Food Biochemistry and Food Processing (2 edition)

BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come

23 Dairy Products 439

growth ofLb. delbrueckii.Initially,S. thermophilusgrows faster

thanLb. delbrueckii, however, at the later stages of the fermen-

tation process, the growth of theS. thermophilusis inhibited

by the reduced pH of the yogurt. The mutual stimulation of the

yogurt cultures through their metabolic activity significantly in-

creases the formation of lactic acid at a rate greater than would

be possible by the individual cultures.

The acid produced by the yogurt cultures decreases the pH

of the milk to cause the aggregation of the casein micelles. As

the pH decreases to less than 6.0, the colloidal calcium phos-

phate that stabilizes the casein micelle structure is solubilized

and micelle undergoes partial rearrangement. Solubilization of

the colloidal calcium phosphate is essentially completed when

the milk reaches a pH of 5.0. With further acid production,

the pH of the milk reaches the isoelectric point of casein (pH

4.6) and electrostatic repulsion of the charged groups on the

casein proteins decreases as these groups become protonated.

The increased casein–casein association through hydrophobic

interactions results in the aggregation of casein micelles and

gel formation (Lucey 2002). Complete solubilization of col-

loidal calcium phosphate prior to the initiation of gel formation

results in the formation of yogurt with stronger gels and re-

duced tendency for syneresis. Higher inoculation rates for the

starter cultures (2%) and a lower incubation temperature (40◦C)

contribute to stronger interactions between casein molecules

and a more stable gel structure and reduced syneresis (Lee and

Lucey 2004).

Two types of yogurt, set-style and stirred-style are produced.

Set-style yogurt is fermented following packaging, with color

and flavors added to the container prior to the addition of the

inoculated milk, resulting in a gel that forms a firm, unbroken

coagulum. Stirred-style yogurt is fermented in a vat prior to

packaging, with the gel structure broken following fermentation

during the addition of flavors and colors, cooling and packaging

stage. For both types of yogurt, either a short incubation period

at 40–45◦Cfor2^1 / 2 –3 hours or a long incubation period at 30◦C

for 16–18 hours may be used to attain a pH of 4.5 or titratable

acidity of 0.9% lactic acid prior to cooling.

The metabolism of the yogurt cultures contributes to the char-

acteristic flavor of yogurt. Both bacteria are heterofermentative

and produce lactic acid from glucose, yet are unable to metabo-

lize galactose. Acetaldehyde, a key flavor component of yogurt

described as having a fruity aroma, is produced by the degra-

dation of threonine to acetaldehyde and glycine by the enzyme

threonine aldolase. AlthoughS. thermophilusforms a major-

ity of the acetaldehyde produced, the proteolytic activity ofLb.

delbrueckiissp.bulgaricusgenerates the precursors for the for-

mation of acetaldehyde (Wilkins et al. 1986). Other volatile

flavor compounds generated by the metabolism of lactic acid

bacteria include diacetyl and 2,3-pentanedione, both of which

have buttery aroma characteristics. The formation of lactic acid

during the fermentation is also a critical contributor to the typical

yogurt flavor (Ott et al. 1997, 2000).

Syneresis, the expelling of interstitial liquid due to association

of the protein molecules and shrinkage of a gel network, is

undesirable in yogurt. Syneresis increases with an increase in

incubation temperature. In stirred yogurt, extensive amounts of

syneresis results in a thin product. Therefore, incubation of the

yogurt at a lower temperature, such as 32◦C, is recommended

to maintain the desirable viscosity. Stabilizers, such as gelatin,

xanthan gum, locust bean gum, andκ-carrageenan, increase the

firmness and viscosity of yogurt and minimize syneresis through

their ability to bind with water and the milk proteins to stabilize

the protein network (Soukoulis et al. 2007).

Acidophilus Milk

Acidophilus milk was developed as a fluid milk product that

provides therapeutic benefits through the alleviation of intesti-

nal disorders. Following homogenization and heat treatment

of the milk to minimize contamination by other microorgan-

isms, the milk is cooled to 37◦C and inoculated with a 1%

culture ofLactobacillus acidophilus(Fig. 23.6). The milk is in-

cubated at this temperature for 18–24 hours until the titratable

acidity reaches 0.63–0.72%.Lb. acidophilusis a thermophilic

starter that grows slowly in milk and has a relatively high acid-

tolerance. To provide the desired therapeutic effects, the milk

Figure 23.6.Processing scheme for acidophilus milk.