Microbiological Aspects of Dairy Ingredients 85cheese fl oats due to pockets of gas (CO 2 )
formed by bacterial fermentation of lactose
trapped within the cheese curd. High coli-
form loads may arise from contaminated
milk, dirty utensils/plant, and poor hygiene.
Sporeforming bacteria also contribute to
spoilage of cheese products. Many can
survive pasteurization, subsequently germi-
nate, and experience growth of vegetative
cells. Clostridium spp. may grow in cheeses
with low oxidation - reduction potential, pro-
ducing a late - blowing defect and off - fl avors
associated with butyric acid and sulfur
product formation. The predominant clostri-
dal species associated with late blowing are
Cl. butyricum , Cl. pasteurianum , Cl. sporo-
genes, and Cl. tyrobutyricum. Late blowing
may also be attributed to heterofermentative
LAB. Sporeforming Bacillus spp. such as B.
subtilis, B. pumilus, B. stearothermophilus,
and B. polymyxa also can contribute to gas-
siness in cheese curds and sliminess in brine -
treated cheeses.
Yeast and mold contamination and subse-
quent spoilage is not uncommon in cheeses.
Defects include the surface growth of molds,
which may produce discoloration and are
associated with musty/bitter fl avors.
Implicated spoilage yeast and molds include
Candida (black discoloration), Penicillium
(green discoloration), and Cladosporium
(green to black discoloration). Cheeses such
as cottage cheese and other unripened soft
cheeses are susceptible to yeast spoilage
during refrigeration. For example, Torulopsis
sphegerica , Candida lipolytica , Sporobo-
lomyces roseus , and Can. valida have been
implicated in spoilage (Fleet, 1990 ). However,
many harder cheeses have waxy coatings or
a developed rind which can minimize the
spoilage from yeast and mold. Excessive
yeast growth can cause softening of cheese
due to lipolytic and proteolytic activity.Yogurt
The Codex Alimentarius (Robinson, 2003 )
defi nes yogurt as a milk product obtained byThe concomitant proteolytic and lipolytic
enzymes associated with some of these
organisms are responsible for many of the
undesirable attributes of spoiled cheese.
Bacterial spoilage can be common during
cheese manufacture and ripening, although
cottage cheese may be susceptible to bacterial
deterioration following its manufacture. This
can be due to removal of lactic acid during
curd washing, low moisture, and salt content.
Many of the Gram - negative bacteria involved
in cheese spoilage are psychrotrophic and
capable of growth under refrigeration.
Pseudomonads are the major psychrotrophs
associated with cheese spoilage. Excessive
lipase and protease activity and sliminess due
to growth on the cheese surface can result
from high counts in cheese milk, thereby
causing cheese off - fl avors. Putrescence, off
odors, and discoloration caused by proteo-
lytic bacteria and Pseudomonas spp. are
common among cheeses such as mozzarella.
Furthermore, excessive proteolytic activity of
starter culture can lead to the development of
a bitter taste in cheeses. In blue - veined
cheeses, the excessive growth of lactic acid
bacteria (LAB) causes high - acid curds to
develop, resulting in brittle cracked rinds
and loss of fl avor in the curds (Scott, 1986 ).
Large numbers of Enterobacteriaceae can
cause gas bubbles (blowing) in the curd of
blue - veined and other cheese varieties, a con-
dition termed gassy cheese. Early blowing is
also caused by other coliforms including
Citrobacter , Escherichia, and Aerobacter.
Aerobacter aerogenes has been reported to
be responsible for blown tins of Domiati
cheese and Klebsiella aerogenes can cause
early blowing and poor cheese quality in
white - brined cheeses (Bintsis and Papademus,
2002 ). Early blowing typically develops up
to and including cheese formation and mani-
fests itself through the production of bubbles
throughout the cheese matrix. Some cheeses
such as feta and other white - brined cheeses
may develop a spongy texture. Early blowing
can become more evident during salting of
hard cheeses such as Grana Padana, when the