Chemistry Reference
In-Depth Information
'system' (LPS) (LP
hydrogen peroxide) irreversibly inhibits the membrane-
energizing D-lactate dehydrogenase in gram-negative bacteria, leading to cell death. In gram-
positive bacteria, the membrane ATPase is reversibly inhibited and this may be the basis of
bacteriostasis, rather than death.
Although all raw milk contains LP and thiocyanate, there is not sufficient natural hydrogen
peroxide to activate the enzyme system (LPS). Several methods have been devised to increase
hydrogen peroxide levels in commercial raw milk supplies to provide a 'cold sterilization'
system for countries with insufficient energy resources for heat treatment to preserve raw
milk before consumption. Its efficacy in eliminating psychrotrophic gram-negative spoilage
bacteria from raw milk stored at 4
◦
C was demonstrated by Reiter and Marshall.
16
Although
hydrogen peroxide alone can be used as a preservative in these countries, it must be used
at 300-500 mg L
−
1
to be effective, and at this concentration it destroys some vitamins and
impairs the functionality of the milk proteins. With the LPS, hydrogen peroxide can be
generated in situ using glucose oxidase and free hydrogen peroxide levels are too low to
damage the milk. Even if chemical peroxide is used instead of glucose oxidase, it needs to be
added at only 10 ppm to activate the LPS. Law and Goodenough
11
+
thiocyanate
+
have summarized these
options.
5.4
CHEESE-RIPENING ENZYMES
5.4.1 Types of enzyme available commercially
The enzymes and enzyme 'packages' used to modify, enhance or accelerate the maturation
of cheese are generally composed of more than one class of enzyme, and for the sake of
clarity they are discussed here as a technological group, rather than as individual classes.
The classes used in commercial ripening technology include many hydrolases represented by
proteinases, peptidases and lipases, and if current research is successful, this list may soon
extend to metabolic enzymes such as acetyl-CoA synthases and amino acid-catabolizing
enzymes to generate volatile esters and sulphur compounds.
Considering the very extensive worldwide research effort and literature on the enzymol-
ogy of cheese ripening,
17
it is remarkable that only a few enzyme companies have success-
fully developed commercial enzyme packages for cheese technology, other than the ageing
enzyme-modified cheese (EMC) production methods used to make flavour ingredients for
processed cheese and cheese-like foods.
18, 19
This state of affairs is partly due to the poor availability of commercial enzyme prepara-
tions that are both dedicated to cheese ripening and of proven efficacy. The research literature
contains hundreds of reports of small-scale and pilot-scale enzyme trials with well-known
cheese varieties but few reach the market. Thus, in contrast with the widespread use of animal
(bovine, porcine, caprine) and fungal (
Aspergillus
,
Candida
,
Rhizomucor
) lipases and pro-
teolytic enzymes in the manufacture of EMC, only one commercial system, Accelase
R
has
been widely trialled in the manufacture of the established cheese varieties and their reduced
fat variants.
This product was developed by IBT Ltd., now part of Danisco, from the author's basic and
applied research into the role of starter enzymes and cell lysis in flavour development.
20, 21
Smith
22
has described the efficacy of the commercial system. It is made up of a food-grade
microbial endopeptidase (proteinase) active against all of the casein components in cheese,
together with general and specific lactic acid bacteria (LAB) aminopeptidases, and undefined
