentation Using Spectroscopic Techniques. Journal of Physical Chemistry.
4. Souza, J. M., Silva, P. H., & Pereira, A. C. (2018). Dynamics of Fungal Communities during Coffee Bean Fermentation and Their Impact on Quality. Journal of Agricultural and Food Chemistry.
5. Carvalho Neto, A. L., da Silva, M. R., & de Souza, J. P. (2021). Microbial Succession and Metabolite Changes during Coffee Bean Fermentation. International Journal of Food Microbiology.
6. Silva, C. F., Batista, D. M., & Schwan, R. F. (2020). Influence of Yeast Inoculation on the Sensory Quality of Coffee Fermentation. Food Microbiology.
7. Pereira, M. G., Oliveira, L. S., & Franco, F. C. B. (2019). Characterization of Lactic Acid Bacteria Isolated from Coffee Fermentation and Their Potential Probiotic Properties. Food Research International.
8. Santos, L. M., Lima, F. A., & Silva, R. G. (2017). Metagenomic Analysis of Fermented Coffee Beans Reveals Novel Bacterial and Fungal Diversity. International Journal of Food Microbiology.
9. Almeida, B. L., Ferreira, J. V., & Santos, M. T. (2016). Impact of Fermentation Conditions on the Microbial Community and Quality of Coffee. International Journal of Food Microbiology.
10. Gomes, R. P., Costa, F. A., & Silva, D. M. (2015). Role of Non-Saccharomyces Yeasts in Coffee Fermentation: Metabolic and Sensory Implications. Food Microbiology.

Podcasts

1. Solis, L. (Host). "Making Coffee with Lucia Solis."
2. Harper, J. (Host). "Filter Stories Podcast."
3. CoffeeMind. "CoffeeMind Podcast."
4. Hoffmann, J. (Host). "Coffee Podcast by James Hoffmann."
5. Sprudge Media Network. "Coffee Sprudgecast."
6. Feran, C. (Host). "Coffee People Podcast with Christopher Feran."
7. Hendon, C. (Host). "The Coffee Science Podcast with Christopher Hendon."