Handbook of Herbs and Spices by K. V. Peter

Author:K. V. Peter [Peter, K. V.]
Language: eng
Format: epub, pdf
ISBN: 9781601191021
Publisher: IB Dave's Library
Published: 2002-06-15T07:00:00+00:00

CH2=CH—CH—

-called (+)-S-(2-propenyl)

© 2001 Woodhead Publishing Ltd.

When the fresh tissue is damaged, the flavour precursors react under the control of the enzyme alliinase (S-alk(en)yl-L-cysteine sulphoxide Lyase) to release the highly reactive sulphenic acids plus ammonia and pyruvate. Alliin, or S-allyl cysteine sulphoxide, was the first sulphur compound isolated from Allium sativum (garlic). The enzyme alliinase is confined to the cell vacuole, whereas the flavour precursors are confined to the cycloplasm probably within small vesicles associated with their presence in the cell. Hence the enzyme has access to the precursors only when cells are disrupted. In garlic, alliinase catalyses the formation of allicin, which gives fresh garlic its characteristic smell (Brewster, 1994).

Alliinase (E.C.4.4.1.4., S-alk(en)yl-L-cysteine sulphoxide cyase is the enzyme ultimately responsible for the development of the flavour compounds. The reaction catalysed by the enzyme is a beta elimination of the S-alk(en)yl sulphoxide group from the substrate:

O

||

R—S—CH2—CH—COOH—R—S=O: + CH3—C—COOH

|

||

NH2

NH

Both products of the reaction are chemically unstable; the Ketimore product spontaneously hydrolyses to pyruvate and ammonia while the reactive sulphur species will combine with any of a number of co-reactants, most often another of the same species, to give a range of flavour components. Thus the reaction is commonly described as:

O

O

O

||

||

||

2R—S—CH2—CH—COOH+H2O>R—S—S—R+2CH3—C—COO+2NH4

|

NH2

Rabinowitch and Brewster (1990) have elaborated the details about the alliinase solubility and stability, purification, physical, chemical and catalytic properties. They report that alliinase from Alliums is less soluble. It is difficult to maintain this enzyme in aqueous solution. A cosolvent is required to maintain the enzyme activity. It is further reported that alliinase enzyme has a tendency to aggregate and precipitate and gets totally inactivated by freezing.

Alliinase from garlic was first prepared by Stoll and Seebeck in 1947; however, the preparation was relatively crude. A common feature of the preparation is the use of a polyalcohol cosolvent, 10% glycerol in the case of garlic enzyme. The purification of garlic enzyme also required the presence of 10M pyridoxal phosphate in the buffers. The garlic enzyme has been described as having a molecular weight of 130,000 with two subunits of 65,000. Spectral studies on alliinase from garlic revealed an absorption peak at 420 nm characteristic of pyridoxal phosphate. Garlic alliinase has been reported to contain no carbohydrate. However, the presence of a subunit band giving a periodic acid–schiff base stain on SDS gels of the purified enzyme indicates that the enzyme is a glycoprotein.

The general catalytic properties of alliinase were well described long before the enzyme was purified to homogeneity. A broad pH optimum of 5–8 was described by Stoll and Seebeck (1947) for the garlic enzyme with a temperature optimum under the

© 2001 Woodhead Publishing Ltd.

condition of assay of 37C. They also made observations on the specification of the reaction: the substrate must be a derivative of (-)L-cysteine, the sulphur atom must be linked to an aliphatic group, the amino group of cysteine must not be substituted and the sulphur atom must be in the sulphoxide form with the + and À configurations both being substrate.

16.3

Processing

Raw garlic no doubt has the ideal flavour and

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