Foodprocessing Properties Of Glycosidases

This section will examine the properties of enzymes, focusing on the specific conditions encountered in the processing of fruit juice and derived beverages. In particular, three important parameters influencing flavor release will be discussed: the effect of pH; aglycone and sugar specificity of glycosidases; and inhibition of activity by sugar and sugar analogues. A summary of the properties of ^-glucosidase from some plants and microorganisms is given in Table 2.

The optimum pH activity of plant ^-glucosidase (55, 59) and endoglycosidase (51) is similar (5.0-6.0) to that of intracellular yeast ^-glucosidase (63, 87, 93, 105). This value is generally lower (4.0-5.0) for extracellular yeast (63, 92, 93) and Aspergillus enzymes (10, 61, 106). The optimum pH of an extracellular endo-^-glucosi-dase from A. niger was found to be unusually low (3.4) (100). Most ^-glucosidases exhibit only 5-15% of their maximum activity in the pH range of fruit juices (2.83.8).

A. niger a-arabinofuranosidase (62, 107) and a-rhamnosidase (47, 108) possess similar pH optima to that of extracellular ^-glucosidase from the same fungus, while the value for A. niger ^-apiosidase is higher (5.0-6.0). (10, 99, 106). Both -arabinofuranosidase and a-rhamnosidase exhibit > 50% of their maximum activities in the pH range of fruit juices.

An important parameter that must be considered for technological applications is the stability of glycosidases in the acidic pH of fruit juices. Glycosidases differ notably in this respect (Fig. 4) (10).

^-Glucosidases from grapes, S. cerevisiae, and C. wickerhamii, are not very stable in these conditions. For example, C. wickerhamii ^-glucosidase was able to hydrolyse ^-D-glucosides of monoterpenes in a wine only when the initial pH (3.0) was adjusted to 3.5 or higher (109). Only 10% of initial ^-glucosidase activity from S. cerevisiae was observed after 90 min incubation (20°C) in a buffer at pH 3.0. (10). This may explain the large decrease in S. cerevisiae ^-glucosidase activity during grape juice fermentation. In fact the optimum pH stability of the enzyme corresponds to the pH of yeast cells (6.0). This decrease is also observed for a-rhamnosidase and a-arabinofuranosi-dase activities from S. cerevisiae during juice fermentation (10, 80). A ^-glucosiase from C. peltata was fairly stable at pH 3.5-6.0 (95). The enzyme from a mutant strain of C. molischiana was shown to be more stable at acidic pH than an enzyme from a wild-type strain (110,111). This difference could be explained by the difference in glycosylation of the relevant proteins.

In contrast, ^-glucosidase, a-arabinofuranosidase, and a-rhamnosidase from A. niger are remarkably stable over a pH range of 2.5-7.0 (Fig. 4) (10, 62, 106). The enzymes exhibit > 80% of their maximum activity after 24 h of incubation in a buffer at pH 3.0. However, A. niger ^-apiosidase was less stable at a pH < 4.0 (10, 106). The same trends were observed when the stability of A. niger glycosidases were monitored during grape juice fermentation at 20°C (pH 3.0)

Table 2 Properties of ^-Glucosidases from Some Plants and Microorganisms

Origin pH

Temperature (°C)

Glucose inhibition

Gluconolactone inhibition

Table 2 Properties of ^-Glucosidases from Some Plants and Microorganisms

Origin pH

Temperature (°C)

Glucose inhibition

Gluconolactone inhibition

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