The elements affecting the coffee sector are appearing before us day by day in different ways. The point it has reached from the days when we heard the concept of specialty coffee and it started to develop and spread all over the world to today is quite surprising. Until recently, coffee lovers all over the world were discussing what specialty coffee is, what are the elements, what are the parameters, but now they find themselves in brand new concepts that are born day by day. The chemistry of the bean and the magic of the unknown it creates are indisputable, but it has scientists running behind it. We, scientists, are determined to solve and illuminate this structure. Every passing day, not us, but the coffee bean teaches us something and winks, as the saying goes, “I have many more secrets”.
The term Specialty Coffee refers to a beverage that is of higher quality than those available at commodity prices, as well as having a unique taste and characteristics.
When it comes to quality coffee, the topics we used to talk about, such as altitude, garden, variety, brewing, roasting, have quickly given way to parameters such as water chemistry, antioxidant activities, development of tasting notes, fermentation and even fermentation vessels. As we said, parameters from the bean chemistry, which is a sea, are changing and developing every day. The subject of today's blog post is the effect of stainless steel tanks on fermentation. We will summarize the information we have collected from this study conducted in 2021 and the new term studies that followed, blend it with our own scientific data and try to present the proven facts of today.
As is known, the fermentation process begins after the coffee beans are washed with water and kept within a certain temperature range for a certain period of time. During this process, chemical and physical changes occur in the coffee beans due to microbial activity, resulting in a different taste and aroma profile.
A bioreactor is a vessel in which the optimum environment is provided to meet the needs of any biological system and to achieve its high efficiency. By means of bioreactors, enzymatic, microbial and cellular reactions occurring in the medium are simulated. Two types of fermentation are carried out in bioreactors: submerged fermentation (SMF) using liquid substrates or culture broths and solid state fermentation (SSF) using solid substrates such as soybean meal, bran, pulp and others.
SSF mimics the natural environment of organisms, requires less energy for sterilization, is less susceptible to contamination, and produces high concentrations of final product. In the food industry, bioreactors mainly produce alcoholic beverages, vinegar, yeast biomass, enzymes, yogurt, other lactic beverages, flavor compounds, and pigments that increase their usefulness for coffee fermentation. Coffee is a non-alcoholic product with natural antioxidant properties and has been pre-fermented to form flavor precursors. Many studies have shown that fermentation changes the sensory profile of beverages. In general, coffee, as we know, ferments naturally because it is exposed to microbial action in the environment. On-farm fermentation is carried out either on open platforms, in concrete tanks, or in polypropylene bioreactors.
Usually, the fermented quantities are small, about 20 to 80 L, to facilitate homogenization of the vessels, and their feeding is done manually. Although the vessels are made of low-cost materials, fermentation control and cleaning are difficult. These problems can be avoided by applying stainless steel automated bioreactors for continuous production of specialty coffees under better process control and better hygienic conditions. In coffee processing, fermentation is an important step for microbial activity that produces enzymes that hydrolyze pectins or other compounds that will be converted into flavor precursors. In addition, other microorganisms can inhibit the growth of mycotoxigenic fungi. During fermentation, a series of microbial groups degrade carbohydrates and other organic compounds that are converted into flavor precursors. Automated bioreactors allow better control of microbial succession, temperature and pH values during coffee fermentation. Specialty coffees can be mass-produced with high quality and reproducibility in different harvest seasons. Of course, the uncontrolled and, so to speak, shameless consumption of quality or standard coffee has insidiously shown everyone that access to quality coffee is under threat in the future. And thus, it has pushed everyone to take such actions and think about improvements. Because while there is a lot of demand, the supply has remained quite low and especially quality coffee production has been forced to grow and break out of its shell, stuck in a boutique area.
Until now, farmers could only produce nano-lots (a term meaning the minimum quantity for export, around 100 kg of green coffee) through a laborious process. The quantities produced are mainly for international competitions, the profit is usually high, but the quantity is small compared to micro-lots (600 kg) or commodities (over 20,000 kg).
Therefore, studies are being conducted with the aim of increasing fermentation performance with natural/paste natural and processed ungrafted/ungrafted coffee forms with new 0.316 m3 stainless steel bioreactors built to increase specialty coffee production under controlled fermentation conditions. In an experimental study, the stainless steel tank was designed in a cylindrical shape with a total capacity of 0.316 m3. Each bioreactor has a height of 900 mm and a diameter of 650 mm. The bioreactor base consists of a conical funnel with a 25° inclination that allows the liquid from the exudation to slide down without interfering with the substrate consumption of the microorganisms. The tanks also had a 4 cm diameter butterfly valve made of stainless steel. This valve allows the coffee to be completely or partially opened and flow out after fermentation and also allows the exudate (permeability) formed during fermentation to dry faster.
To mix the coffee, mechanical mixers were developed to improve internal homogeneity, homogeneity in fermented beans, oxygen transport and heat transfer, and two mechanical mixers were installed in the tank. The first one actually resembles a hoe in design and is designed to mix the coffee without damaging the rim of the chamber and the conical bottom. The second one is designed as a paddle type mixer, aimed to completely homogenize the inside of the mass. It uses a 1 ½ CV motor for the hoe mixer and a 1 CV motor for the propeller type mixer, both of which are set to 50 rpm and are aimed to rotate counterclockwise. The bioreactors are placed in an on-farm facility consisting of a closed room without windows, which maintains a constant temperature during fermentation.
In the established setup, temperature, pH and microbial activity, which are of vital importance for fermentation, were of course continuously monitored.
In the study, coffee beans were kept at a certain temperature range (20-25°C) during the fermentation process. The pH level was adjusted to be between 4.2-4.5. These temperature and pH ranges are commonly used to provide the desired quality and aroma profiles in the coffee fermentation process.
The results of the study are summarized as follows:
This study focused on the natural fermentation of coffee beans, meaning that instead of adding a specific microorganism culture, the beans were relied on to rely on their own natural microbial flora.
The graph showing the pH changes of coffee beans during different fermentation times (12, 24, 36, 48, 60 hours) showed that when fermented in stainless steel tanks, pH values changed more evenly over time.
In samples fermented in plastic containers, larger changes in pH values are observed. Especially after 36 hours, pH values began to decrease rapidly. This situation proved that stainless steel tanks provide a more stable fermentation environment and pH values can be controlled more evenly. It has been shown that fermentation using stainless steel tanks provides a higher humidity rate and a lower acidity rate compared to classic plastic fermentation containers. However, no significant difference was observed between the two methods in terms of alcohol content and pH values resulting from fermentation.
Coffee fermentation using stainless steel tanks has been observed to produce higher levels of volatile compounds, particularly acetic acid, propionic acid, valeric acid, ethanol, 2-phenylethanol, 4-ethylguaiacol, and 4-ethylphenol.
It was also observed that the volatile compounds in coffee fermentation obtained using stainless steel tanks presented a more distinct aroma and taste profile than those in fermentation carried out in classic plastic fermentation vessels. These results show that stainless steel tanks are an important tool in the coffee fermentation process and play an effective role in the production of quality coffee.
The differences between stainless steel tanks and classic plastic fermentation vessels in terms of coffee fermentation can be summarized as follows:
Temperature control: Stainless steel tanks provide better temperature control than plastic fermentation vessels. This contributes to a more consistent fermentation process and ultimately a higher quality coffee product.
Oxygen control: Stainless steel tanks provide better oxygen control than plastic fermentation vessels. This prevents oxidation of the coffee, contributing to a fresher flavor and aroma profile.
Hygiene: Stainless steel tanks are more hygienic than plastic fermentation vessels and can be easily cleaned. This is important for a healthy fermentation process.
Cost: Stainless steel tanks are more costly than plastic fermentation vessels, so they may be less suitable for small-scale coffee producers.
In general, stainless steel tanks may be more suitable for obtaining a higher quality product in coffee fermentation, as they provide better temperature and oxygen control and have a more hygienic structure. At the same time, as mentioned above, one of the most important differences is the formation of high levels of volatile compounds that create flavors that affect the area of interest of us coffee lovers, which seems to increase the interest in stainless steel tanks despite their cost.
DUYGU KURTULUŞ
Co-Founder / Chemist / Nanotechnology Engineer / Hazardous Chemical Consultant / Chemical Evaluation Specialist