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The Microbiology of Coffee Processing.
Part 3. Tertiary Technology.
Ken Calvert.
Processing Dept.
C.I.C. C.R.I.
In the first two articles in
this series we have looked firstly at the wet processing and then at the drying
of coffee parchment, and how the way in which these operations are carried
out can lead to a range of taints and off flavours and major problems in maintaining
a good quality product. However it
does not finish there. Even at the
stages of hulling, grading and shipping coffee, when the moisture content
of the green bean is so low, <12%, that it should be protected from bacteria
and yeast infections in terms of its low >water activity=,
it is still helpful to look at the problems involved, in terms of how they
may be intensified by the presence of microorganisms.
The basic causes of the problems of premature aging and deterioration
of quality of polished green bean are elevated
temperature and humidity. That is, the sweating of coffee in the
drying fields and the overheating of green bean by mechanical drying and
hulling.
Moisture Effects:
The easiest way to understand these problems is to look
at our own bodies. Even on a very
hot day, if we are out in the breeze, our skin remains dry despite the fact
that we are keeping cool by evaporating a lot of water. The thing is that the moisture evaporates from under the skin, it
is under the skin that the cooling effect occurs, and only a gaseous vapour
of nearly pure water leaves us. However,
restrict the flow of air around us to raise its relative humidity, and wrapping
ones self up in a plastic raincoat is the extreme case, then we still lose
the same amount of water but it comes out through the skin as the liquid we
call sweat, and it has no cooling effect.
Sweat however is far from being pure water. It is a solution of salts, sugars
and other nutrients which then deposit on the skin surface, and makes ideal
food for microorganisms. All the stale
sweaty odours of the great unwashed, and the unpleasant sticky feel of our bodies
when we are not clean, are the obvious result of a lot of unwanted microbial activity. It is also pertinant to mention at this point that
coffee which is left to soak in water for overly long periods is also subject
to this same leaching process. With water outside as well as inside the bean,
the transfer of soluble sugars and salts accross the surfaces of the bean
can also happen even quicker. Some knowledgable sources give a figure of 1-2%
loss in the eventual weight of the green bean for every day that coffee parchment
is unneccessarily in contact with water. Therefore fermentation washing and
soaking times should be kept as short as possible. Nevertheless, do not do
away with any of these processes, especially the washing and soaking, or there
will be an even more costly loss in ultimate liquoring quality.
As stated, coffee beans are no
different to the human body. The
ideal way to dry coffee is to use conditions where the air surrounding the
beans has such a low relative humidity that moisture is lost as a vapour only,
leaving all the nutrients back inside the bean, to maintain its maximum weight for sale.
However, let your coffee sweat, by leaving it wrapped up in a coffee
sail when the sun is shining, and everywhere a droplet of coffee sweat forms,
under the parchment and the silverskin, and on the surface of the bean itself,
and there within a matter of days will be a patch of microbial activity which
shows up as a spot of pale blue fluorescence on the surface of the polished
bean when it is placed under an ultraviolet light.
A small U.V. fluorescent light, such as the banks use for checking
signatures, is a very useful tool for a Factory Manager.
A further cause of U.V. pale blue fluorescent areas on polished
green bean, is when the beans are crushed or damaged by impact with something
solid. Overactive bag pulping, with
a stone or a billet of wood is a major problem, especially when the bag is
placed on top of something solid like a road.
Older type Kivu pumps with open impellor blades are bad for both skinning
and impact damage of wet parchment, particularly when they have been speeded
up by use of a four pole or even a two pole electric motor. If there is even a small gap between the sides
of the impellor blades and the front and back of the pump casing, then the water sweeping back through the spaces
will trap beans, so that they are pressure rolled through the gap by the moving
impellor. Being soft and juicy they will spring back into shape, often minus
their parchment coating. When there is lots of skinned parchment appearing,
check these kind of clearances in all your machinery.
However, wet beans that are crushed in this way will extrude watery
juice from the damaged cells that is full of nutrients, just right for microorganisms
to grow on.
Over time, these UV fluorescent pale blue areas will be lost in
the overall build up of the frosty coating that typifies aged beans and which
glows white rather than blue under the
same UV light. That frosty coating is a
mix of microorganisms and oxidation products which inevitably build up on the
exposed surface of green bean over a
period of months. The best that can be
done is to understand how to delay its onset as long as possible, or at least
until the green bean has been further processed into roast and ground
coffee.
Oil extrusion:
The major cause of that white fluorescence, is not moisture damage
but oxidation of oilly products. Green
bean coffee has almost as much oil content as do peanuts. We are all familiar with the oily appearance of roasted peanuts
or of over roasted coffee. The major
problem for green coffee however is a bit more insidious. When coffee parchment is dried down to about
15% moisture content, the black colour of the bean starts to disappear, as
the 'water/oil' emulsion within the bean cells changes over from a predominantly
water based system to an oil based 'oil/water' one. From that point on, damage to the surface of the bean during hulling
and polishing, will extrude oil rather than water based products, and the
microbial damage to oily products fluoresces with a different white colour
instead of blue. Using a portable
battery U.V. light, an astute factory manager can diagnose a lot of his own
quality problems in this way.
Apart from over vigorous mechanical
abrasion during hulling and polishing, just overheating the coffee in a hulling
operation or in a dryer fired to above
50oC, will start to extrude
sufficient oil emulsion on the surface of the beans to set those resident
mould spores and other microbiologicals into intensive activity with an especially
rich supply of food. This is primarily
a problem of case hardening and micro cracking.
When moisture is pulled out of the surface layers of the drying bean faster
than the moisture can move accross from the still damp interior, the surface
of the bean is hardening and shrinking over a still swollen interior. So,
just like the cracking of parchment through too the too rapid skin drying
of wet parchment, the actual horny surface of the green bean itself is subject
to 'micro cracking'. Even though the cracks and indeed the horny layer itself
will dissappear as the moisture evens up, those micro fractures remain. And
that will allow nutrients to seep out onto the surface to cause lots of problems
down line. It is overheating in the huller however that is the major
cause of premature aging. Over intensive polishing activity combines
with the temperature rise, to scarify and remove the naturally antibiotic
surface layer of cells on the green bean and allow fluids from damaged cells to leak out on the surface. Some experts may want to argue that the subsequent
reactions are purely the chemical oxidation and rancidity reactions of exposed
oily biochemicals, but there is little doubt that those oxidative chemical
reactions are stimulated by the enzymes produced from living microorganisms.
Cross coupled experience, in the food industry, especially in the processing
of nuts, such as walnuts, almonds, roasted peanuts
etc, where broken and polished nuts are protected from such oxidation, usually
termed rancidity, by application of a minute amount of a commercial antioxidant
such as vitamin C, would lead one to make the suggestion that premature aging,
and indeed any aging of coffee is preventable. However coffee, having several hundred more
years of history in its traditional
practices, tends to be very conservative
in matters like this.
When freshly hulled green bean has a warm greasy feel, then it
is already a major step on the way towards premature aging. There is often
enough oil on the surface of such coffee, to react with the fine dust in the
grader and make a kind of varnish which will build up on the screens and slowly reduce the hole size, unless they
are checked and scraped clean at
regular intervals. One has only to be
accused of trying to pass an A-B coffee off as A grade, or a mixed A as
straight AA, to learn a swift lesson about the need to continually check the
screens in ones grader for build up of such deposits.
Bentall Okrassa hullers, which have the hulling and polishing operation
combined, are well known in the industry for overheating coffee. Separate the hulling operation from the polishing,
and allow the green bean to cool between each operation is good practical
advice to new players. Despite the
number of Kaack impact hullers which are lying derelict in older factories,
there is some method in their use, to minimise both the temperature and the
scarifying action from intensive polishing, and the destruction of that naturally microbial
resistant surface layer of intact cells. The thing is that impact hullers tend to leave not only the silverskin but that protective layer
intact for as long as possible. Hence
their use in the coffee Benificios
deep inland in Brazil. This reduces
the bulk and volume for storage and transportation out to the coast, but preserves
the quality as long as possible, until that final polish and pack just before
shipping.
Lets get back to >Impact= primary hullers:
A further good reason for
processors of Y grade coffee to look again at the use of impact type machines
as the first stage in their hulling operation, concerns the problem of earthy
tastes, and discolouration of their green bean product. These problems are largely caused by
screwing up the essentially clean and protected beans with large quantities of dirty foxy coloured hulls and fragments of dried pulp.
Get rid of as much dirt and stained hulls as possible, by impact
hulling with a good screening and
recycle operation, and then follow on with the SM14 or the old Okrassa
huller in a minimal polishing operation on the separated beans, minus all the
dirt. With the Okrassa, one should use
either the steel end or the bronze end, but not both, to minimise the rise in
temperature.
It is a mute point as to whether strongly attached
silverskin, as in much Y2 grade
parchment, will detach and catch fire in the roaster or not, but there is much to be said for minimal
polishing even if the silverskin is very hard to detach. Papua New Guinea is one of the few countries
that has to ship its Arabica coffee across the equator in the stuffy hold of a
ship, and anything that will allow our coffee to open up in the northern
hemisphere in as good a condition as that of our major competitors, can only be
beneficial to our international image
Storage conditions:
In the light of these points it should be clear that the only way to preserve coffee for as long as possible is to keep it in parchment form. Once coffee is hulled and more particularly polished, the aging process leading on to surface oxidation aided by microorganisms is inevitable. It can be however be minimised, by storing the processed green bean in conditions of as low a temperature and humidity as possible, to slow down the metabolism of that surface coating of oily foodstuff for bacteria, yeasts and moulds. Moulds in particular are adept at growing on the surface of dry and concentrated foodstuffs by using the humidity of the air as their source of moisture, and pulling their food in some way up through their waterproof boots. The typical illustration is the white fluffy mould that grows on the surface of an open jam jar where the jam is so concentrated that the sugar would suck the moisture out of anything porous in contact with it. Keep the lid on to control the moisture in the air, and the mould cannot grow. Nobody has yet built an air conditioned coffee store in PNG, but one day that will happen. Its not the temperature control that is effective, but the humidity. In the meantime, there is much to be said for storing containers filled for export up in the highlands, and only taking them down to the coast as late as possible. A further possibility is to minimise on the use of bags by bulk filling of containers with an internal plastic liner which can be hermetically sealed. That sealed but still semi permeable liner will minimise the levels of both oxygen and moisture, but concentrate the percentage of carbon dioxide. Indeed, in the major towns that have a plant for making welding gases, those plastic lined containers could well have the warm air purged out of them with cold and very dry nitrogen gas that will bring a rapid halt to all of these aging processes. These changes will all cut down on the metabolic rate of the resident microorganisms and so prolong the onset of the aging process.
The final word on the quality
of our product, is not so much when it is drunk by the consumer, but as when
those experts who count, the overseas buyers,
make their assessment when they open up our product on arrival at their
warehouse. There is so much that we could yet do, to
maintain quality up until the container seals are broken, at that point.
---ooooOOOooo---
.