166 Modern Food Microbiology
the products studied by these workers were as follows: 3.65–4.40 for yogurts, 4.1–4.9 for buttermilk,
4.18–4.70 for sour creams, and 4.80–5.10 for cottage cheese samples. In another study, commercially
produced yogurts in Ontario were found to contain the desired 1:1 ratio of coccus to rod in only 15%
of 152 products examined.
5 Staphylococci were found in 27.6% and coliforms in around 14% of these
yogurts. Twenty-six percent of the samples had yeast counts more than 1,000/g and almost 12% had
psychrotroph counts more than 1,000/g. In his study of commercial unflavored yogurt in Great Britain,
Davis
11 found counts of the two starters to range from a low of around 82 million to a high of over
1 billion/g, and the final pH to range from 3.75 to 4.20. The antimicrobial activities of lactic acid
bacteria are discussed further in Chapters 3 and 13.
Kefir is prepared by the use of kefir grains, which contain one or more bacterial species of the
genera Acetobacter, Lactobacillus, Lactococcus, Leuconostoc, and one or more yeast species of the
genera Candida, Kluyveromyces, and Saccharomyces. These symbionts are held together by coagulated
protein.
18 The important Lactobacillus spp. in kefir are: L. kefiri, L. parakefiri, L. kefiranofaciens
subsp. kefiranofaciens, and L. kefiranofaciens subsp. kefirgranum.
75 The last two are responsible for
the production of kefiran (a water-soluble polysaccharide), which accounts for about 24% of kefir
grains.
75 Kumiss is similar to kefir except that mare’s milk is used, the culture organisms do not form
grains, and the alcohol may reach 2%.
Acidophilus milk is produced by the inoculation of an intestinal implantable strain of L. acidophilus
into sterile skim milk. The inoculum of 1–2% is added, followed by holding the product at 37
◦ C until
a smooth curd develops. A popular variant of this product that is produced commercially in the United
States consists of adding a concentrated implantable strain culture of L. acidophilus to a pasteurized
and cold vat of whole milk (or skim or 2% milk), and it is bottled immediately. It has the pH of normal
milk and is more palatable than the more acidic product. The numbers of L. acidophilus should be
in the 10
7 –10
8 /ml range.
32 Bulgarian buttermilk is produced in a similar manner by the use of L.
bulgaricus as the inoculum or starter, but unlike L. acidophilus, L. bulgaricus is not implantable in
the human intestines. A summary of fermented milk is presented in Table 7–4.
Butter contains around 15% water, 81% fat, and generally less than 0.5% carbohydrate and protein.
Although it is not a highly perishable product, it does undergo spoilage by bacteria and molds. The main
source of microorganisms for butter is cream, whether sweet or sour, pasteurized or nonpasteurized.
The biota of whole milk may be expected to be found in cream because as the fat droplets rise
to the surface of milk, they carry up microorganisms. The processing of both raw and pasteurized
creams to yield butter brings about a reduction in the numbers of all microorganisms, with values
for finished cream ranging from several hundred to over 100,000/g having been reported for finished
salted butter. Salted butter may contain up to 2% salt, and this means that water droplets throughout
may contain an effective level of about 10%, thus making this product even more inhibitory to bacterial
spoilage.
32
Bacteria cause two principal types of spoilage in butter. The first is a condition known as “surface
taint” or putridity. This condition is caused by Pseudomonas putrefaciens as a result of its growth on
the surface of finished butter. It develops at temperatures within the range 4–7
◦ C and may become
apparent within 7–10 days. The odor of this condition is apparently due to certain organic acids,
especially isovaleric acid. Surface taint along with an apple odor is caused also by Chryseobacterium
joostei.
34 The second most common bacterial spoilage condition of butter is rancidity. This condition
is caused by the hydrolysis of butterfat with the liberation of free fatty acids. Lipase from sources other
than microorganisms can cause the effect. The causative organism is Pseudomonas fragi, although P.
fluorescens is sometimes found. Bacteria may cause three other less common spoilage conditions in
butter. Malty flavor is reported to be due to the growth of Lactococcus lactis var. maltigenes. Skunklike
the products studied by these workers were as follows: 3.65–4.40 for yogurts, 4.1–4.9 for buttermilk,
4.18–4.70 for sour creams, and 4.80–5.10 for cottage cheese samples. In another study, commercially
produced yogurts in Ontario were found to contain the desired 1:1 ratio of coccus to rod in only 15%
of 152 products examined.
5 Staphylococci were found in 27.6% and coliforms in around 14% of these
yogurts. Twenty-six percent of the samples had yeast counts more than 1,000/g and almost 12% had
psychrotroph counts more than 1,000/g. In his study of commercial unflavored yogurt in Great Britain,
Davis
11 found counts of the two starters to range from a low of around 82 million to a high of over
1 billion/g, and the final pH to range from 3.75 to 4.20. The antimicrobial activities of lactic acid
bacteria are discussed further in Chapters 3 and 13.
Kefir is prepared by the use of kefir grains, which contain one or more bacterial species of the
genera Acetobacter, Lactobacillus, Lactococcus, Leuconostoc, and one or more yeast species of the
genera Candida, Kluyveromyces, and Saccharomyces. These symbionts are held together by coagulated
protein.
18 The important Lactobacillus spp. in kefir are: L. kefiri, L. parakefiri, L. kefiranofaciens
subsp. kefiranofaciens, and L. kefiranofaciens subsp. kefirgranum.
75 The last two are responsible for
the production of kefiran (a water-soluble polysaccharide), which accounts for about 24% of kefir
grains.
75 Kumiss is similar to kefir except that mare’s milk is used, the culture organisms do not form
grains, and the alcohol may reach 2%.
Acidophilus milk is produced by the inoculation of an intestinal implantable strain of L. acidophilus
into sterile skim milk. The inoculum of 1–2% is added, followed by holding the product at 37
◦ C until
a smooth curd develops. A popular variant of this product that is produced commercially in the United
States consists of adding a concentrated implantable strain culture of L. acidophilus to a pasteurized
and cold vat of whole milk (or skim or 2% milk), and it is bottled immediately. It has the pH of normal
milk and is more palatable than the more acidic product. The numbers of L. acidophilus should be
in the 10
7 –10
8 /ml range.
32 Bulgarian buttermilk is produced in a similar manner by the use of L.
bulgaricus as the inoculum or starter, but unlike L. acidophilus, L. bulgaricus is not implantable in
the human intestines. A summary of fermented milk is presented in Table 7–4.
Butter contains around 15% water, 81% fat, and generally less than 0.5% carbohydrate and protein.
Although it is not a highly perishable product, it does undergo spoilage by bacteria and molds. The main
source of microorganisms for butter is cream, whether sweet or sour, pasteurized or nonpasteurized.
The biota of whole milk may be expected to be found in cream because as the fat droplets rise
to the surface of milk, they carry up microorganisms. The processing of both raw and pasteurized
creams to yield butter brings about a reduction in the numbers of all microorganisms, with values
for finished cream ranging from several hundred to over 100,000/g having been reported for finished
salted butter. Salted butter may contain up to 2% salt, and this means that water droplets throughout
may contain an effective level of about 10%, thus making this product even more inhibitory to bacterial
spoilage.
32
Bacteria cause two principal types of spoilage in butter. The first is a condition known as “surface
taint” or putridity. This condition is caused by Pseudomonas putrefaciens as a result of its growth on
the surface of finished butter. It develops at temperatures within the range 4–7
◦ C and may become
apparent within 7–10 days. The odor of this condition is apparently due to certain organic acids,
especially isovaleric acid. Surface taint along with an apple odor is caused also by Chryseobacterium
joostei.
34 The second most common bacterial spoilage condition of butter is rancidity. This condition
is caused by the hydrolysis of butterfat with the liberation of free fatty acids. Lipase from sources other
than microorganisms can cause the effect. The causative organism is Pseudomonas fragi, although P.
fluorescens is sometimes found. Bacteria may cause three other less common spoilage conditions in
butter. Malty flavor is reported to be due to the growth of Lactococcus lactis var. maltigenes. Skunklike