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MICROBL1
The Microbiology of Coffee Processing.
Part 1 of a three part series , published in the PNGCRI Coffee Research Newsletter.
For generations the processing of coffee cherries into high quality ‘green bean’, has been an art and not a science.
It was carried out according to rules of hygiene and basic cook book art, keeping everything clean and doing it the way you were taught, by your predecessor, or you read about it in ‘Wellman’, ‘Sivetz’ or ‘Wilbeau’. It was less than forty years ago that that scientists in the EAIRO, the East African Industrial Research Organization, the combined research facilities of the major African coffee producing countries, began to write that the fermenting of coffee mucilage was not just a chemical reaction that happened, but that it was all controlled by microorganisms, and the enzymes that they produced. Despite the forty years, that knowledge has not yet seeped down into the coffee text books. The only quick reaction was the appearance on the coffee market of pectolytic enzymes such as Cofepec and Ultrazym. These products however, were not researched and designed for quickly fermenting coffee, they were repackaged products which were already on the shelves of the chemical companies, designed for other purposes such as clarifying beer and apple juice, and so priced accordingly. Of that more will be said later.
Virus’, Bacteria, Yeasts and Moulds,
We are all familiar with the processes of softening fermenting, rotting or putrefaction and disease caused by the various microorganisms which are usually classified into the above mentioned four major types. However, for the production of coffee the smallest in size, the virus’, can be disregarded.. Bacteria are the second smallest, they like to live and work in damp or liquid places, and in good conditions they will each divide and double their numbers every 15-20 minutes. It is certain types of bacteria which we need to encourage for the fermentation of quality coffee.
Only slightly larger than the bacteria and slower to reproduce themselves, around 30 minutes per generation, are the next biggest category the Yeasts. To those people who home brew and make their own bread, yeast appears in mass as a white pasty solid, or as dried granules which disperse in the brew to make a cloudy sediment. Individual yeast cells are right on the limits of human vision. If the conditions are not right for bacteria to grow quickly, then it is often the slower growing yeasts which will take over the action. And this spells lots of problems for the coffee processor.
Yeasts will first of all produce alcohol, but from a diet of mucilage rather than straight sugars they will also produce larger quantities of the various aldehydes and ketones that in brewing terms creates bouquets or the ‘nose’, and in coffee terms spells ‘fruity flavour.’ Then as we all know if the brew is left for too long the alcohol goes on through to vinegar and we are left with ‘sour coffee’.
The largest in size of the microorganisms are the moulds or fungi. We are all familiar with the white cotton wool fuzz that grows on old fruit or stale bread. Within a few days the white then changes colour to black, green or hues of red and yellow as the mould produces thousands of millions of coloured spores, which one puff of air will disperse into the atmosphere to drift for miles before they settle. The fact that it takes 3 or 4 days for those mould spores to germinate, grow and reproduce, albiet in such large numbers, means that anywhere where the conditions are alright for bacteria and yeasts to grow, then the moulds just cannot compete. They are still there in their original numbers but you just don’t see them.
It is not only the mould spores however that can travel. When those lovely liquid growing conditions for bacteria and yeasts start to dry up or become less habitable, then both of these microorganisms will go into a kind of hibernation and also produce very resistant spores, just like the moulds. Every breath of air accross a garden or plantation will carry millions of these microscopic embryos which will be caught on the sticky surface of ripe fruits. Most of these spores actually come from the soil and are an intrinsic part of the dust. Good dark healthy soil has millions of micro-organisms per cubic centimetre. It only takes a few minutes in the warm humid atmosphere of a picking bucket or bag with damaged cherries oozing sugary juice everywhere to create an enzymatic powerhouse of germinating spores of all kinds, each eager to outdo the other take control of the situation and reproduce themselves ahead of the rest.
So, there is little point in trying to keep a factory super clean, although basic cleanliness is still desirable, because both the good and the bad microorganisms are all there on the cherry itself, a dynamic equilibrium just waiting to happen.
Achieving Good Fermentation.
For good fermentation the kind of bacteria that we want to encourage to outstrip the rest are what in plant disease terms are called the soft rot or brown rot bacteria. These types principally belong to the families of Erwinea, Kliebsiella and Bacillus, all of which which cause rots and decay. They also can ferment Lactic acid. Once the cherry skin is broken then the firm texture of the fruit is destroyed by liquid enzymes, ie, bacterial saliva, and the organisms can then spread quickly through the soft liquid environment which they have created for themselves. The fact that we remove the skin and expose the mucilage in the coffee pulping process just makes it happen all the quicker.
Part of the confusion in the minds of earlier research people was that in the process of normal development, fruit produce their own internal supply of pectolytic enzymes which softens the fruit naturally as it ripens..
Although these ‘natural’ enzymes will act in concert with the bacterial ones, they do act much slower and under different conditions. The inside of an intact fruit is anaerobic, and so the natural fruit enzymes tend to act better under water. The subject of ‘enzyme kinetics’, finding out the speed with which various enzyme systems can react and work was the breakthrough in research which has put emphasis very heavily on the side of micro-organisms for achieving quality fermentation of coffee. Bacterial pectolytic enzymes starting on the fruit surface act much faster in aerobic conditions, or where there is oxygen dissolved in the solution. Yeast enzymes are somewhat ambivalent, but tend to prefer anaerobic conditions. So, if it is necessary to keep fermenting coffee under water for other reasons, then don’t let the water stagnate for more than a few hours 7-8 at the maximum. The dissolved oxygen content in the water is vital.
What is an Enzyme.
An enzyme is a biological catalyst, a complex organic chemical which enables other chemicals to react or change but does not get used up or converted itself. Therefore, even a very small of an enzymes can continue to work over and over again creating changes at low energy levels and without any apparent effort. Depending on their origin enzymes differ remarkably in their preferred working conditions. We have talked about oxygen levels or aerobicity, and the two other major factors involved are temperature and acidity.
Looking at Temperature.
A major rule of thumb for chemists is that between say 0 and 50oC, the range of most living things a rise of 10 degrees will essencially double the rate of any chemical reaction. So, the warmer the water supply the quicker the quicker the coffee will ferment. If it is possible to get the factory water supply up only 4-5 degrees to around 24-25 degrees then it should be possible to ferment coffee in 12-16 hours. The easiest way to do this is to recirculate ones pulping water.
By recirculating the water both the temperature and the enzyme concentration are raised and whatever hour pulping is finished, as long as it is before midnight, you can start washing it our first thing next morning. Even though the pulping is started with cold water that first pulpage is in the bottom of the tank, and all the warmed water put through later will have drained down through it to bring conditions into line with the rest of the tank. However, if the tank is too deep, then despite the continued drainage, conditions may become anaerobic at the bottom and fermentation will be slowed down. Nevertheless by constantly recirculating the pulping water, that which is draining down through the tank will carry entrained and dissolved oyxgen down with it, sufficient to keep the enzymes working right to the bottom. Letting pulped coffee go solid in the tank will slow down the bacteria and encourage the yeasts to take over and fruity flavour will be sure to result. Prolonged fermentation times are usually a temperature problem, but acidity and lack of oxygen may also be involved.
If the the fermentating pulpage that has stalled is washed, that is, pumped from one tank to another preferably with warmed water, then the mix will be reaerated, the liquid mucilage allowed to drain away, the soft rot bacteria will be encouraged to win their battle against the yeasts and normal fermentation will recommence.
A good ploy to both warm the pulping water and aerate it is to pump the clean water supply early in the day and run it over a roof, preferably painted black., and then down through the gutters and spouting into a low level holding tank. From there the normal pumping systems can use it and recirculate it until pulping is finished. By the time that the cherry picked in the heat of the day comes into the factory, it is usually 25 degrees centigrade or more. The aim should be to keep it as close to that temperature as possible by water recirculation. However, after pulping is finished that very sugary and highly discoloured water must be discarded. Also, any tanks and pipelines that have carried this recycled water must also be washed clean at the end of the day. Failure to do this will allow the buildup of a mucilage coating on the walls. By the next day it will not only be vinegar, but it will also be highly anaerobic, and therefore nasty bacteria like the Clostridium family may have started to multiply. Clostridium normally spells ‘onion flavour’ The vinegar aspect however leads us into the last point of acidity.
Fermentation and Acidity.
Pectin degrading enzymes from bacteria can only work in neutral to slightly acid conditions. In contrast, yeast and fungal or mould pectolytic enzymes prefer much more acid conditions. If one reads the technical literature that comes with commercial fungal based enzymes like Ultrazym 100, see figure 1, to get the best effect the pulpage should be at a pH of 5 or less. This is quite acidic. At a normal pH of 6 to 7, nearly half of its effectiveness is gone. In contrast bacterial enzymes like to work at near to neutral conditions ie.a pH of around 7. The usual causes of a very slow or a halted fermentation are low temperatures, low pH, and anaerobic conditions. So, a tank of coffee that hangs up on the fermentation then becomes an ideal case for the use of the acid loving fungal ‘Ultrazym’. A surprisingly small amount, less than half of a 50gm packet costing around US$10-00, if well mixed and aerated into up to 20 tonnes of coffee by pumping the coffee from one tank to another, will really work wonders in the space of 2-4 hours, depending on that final factor the temperature.
To explain this figure of pH, the chemical measure of acids and alkalis, is a logrithmic number. So that a pH of 5 is 10 times more acidic than 6 and 100 times more than a pH of 7, which is considered to be neutral. Pure water has a pH of 7 and past that, pH 8, 9 or 10, is becoming very rapidly an increasingly strong alkali.
Coming Up!
In the next article we will show how easy it is to make up ones own enzyme mixture using lights and floater beans. This will work better than the expensive fungal types under normal acidity conditions. We will also look at the way in which taints and flavours, including the dreaded Rio flavour are often caused by microorganisms and how they can be prevented. And we will look at the problems and the potentials of microbes in the washing and drying of coffee.
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