Chemistry Reference
In-Depth Information
N-acetyl muramoylhydrolase, available commercially from hen egg white or
Micrococcus
lysodeikticus
. The food-grade preparations are from egg albumin.
Lysozyme is sold by the major dairy enzyme suppliers as an alternative control agent
for 'late blowing', the textural defect of slits and irregular holes caused by the butyric
fermentation in Gouda, Danbo, Grana Padano, Emmental and other important hard and
semi-hard cheese varieties. Traditionally the defect, caused by
Clostridium tyrobutyricum
in raw milk, has been controlled by the addition of potassium nitrate to the cheese milk.
However, this practice will be phased out because it is associated with the production of
carcinogens, and lysozyme has become the preferred control agent.
11, 33
C. tyrobutyricum
is
a spore former and as such cannot be killed by pasteurization, hence the need to treat the
milk by alternative methods. Lysozyme kills vegetative cells and also inhibits outgrowth of
spores in cheese; it is stable for long periods in the cheese matrix and because it binds to
the cheese curd, little of the enzyme is lost on whey separation. Although lysozyme also
inhibits the LAB used as starters in cheese making, they are less sensitive than the Clostridia
and a typical enzyme dose rate of 500 units mL
−
1
is sufficiently selective (commercial
lysozyme preparations contain about 20 000 units mg
−
1
). Nevertheless, some thermophilic
lactobacilli used in Grana cheese making are very sensitive, but can be 'conditioned' by
unknown mechanisms by repeated growth on lysozyme-containing media.
33
Lysozyme also inhibits the growth of
Listeria monocytogenes
in yoghurt and fresh cheese
with high acidity (
pH 5.0), but the effect is not consistent enough to rely on in commercial-
fermented milk products, and in any case high acidity is usually sufficient in itself to inhibit
these pathogens.
<
5.6
TRANSGLUTAMINASE
With the recent availability of commercial microbially derived transglutaminase (protein-
glutamine
-glutamyltransferase; EC 2.3.2.13) preparations, there has been considerable
interest in their application to the gelation of caseins and whey proteins, though this is not
a widespread technology yet. However, transglutaminase is effective in reducing syneresis
in acid milk gels and has been investigated as a method of improving the texture and shelf
life of yoghurt.
34
Specifically, transglutaminase has been shown to improve the emulsifying
properties of milk proteins
35
γ
and increases the viscosity and water-holding capacity of
yoghurt.
36
5.7
LIPASE
Although lipases are used in cheese flavour technology as components of the ripening systems
already discussed in this chapter, they are also used to produce modified milk fat products
for other food applications.
37
5.7.1 Lipolyzed milk fat (LMF)
LMF has a creamy, buttery and cheesy aroma derived from short to medium chain fatty acids
and fatty acid chemical derivatives released from milk fat by lipases. The raw material sub-
strate for manufacturing LMF is either condensed milk or butter oil emulsified to maximize
