Chemistry Reference
In-Depth Information
salting and moulding. During manufacture or shortly thereafter, curd pH
reaches
5.0, but the rate is characteristic of the cheese variety (6-24 h). Even
after losing
98% of the total milk lactose in the whey as lactose or lactate,
the cheese curd still contains 0.8-1.5% lactose at the end of manufacture
(Huffman and Kristoffersen, 1984).
The pH at whey drainage largely determines the mineral content of a
cheese. The loss of Ca
2+
and phosphate from casein micelles determines the
extent to which the micelles are disrupted and this largely determines the basic
structure and texture of a cheese (Lawrence et al., 1983). In general, curds
with a low pH at drainage have a crumbly texture, e.g., Cheshire, while high
pH curds tend to be more elastic, e.g., Emmental.
The racemization of
L
-lactate is probably not significant from a flavour
viewpoint, but
D
-lactate may have undesirable nutritional consequences in
infants. Calcium-
D
-lactate is believed to be less soluble than calcium-
L
-lactate
and may crystallize in cheese, especially on cut surfaces (Dybing et al., 1988).
Consumers may mistake the crystals as spoilage, and crystal formation is
generally considered negative.
Lactic acid is metabolized by propionic acid bacteria, e.g., in the pro-
duction of Swiss-type cheeses, to propanoic and acetic acids, H
2
O and CO
2
.
The production of CO
2
is responsible for the eye formation which is a
characteristic of Swiss-type cheeses (Fox et al., 1995). Oxidation of lactate
may also occur in cheese to a very limited extent. During this process, lactate
is converted to acetate and CO
2
. Acetate is present at fairly high concentra-
tions in Cheddar and is considered to contribute to cheese flavour, although a
high concentration may cause off-flavour (Aston and Dulley, 1982).
14.2.3.2.
Citrate Metabolism
Bovine milk contains relatively low levels of citrate (
8 mM). Approxi-
mately 90% of the citrate in milk is soluble and most is lost in the whey;
however, the concentration of citrate in the aqueous phase of cheese is
3
times that in whey (Fryer et al., 1970), presumably reflecting the concentra-
tion of colloidal citrate. Cheddar cheese contains 0.2-0.5 % (w/w) citrate,
which is not metabolized by L. lactis ssp. lactis or L. lactis ssp. cremoris, but is
metabolized by citrate-positive lactococci and Leuconostoc sp. with the pro-
duction of diacetyl and CO
2
. Due to CO
2
production, citrate metabolism is
responsible for the characteristic eyes in Dutch-type cheeses. Diacetyl and
acetate produced from citrate contribute to the flavour of Dutch-type and
Cheddar cheeses (Aston and Dulley, 1982). Several species of mesophilic
lactobacilli metabolize citrate with the production of diacetyl and formate
(Fryer, 1970); the presence of lactose influences the amount of formate
formed.
