The Microbiology of Coffee Processing.

Part 4.

Biomass Wastes Treatment in the Coffee Industry.

Making the Industry Energy Self Sufficient.

Ex. Principal Research Scientist. Coffee Processing.

Papua New Guinea Coffee Research Institute.

In the first three parts of this series we have looked at how Microbiology can help us to understand the problems of ; 1/. Picking, pulping and fermenting coffee. 2/. Washing and drying parchment coffee. 3/. Hulling polishing and packing green bean coffee. This new biological view point is in terms of understanding how bacteria yeasts and moulds participate in what previous generations of Coffee Millers took to be basic “house keeping” processes, and just a matter of keeping everything clean. Now, by understanding these processes better, we can convert coffee cherries on the tree to export green bean better, quicker and to a much higher liquoring quality than ever before. And furthermore we can use the waste products to provide the energy to do it. The Coffee industry now has the potential to be totally self sufficient in Energy!

Coffee Wastes:

The microbiological view point can also help us to treat our coffee wastes in new and innovative processes. For 1000 years now the coffee industry has stayed with historical traditional ways which have looked only at producing green bean for export and the rest as waste. What the Coffee Industry in the Third World needs is additional sources of income from coffee byproducts which up to now have been virtually ignored. Let us just remind ourselves, that if we go along with the traditional origins of coffee as a drink, then we have to remember that what Kaldi the Goat herd passed onto the Abbot who wanted to keep his Monks awake, was all about the coffee cherry, not just the beans inside. (Q.V. The Great Millennial Mistake). Because it is only the beans that we know how to preserve for export, the rest has been thrown away. For every tonne of green bean prepared for export, the local country side and its waterways have to reabsorb around three tonnes of wet fruit pulp and up to 6 tonnes of high BOD heavily polluted water.

The only waste that has been used at all is the dry husk removed from coffee parchment. This makes good fuel for drying coffee. However it is been burnt and used so inefficiently, that today, nobody believes that we can get enough energy from the husk or hulls to actually completely machine dry the same volume of wet coffee parchment from which it came. With efficient countercurrent dryers this is entirely possible. However, lets start at the beginning with the water.

Coffee Waste waters:

First thing to do is to minimize processing water usage as much as possible by intensive recycling. Q.V.Part 1, This will reduce coffee fermentation time to 6- 8 hours by raising the temperature of the water and by increasing its level of fermentable sugars and enzymes. It will also greatly improve the colour of the dried parchment in the centre cut area, by fermenting or digesting out mucilage that mechanical or semi washed processes can not get at.

The biggest advantage however, is reduction in factory size. When each daily batch of cherry can be treated in less than 24 hours, before the next batch comes in, that is a great saving in tankage and working space.

After about 4-5 hours of pulping, or when starting on a new receiving tank, the very dark batch of processing water should be completely discarded and pulping started off afresh with clean water. There is no need to add any bacteria, cherry that has been picked for several hours, kept in a bag or in bulk and allowed to heat up, will be a seething mass of micro-organisms of all kinds working on the released sticky fruit juices. The trick to good processing is to encourage the good ones, principally Kliebsiella & Erwinea species, and suppress the bad, Yeasts and Clostridium species. Q.V. part 1.

Biogas from Waste Water:

All waste water should be collected in an ‘acid’ tank or pond, preferably elongated, so that passage from one end to the other gives a set time of residence, usually 12-24 hours, depending on ambient conditions, and factory throughput. During that time, aided by about 5% of feedback, all the mucilage be it biologically or mechanically, retted or removed, is broken down to short chain oligosaccharides which cannot be digested any further. (Q.V. Soluble Dietary Fibre, and “The Great millennial Mistake” ) However, as the sugars are fermented down to alcohol and then vinegar, the acidity or pH drops to 3.8 and that will throw all the mucilage/pectins out of solution to float on the surface as a yellow scum, which rapidly goes black and solid on exposure. These solids should be raked off the surface of the acid pond as required and returned to the cherry skins and pulp solids for compost production.

From the far end of the acid pond one can take out a clear yellow solution of

scvfa’ or ‘vinegar’. This acid solution should now be intermittently pumped up through an open topped tank of 5-10mm limestone or marble chips. This will neutralize the acids to mainly acetate salts and raise the pH from 3.8 to 6.1. The CO2 foam so formed will float out more solids, principally dark coloured tannins and polyphenolics. Evolution of carbon dioxide, CO2, at this point enables the later production of a highly methane enriched biogas with only half of the usual level of inert CO2. Some Biogas Experts will also want to know where the hydrogen goes which is usually also evolved at this point, because it should be contained and converted on into more methane. However, the reaction in the acid pond is more akin to the rapid production of fermented silage or vinegar, rather than highly anaerobic ‘acetogenesis’ , which runs in parallel to methanogenesis, and minimal hydrogen is evolved.

The clear acetate solution can then be passed through a UASB digester to make biogas, or, dripped over a suspended curtain as in the aerobic ‘Fungi Gulp” process to make Single Cell Protein for animal feedstuff. Biogas production levels depend very much on the amount of original recycling and concentration, but a good ball park figure is around 5 litres of gas per litre of strong acetate solution. Do also note that the Renertech UASB/EGSB sludge blanket is different to the usual kind in that it has a lot fewer acetogenic bacteria, and lots more methanogenic ones. The original batch of this kind of sludge took a long period of enrichment processing to develop but supplies of seed sludge can be made available to short circuit the enrichment process for those in a hurry. http://www.coffee.20m.com/CoffeeWasteWater.pdf

The biogas produced can be best be used by running an engine on it to generate electricity, and all the lower grade waste heat from cooling and exhaust can still be used for drying coffee as before. The engine, preferably a diesel dual fuel model, can also be used for burning producer gas from coffee husk, as outlined below.

Passage through the biogas digester will reduce the BOD by over 90%, but what remains contains remnants of the indigestible fruit colour compounds, which as the acidity falls suddenly reappear as the familiar black colouring in the water that indicates the presence of a coffee factory for kilometers down stream. At this stage, all that can be said is that this colour, akin to the brown colour of swamp water, is harmless to fish, and intensive research is going on in the wine and olive oil industries, who have a lot more money than does coffee to enhance present methods of removal , as below.

Tertiary cleanup of Coffee waters:

From the biogas digester the neutral effluent water should be discharged into an artificial wetlands created by growing hollow stemmed water plants in a series of shallow ponds. Reeds and rushes can grow in highly anaerobic water because they pump enough oxygen down to their roots through those hollow stems to keep them alive, along with their symbiotic bacteria, the bugs that can start changing the black water into brown swamp water and begin the process of re-oxygenation.

In areas where water hyacinth is available, The reed pond can be supplemented with a smaller but deeper pond of water hyacinth which will create the most effective biological tertiary filter system known to ‘mankind’. Instead of outputting water with a dull leaden coloured surface, due to a remaining high bacterial count, that wonderful filter of filamentous hyacinth roots, are what can make the water really sparkle once again. Do note however that water hyacinth does not like anaerobic water, nor salts in solution, so that first large reed pond, or ponds, is a vital part of the system.

http://www.coffee.20m.com/RenerTechWWSystem.pdf

Coffee Pulp Solids to Silage:

A major instigation for the writing of these articles, is the attempt to educate coffee management to the possibilities of creating additional income by developing byproducts along with the seasonal production of green bean. Coffee pulp is really a very versatile substance, but the presence of caffeine has up to now been seen as a negative factor making it unusable as an animal feedstuff. Only in the last few years has it been discovered how to convert it into animal feed silage in a way that digests out the alkaloids and tannin antinutritionals by using them as a source of nitrogen for protein synthesis by the bacteria concerned, usually lactobacillus species, and Lactobacillus plantarum in particular. By a slight dewatering of the pulp, inoculation with commercial silage additives and packing it into plastic liners within FIBCs, or one tonne flexible bulk containers, within 3-4 months an excellent feedstuff suitable for cattle feedlots is achievable, bringing an extra cash flow during the off season period.

Mushrooms:

In contrast to the larger scale operations required for waste water treatment and silage making, coffee pulp can also be handled on the small scale family level operation with ease. Fermented and partially dried, pulp used as a substrate for growing exotic mushrooms. Of particular interest is the remixing of husk and semidried pulp to fast grow Shiitake, Linchi and other mushrooms that traditionally take many months to grow on billets of cut oak wood. With coffee wastes it is weeks rather than months or years.

Even quicker is the production of Pleurotus or Oyster mushrooms which normally grow on rotting trees in the bush. In areas where mushrooms are a prized food delicacy, smallholder coffee growers can bring in a significant cash flow from their local markets. All that is required are plastic shopping bags, two cookers made from a cut down oil drum, strong hands to squeeze out the water, and a quiet corner of their house or a shed. Propagation from local mushrooms found in the bush is possible, but if the local Agriculture Depts., do make mushroom spawn available that is usually grown on rice or wheat grains, then that is indeed a lot easier. Seminars to teach how this can be done, should be a priority for National Coffee Administrations, to create Demonstrators/Teachers in every district area.

Coffee Husk:

Husk is practically pure lignocellulose and has no fertilizer value at all. Yes, we all know how to get rid of the mountains of husk outside our dry factories, we burn it in crude furnaces to dry our coffee parchment. If most of the parchment is partially sun dried for quality reasons then, even with today’s crude single pass hot air driers, it is still possible to have a surplus of fuel after a finish drying operation. What more do we want? What we could do is burn the husk in a gas producer, and then run an engine on that producer gas to produce electricity. Once again as with biogas, the waste heat from the gas producer and the engine can be used to heat a clean air stream, and that can still be used to dry even more coffee than before.

Countercurrent Drying of Coffee:

The grain drying industry in Europe and the USA has developed techniques of extracting every drop of useable heat by passing the hot air and the grain in opposite directions. The problem with using

most grain dryers for coffee, is that grain only needs the final 5-10% moisture removed after harvesting from the field. Whereas, coffee goes into the drier at 55% moisture and needs 43% of that moisture removed. So much moisture, along with the remaining mucilage causes sticking and bridging of the coffee beans, which will not flow through the contorted pathways and holds up in the dryer. It is usual therefore to ‘skin dry’ coffee in a flat bed dryer , where stickiness is not a problem, and once the parchment is skin dry, then other types of dryers can be used for the finish operation. If however, the coffee is very well fermented and washed an soaked, as outlined above, then most of the stickiness is alleviated, and drying in one operation can be attempted. It must be stressed that partial sun drying, particularly in the early stages, is very important for final liquoring quality, and therefore multiple handling in the drying process is usual. However, during times of bad weather it is essential to have machinery that can cope with sopping wet parchment if required.

The major problem with single pass flat bed dryers is that the air is far too hot in the first instance, and unless the wet parchment coffee is vigorously stirred then the bottom half of the bed is dried far too quickly and parchment is cracking, before the top is even half dry. In contrast, a counter current operation means that there is no great difference in relative humidity between the stream of air and the coffee that it is passing through at that point. Therefore all the problems caused by overly rapid drying, cracking parchment, case hardening of green bean, micro cracking , and release of oil causing premature aging of green bean can all be alleviated . (Q.V. Part 3.) There is one model of grain dryer which does seem have all the attributes that would allow it to be also used for coffee, and at a very reasonable price. However, the all important field trials have yet to be done.

http://drzewicz.home.pl/english/ and the SP100 model.

Making Compost:

Coffee pulp solids contain only one fifth of the nutrients taken out of the soil by export of the green bean. Therefore, although it is a good source of humus and organic soil carbon , it is not an answer in itself to make a complete fertilizer. However, just piling it up in a heap and forgetting it for a couple of years is no way to get anything like the value that it does have. We are all familiar with the results of such neglect. A black crust 100mm deep over the pile, loaded with vinegar flies and inside the bright pink highly acid sticky silage or sauerkraut, which goes brown the moment it is opened up and exposed to the air. Direct use of that acid material will sicken coffee roots unless it is opened up and exposed to air until it too has gone not just brown but intense black, when all the toxic tannins and other polyphenolics have condensed into humus.

Just leaving heaps of pulp for such long periods allows most of the nutrients

to drain away as a thick black liquid which is highly polluting should it get into any water ways because it contains most of the caffeine content of the original pulp. If coffee pulp is turned over every few days, as in conventional compost making, it will compost in three weeks into one fifth of the original volume of a stable earthy smelling material which does not attract flies. Left to mature for three months under cover, it will reduce further to become a very nice dry earthy compost which is a good soil improver and conditioning agent but not a fertilizer in the real sense. It is at the point where the pile begins to heat for the second time, after the first turnover, that the collapse of structure occurs with a massive release of black sticky liquid which contains most of the nutrients and is the real fertilizer material. This liquid should not be allowed to flow away, it should be collected and sold as a high value organic plant nutrition agent and pest deterrent to give an extra source of cash flow.

The whole process can be speeded up even further and the production of liquid and alkaloid increased, by the addition of 0.05% of the weight of the pulp as urea, or better DAP fertilizer. That fertilizer content will be recovered as technical organic nitrogen in the form of bacterial protein within both the liquid and the solid compost.

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