| dc.description.abstract | The presence of galacturonic acid within the
reported pectic component of coffee mucilage has been
confirmed. The insoluble mucilage material (protopectin)
contains arabinose, glucose and xylose units in addition
to the uronic acid. Most of the arabinose is probably
associated with the galacturonic acid within the middle
lamella whereas the glucose and xylose units predominate in
the cell wall. The structures containing a high proportion
of glucose and xylose are particularly resistant to degradative
reactions.
Earlier studies of mucilage composition have failed
to distinguish between the insoluble protopectin fraction and
the water-soluble material. An awareness of this distinction
serves to clarify many of the problems of fermentation. The
sugars sucrose, fructose and glucose are dominant components
of the soluble fraction.
Fermentation Reactions
In coffee fermentation, it might be expected that
one result of the pulping operation, involving as it does
cellular rupture and exposure of mucilage to oxygen, is to
initiate a number of reactions dependent upon enzymes present
within the fruit flesh. Apart from the polyphenoloxidase
system known to occur in coffee, however, no evidence exists
to show that such enzyme reactions are of great importance
to coffee fermentation. The great majority of the pectic
enzyme activity has been shown recently to be elaborated
by micro-organisms.
The open, cellular, physical structure of the
mucilage of freshly pulped coffee beans is very retentive of
water; the retained liquor is at a pH of about 5.5 and
contains simple sugars. Such a system forms an attractive
substrate to a wide variety of micro-organisms. Microbial
activity is largely responsible for the essential reaction
during fermentation, the degradation of the insoluble
protopectin material left adhering to the endorcarp after
pulping. Free arabinose is released early on during
fermentation; free galacturonic acid is not normally
observed at any stage except at very low concentrations.
The insoluble residual material present at the end of
fermentation, detached from the endocarp, is composed
predominantly of glucose and xylose units.
The sugars sucrose, fructose and glucose,
constituents of the cherry juice, are also attac~d by
micro-organisms. Since these sugars are soluble, their
initial concentrations, besides being dependent upon the
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maturity of the fruit, are greatly affected by the degree
of water-washing involved during o~ immediately after the
pulping stage. The more important products of the degradation
of the sugars are acetic and lactic acids. Under the
conditions of a very heavy initial w3shing, the concentrations
of sugars are very low and the production of acetic
and lactic acids is correspondingly low; after forty hours
or so of undisturbed development, these acids can be
accompanied by butyric and propionic acids, the lactic acid
already formed being destroyed. A close correlation exists
between the level of propionic acid and the incidence of a
taint known as "onion flavour".
The acids produced Lower the pH of the fermenting
aqueous mass which in turn slows the rate of mucilage
degradation; evidence obtained from buffered micro-fermentations
is presented to demonstrate this important pH effect. The
efficiency of acid production from the sugars is greater if
fermentation proceeds under water insteud of under the normal
conditions of free drainage; the low pH attained under these
circumstances probably partly accounts for the relatively slow
rate of mucilage decomposition observed.
The overall pattern of change is complex and
incorporates numerous trivial reactions, one of which leads to
the evolution of hydrogen gas as a result of bacterial attack
upon sugars. Another releases an unidentified compound
very similar to galacturonic acid.
Effects upon the Coffee Bean
During the fermentation process, it can be expected
that diffusion of material into and out of the bean will occur.
Overall, there is a net loss of bean dry matter of the order
of one per cent during a normal fermentation, much of which
is in the form of simple sugars. It is shown that the quality
of the bean improves during fermentation and, in particular,
the bean loses much of its potential to undesirable brown
colorations which become evident when dry.
The improvement of coffee bean quality is
particularly evident in the raw appearance of the bean itself.
Statistical evidence is available to demonstrate the closeness
of the correlations of raw appearance with both the liquor
quality and the overall quality assessment of coffee.
It is evident that two desirable changes are proceeding
concomitantly during a normal fermentation process, the
degradation of mucilage and the diffusion effect which improves
quality. It can be reasoned that these two changes can be
carried out more effectively if they are encouraged to proceed
independently. A rapid removal of mucilage followed by a
simple immersion of the clean bean in water forms the basis
of a procedure which produces excellent coffee successfully
and efficiently. | en |
