2009_Book_FoodChemistry

10.2 Dairy Products 529
For the production of ice cream, the mixture
of components is subjected to high-temperature
short-time pasteurization (80–85 ◦ C, 20–30 s),
high-pressure homogenization (150–200 bar) and
cooling to ca. 5 ◦ C. Air is then mixed into the
mixture (60–100 vol%) while it is frozen at temperatures
of up to −10 ◦ C and then hardened. The
freezers used are mainly continuously working
systems furnished with coolants which evaporate
at −30 ◦ Cto−40 ◦ C.Theprocessiscontrolled
in such a way that the core temperature of the ice
cream production is ca. −18 ◦ C.
The structural elements of ice cream are ice crystals
(∼50 µm), air bubbles (60–150 µm), fat globules
(<2 µm), and aggregated fat globules (5–
10 µm). The fat is mostly attached to the air bubbles.
The air bubbles have a three fold function:
they reduce the nutritional value, soften the product,
and prevent a strong cold sensation during
consumption.
10.2.8 Cheese
Cheese is obtained from curdled milk by removal
of whey and by curd ripening in the presence of
special microflora (Table 10.29). The great abundance
of cheese varieties, about 2000 worldwide,
can be classified from many viewpoints, e. g., according
to:
• Milk utilized (cow, goat or sheep milk).
• Curd formation (using acids, rennet extract or
a combination of both).
• Texture or consistency, or water content (%) in
fat-free cheese. Following the latter criterion,
the more important cheese groups are (water
content in %):
Extra hard: <51%
Hard: 49–56%
Semihard: 54–63%
Semisolid: 61–69%
Soft: >67%
• Fat content (% dry matter). By this criterion,
the more important groups are:
Double cream cheese (60–85% fat);
Cream cheese (≥50);
Whole fat cheese (≥45);
Fat cheese (≥40);
Semi fat cheese (≥20);
Skim cheese (max. 10).
Within each group, individual cheeses are characterized
by aroma. A small selection of the more
important cheese varieties is listed in Table 10.30.
Cheese manufacturing essentially consists of curd
formation and ripening (Fig. 10.26).
10.2.8.1 Curd Formation
The milk fat content is adjusted to a desired
level and, when necessary, the protein content is
also adjusted. Additives include calcium salts to
improve protein coagulation and cheese texture,
nitrates to inhibit anaerobic spore-forming
microflora, and color pigments. The prepared
raw or pasteurized milk is mixed at 18–50 ◦ C
in a vat with a starter culture (cf. Table 10.29)
(lactic acid or propionic acid bacteria; molds,
such as Penicillium camemberti, P. candidum,
P. roqueforti; red- or yellow-smearing cultures,
such as Bacterium linens with cocci and yeast).
The milk coagulates into a soft, semi-solid mass,
the curd, after lactic acid fermentation (sour milk
cheese, pH 4.9–4.6), or by addition of rennet
(sweet milk cheese, pH 6.6–6.3), or some other
combination, the most common being combined
acid and rennet treatment. This protein gel is
cut into cubes while being heated and is then
gently stirred. The whey is drained off while
the retained fat-containing curd is subjected
to a firming process (syneresis). The firming
gets more intense as the mechanical input and
the applied temperature increase. The process
and the starter culture (pH) determine the curd
properties. When the desired curd consistency
has been achieved, curd and whey separation is
accomplished either by draining off the whey or
by pressing off the curd while simultaneously
molding it.
New methods of cheese making aim at including
the whey proteins in the curd, instead of removing
them with the whey. Apart from giving higher
yields (12–18%), these processes help to economize
on waste water costs or elaborate whey
treatments (cf. 10.2.10).
The use of ultrafiltration steps as compared
with conventional cheese making is shown
in Fig. 10.26. Alternatively, conventionally
produced whey can be concentrated by ultrafiltration
and then added to the curd or milk
can be soured with starter culture and/or rennet