BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come
430 Part 4: Milk(2,3-butanedione), acetoin, and 2,3-butanediol, which are key
volatile flavor compounds in cultured dairy products. Diacetyl
is formed through the chemical oxidation ofα-acetolactate. Di-
acetyl reductase (also referred to as acetoin reductase) catalyzes
the irreversible reduction of diacetyl to acetoin and the reversible
reduction of acetoin to butanediol in which NAD(P)H acts as the
coenzyme (Hugenholtz 1993, Rattray et al. 2000, Østlie et al.
2003). Diacetyl contributes to the characteristic buttery flavor
of many cultured dairy products. There is significant interest in
adapting processing methods that will enhance the formation of
diacetyl or decrease diacetyl reductase activity to maintain these
desirable flavor characteristics (Rattray et al. 2000).
Many commercial dairy processors now use the direct vat in-
oculation (DVI) process for frozen or freeze-dried cultures (up
to 10^12 bacteria per gram of starter) in the processing of cultured
dairy products. The use of DVI cultures allows the dairy proces-
sor to directly add the cultures to the milk and by-pass on-site
culture preparation. This recent progress in the development of
starter cultures has also increased phage resistance, minimized
the formation of mutants which may alter the characteristics
of the starter cultures, enhanced the ability to characterize the
composition of the cultures, and improved the consistent quality
of cultured dairy products. However, the DVI process is lim-
ited by the additional cost of these cultures, the dependence of
the cheese plants on the starter suppliers for the selection and
production of the starters, and the increased lag phase of these
cultures in comparison to on-site culture preparation (Stanley
1998, Tamime and Robinson 1999a, Canteri 2000).Coagulation of Milk ProteinsThe production of lactic acid by lactic acid bacteria decreases
the pH of the milk to cause coagulation of the casein. As the
pH decreases to less than 5.3, colloidal calcium phosphate is
solubilized from the casein micelle, causing the micelles to dis-
sociate and the casein proteins to aggregate and precipitate at the
isoelectric point of casein (pH 4.6). The resulting gel, which is
somewhat fragile in nature, provides the structure for sour cream,
yogurt, and acid-precipitated cheeses, such as cream cheese and
cottage cheese (Lucey 2002).
The casein micelles are also susceptible to coagulation
through enzymatic activity. Rennet, a mixture of chymosin and
pepsin, obtained from calf stomach, is most commonly recog-
nized as the enzyme for coagulation of casein. However, pro-
teases from microorganisms and produced through recombinant
DNA technologies have been successfully adapted as alterna-
tives to calf rennet (Banks 1998). Chymosin, the major enzyme
present in rennet, cleaves the peptide bond between Phe-105
and Met-106 ofκ-casein, releasing the hydrophilic, charged ca-
sein macropeptide, while thepara-κ-casein remains associated
with the casein micelle. The loss of the charged macropeptide
reduces the surface charge of the casein micelle and results in
the aggregation of the casein micelles to form a gel network
stabilized by hydrophobic interactions. Temperature influences
both the rate of the enzymatic reaction and the aggregation of
the casein proteins, with 40–42◦C, the optimal temperature for
casein coagulation. The use of rennet to hydrolyze the peptidebond and cause aggregation of the casein micelles is used in the
manufacture of most ripened cheeses (Lucey 2002).HomogenizationMilk fat globules have a tendency to coalesce and separate upon
standing. Homogenization reduces the diameter of the fat glob-
ules from 1–10μm to less than 2μm and increases the total
fat globule surface area. The physical change in the fat globule
occurs through forcing the milk through a small orifice under
high pressure. The decrease in the size of the milk fat globules
reduces the tendency of the fat globules to aggregate during the
gelation period. In addition, denaturation of the whey proteins
and interactions of the whey proteins with casein or the fat glob-
ules can alter the physical and chemical properties of the milk
proteins to result in a firmer gel with reduced syneresis (Tamime
and Robinson 1999b, Fox et al. 2000). Milk to be used to process
yogurt, cultured buttermilk, and unripened cheeses is commonly
homogenized to improve the quality of the final product.PasteurizationThe original cultured dairy products relied on the native mi-
croorganisms in the milk for the fermentation process. Current
commercial methods for all cultured dairy products include a
pasteurization treatment to kill the native microorganisms, fol-
lowed by inoculation with starter cultures to produce the de-
sired product. The heat process, which must be sufficient to
inactivate alkaline phosphatase, also destroys many pathogenic
and spoilage microorganisms, and enzymes that may have a
negative impact on the quality of the finished products. The
time-temperature treatments for the fluid milk pasteurization
have been adapted for the milk to be used in the processing of
cultured dairy products (62.8◦C for 30 minutes or 71.1◦Cfor
15 seconds). More severe heat treatments than characteristic of
pasteurization causes denaturation of whey proteins and inter-
actions betweenβ-lactoglobulin andκ-casein. In cheeses, this
interaction decreases the ability of chymosin to hydrolyze the
casein molecule and initiate curd precipitation and formation.
Pasteurization has a significant effect of the flavor profile of
the milk. Cultured dairy products produced from pasteurized
milks tend to have less intense flavor characteristics due to the
heat inactivation of the naturally occurring microorganisms and
enzymes in the milk that contribute to flavor formation (Buchin
et al. 1998). Lactones and heterocycles are also formed dur-
ing the heat treatment of raw milk to contribute cooked flavors
(Friedrich and Acree 1998).CoolingThe processing of cultured dairy products relies on the metabolic
activity of the starter cultures to contribute to acid formation and
flavor and texture development. Once the desired pH or titratable
acidity is reached for these products, the products are cooled to
5–10◦C to slow the growth of the bacteria and limit further acid
production and other biological reactions.