86 Chapter 3
Kluveromyces marxianus var. marxianus and
K. marxianus var. lactis, Saccharomyces
cerevisiae, Torulopsis spp., and Candida
spp., which cause spoilage by producing gas,
thus causing the cartons to burst or the foil
seals to “ dome. ” Molds grow on the surface;
examples of spoilage fungi are species of
Mucor, Rhizopus, Aspergillus, Penicillium,
and Alternaria (Robinson, 2003 )Probiotic Cultures
Probiotic organisms have been defi ned as
live microorganisms which when adminis-
tered in adequate amounts confer a health
benefi t on the host. They are predominantly
comprised of lactic acid bacteria, though not
exclusively so (e.g., Saccharomyces boular-
dii ). Probiotic strains have mainly been
incorporated into yogurts and fermented
milk, though there are probiotic cheeses, ice
creams, and frozen yogurts. The cultures can
act by:- Competing with pathogenic organisms for
limited nutrients - Inhibiting epithelial invasion by pathogens
- Producing antimicrobial substances
- Stimulating mucosal immunity
- Reducing serum cholesterol, particularly
LDL - cholesterol, possibly due to produc-
tion of hydroxymethyl - glutarate which is
reported to inhibit hydroxymethyl - glutarate
CoA reductases required for the synthesis
of cholesterol - Aiding in the management of lactose
malabsorption - Preventing rotaviral diarrhea via the phe-
nomenon of competitive exclusion by
modifying the glycosylation state of epi-
thelial cells through the action of excreted
soluble factors - Reducing the incidence and severity of Cl.
diffi cile diarrhea which can occur as a
result of disturbance of the normal gut
microfl ora after antibiotic treatment
fermentation by the action of a synbiotic
culture of Lactobacillus delbrueckii subsp.
bulgaricus and Streptococcus thermophilus.
Alternatively, any Lactobacillus spp. can be
used. Cow ’ s milk yogurt typically has a milk -
solids - non - fat of 8.5% to 9% comprised of
approximately 4.5% lactose, 3.3%, protein
and 0.7% mineral salts. In the fi rst step in
yogurt manufacture, the solids content of
milk is raised by evaporation under vacuum
or, more commonly, by addition of skim milk
powder. Ultra - fi ltration can also be used to
achieve this, but its use is rare. The standard-
ized milk is homogenized before being
heated to 90 ° C to 95 ° C for 5 to 10 minutes
or 80 ° C to 85 ° C for 30 minutes. After cooling
the milk is inoculated with a co - culture of
Strep. thermophilus and Lb. delbrueckii
subsp. bulgaricus and either dispensed
immediately into cartons and held at 42 ° C to
43 o C for 3 to 4 hours before refrigeration in
the case of set yogurts, or pumped to insu-
lated tanks and held at the same temperature
before being fi lled, usually along with fruit
or other fl avorings, into cartons and refriger-
ated to halt any further fermentation.
Yogurt Safety
Salmonella spp., L. monocytogenes, and
Campylobacter spp., along with coliforms,
die off and survival of Staph. aureus, Y.
enterocolitica, and Aeromonas hydrophila is
questionable. There have been two major
food poisoning outbreaks involving yogurt in
the United Kingdom. One was due to growth
and toxin production by Cl. botulinum in a
hazelnut fl avoring added to yogurt and the
other was due to E. coli O157 : H7, phage
type 49, which produced HUS characterized
by acute renal failure in some consumers
(Morgan et al., 1993 ).
Yogurt Spoilage
Spoilage is usually due to the growth of
yeasts and molds. Examples of the former are