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PC 1.04 Fruit Softening

As with most fruits, coffee cherries become softer as they ripen due to the breakdown of structural polysaccharides, including protopectin and cellulose. Tissue softening is attributed to the enzymatic degradation and solubilization of the protopectin (Prasanna, V. et al, 2007). As cherry ripening continues, protopectin in the inner and outer layers of the mesocarp is broken down into pectin. Pectin (discussed below) is, in turn, converted into pectic acid as fruits become overripe and/or fermented. Anyone who has made jam knows that only ripe fruits produce a jam that gels properly. This is because protopectin (in underrripe fruit) and pectic acid (in overripe fruit) are water insoluble; only the pure pectin found in ripe fruit is soluble enough to produce the desired gelling effect for jam. The fermentation process used in wet processing (see Chapter 2) helps to carefully control the breakdown of pectin, with the help of enzymes (pectinases and pectase) found naturally in coffee cherries. 

If you squeeze a ripe coffee cherry between your fingers, the exocarp (skin) and the outer layer of the mesocarp slide off quite easily from the inner layer. The inner layer, commonly called the mucilage, remains firmly attached to the inside of the cherry. It contains over 30% crude pectin content (S. Avallone et al, 2000). The mucilage remains on the beans after the skin is removed during depulping. (See Lesson 2.03.) Mucilage is composed of translucent tissues that are rich in sugars and pectins but, interestingly, is free of caffeine and tannins (Wintgens, 2004). The outer layer of the mesocarp, usually called the pulp, remains attached to the exocarp (skin) and is usually either composted or, occasionally, made into cascara. It can also bring income to farmers as an ingredient in silage, used to feed livestock.

Coffee pulp and mucilage present a relatively untapped source of pectin that has commercial value as a gelling agent. The word “pectin” is derived from the Greek pectos, which means ‘gelled’. Uronic acids are extracted from lemons to produce commercial pectins.