Fermented Dairy Ingredients 353(lactose), and grow at around 10 o C (50 o F).
The lactic - acid produced by the acid -
producing bacteria is around 0.8% depending
on the strain used (Cogan 1995 ). The produc-
tion of lactic acid decreases the pH of the
fermented cream and provides the character-
istic mild, sour taste.
The fl avor - producing culture converts
lactose into D - lactate, ethanol, and carbon
dioxide through the heterofermentative
pathway. Acetate also can be produced when
external electron acceptors such as O 2 and
citrate are available. The citrate content of
milk is around 8 mM and varies throughout
lactation. Citrate metabolism is an important
property of some mesophilic cultures such as
Lc. lactis subsp. lactis biovar diacetylactis.
This metabolism produces acetate, diacetyl,
acetoin, 2,3 - butanediol, and carbon dioxide.
Diacetyl is one of the major fl avor com-
pounds responsible for the typical fl avor of
cultured buttermilk, ripened cream butter,
sour cream, fromage frais, quark, and cheddar
cheese. Acetate also contributes to the fl avor,
but acetoin (a derivative of diacetyl) and
2,3 - butanediol do not (Monnet et al. 1995 ).
The changes during milk cream fermenta-
tion also include an increase in viscosity as a
result of lowering the pH and production of
other metabolites by the bacterial culture.
This increase is due to disaggregation of
calcium submicelles as the colloidal calcium
phosphate solubilizes and aggregates into a
more ordered system (Fox et al. 2000 ). The
production of other metabolites such as exo-
polysaccharide also alter the viscosity. This
polysaccharide contains galactose, glucose,
fructose, mannose, and other sugars, depend-
ing on strain and growth conditions (Nakajima
et al. 1990 , Cerning et al. 1992 , Tamime and
Robinson 2007 ).Quality of Sour Cream
The quality of sour cream depends on the
quality of the raw material and the manufac-
turing conditions, and can be grouped intotains both acid - producing and fl avor -
producing bacteria (Folkenberg and Skriver
2001 ), and to a lesser extent a small amount
of rennet (Kosikowski and Mistry 1997 ).
The acid - producing bacteria commonly
used include Lc. lactis subsp. lactic and Lc.
lactis subsp. cremoris ; the common fl avor -
producing bacteria are Lc. lactis subsp. lactis
biovar diacetylactis (or Cit^ +^ Lactococci)
and Leuc. mesenteroides subsp. cremoris.
A small amount of rennet gives sour cream
its heavy body. The viscosity of sour cream
is usually altered by an addition of milk
powder (Kosikowski and Mistry 1997 ).
Exopolysaccharide - lactic - acid - producing
bacteria and low - fat cream (6% fat) also have
been used (Adapa and Schmidt 1998 ).
Manufacture of Sour Cream
Sour cream manufacture consists of several
key steps such as mixing the milk cream with
stabilizer, homogenization, pasteurization,
cooling to 22 ° C (72 ° F), mixing in the culture,
fermentation, breaking of the coagulum,
homogenization, and storage (Figure 13.3 ).
The ingredients can be added directly into
standardized cream or before it is standard-
ized when dry ingredients are used. The
ingredients are then mixed, preheated, and
homogenized before cooling to 22 ° C (72 ° F).
Starter cultures are then mixed in and the mix
is fermented at 22 ° C (72 ° F) for 14 to 18
hours. The sour cream is further cooled to
12 ° C (54 ° F), homogenized, packed, and
stored at refrigerated temperature (Meunier -
Goddik 2004 ).
Physicochemical Changes
During Fermentation
The addition of two different cultures in
milk cream fermentation lead to the produc-
tion of various lactose derivatives through
two different pathways. The acid - producing
cultures follow the homofermentative path-
way, mainly produce L - lactate from sugars