Dairy Ingredients for Food Processing

Microbiological Aspects of Dairy Ingredients 73

during cheese ripening, especially in the

surface - mold - ripened varieties. This has

been observed in the growth of L. monocyto-

genes parallel to the increase in pH (7.5)

during the ripening of Camembert (Ryser and

Marth, 1987 ). In contrast, feta cheese, with

similar composition in terms of moisture

content, a w , and ripening temperature to

Camembert but with a fi nal pH 4.4, inhibits

L. monocytogenes growth (Donnelly, 2004 ).

The control of pathogenic and spoilage

microfl ora depends on good farm/animal man-

agement and physical processes applied to

cheese milk. The growth or persistence of

microbial pathogens is infl uenced by intrinsic

and extrinsic parameters (Table 3.2 ). The safety

of the product and ingredient depends on

the dynamic interaction between parameters.

In many cheese varieties, cooking (scald-

ing) is required to reduce moisture, but it may

have a minor infl uence on survival of

pathogenic/spoilage organisms. Cheeses pro-

duced using mesophilic starter cultures have

a maximum cooking temperature of 39 ° C,

which ensures an optimum rate of acid devel-

opment and moisture (whey) release. The

low moisture content and pH of such cheeses,

for example Cheddar, strongly infl uences

pathogen survival. Considerably fewer food-

borne disease outbreaks are associated with

semi - hard and hard (low - moisture) cheeses

compared to the high - moisture varieties.

Thermophilic cheeses, such as many Italian

and Swiss varieties, rely on extended high

cooking temperatures (50 ° C to 55 ° C) for

development of typical texture and are rarely

the inhibitory effect of raw milk on the sur-
vival of foodborne pathogens (Pitt et al.,
2000b ).

Cheese Safety

Cheese is most vulnerable to the growth of
pathogenic bacteria during the early stages
of cheese making, when milk is warm.
Therefore, the growth and activity of the
starter culture (mesophilic, thermophilic, or
a mixture of both), with the concomitant pro-
duction of lactic acid, other organic acids,
and anti - microbial agents, creates an envi-
ronment inhibitory to pathogens. Lactic acid
lowers the pH and inhibits most bacteria
eventually, including the starter lactic acid
bacteria (LAB).
Typical cheese pH values measured at
three to seven days after manufacture are 4.9
to 5.5 in most hard - ripened varieties, and 4.4
to 4.8 in fresh lactic and most soft - ripened
varieties. Most pathogenic bacteria can grow
at pH 5.6 to 7, some grow at pH less than 5.6,
and some may survive but few grow at pH
values less than 4.6.
For most cheese varieties the pH profi le
during manufacture and ripening is an impor-
tant process control. If the starter culture
growth is slow, for example, because of bac-
teriophage contamination or the presence of
inhibitors (antibiotics) in the milk, the resul-
tant slow decrease in acidifi cation may enable
the growth of undesirable bacteria. Pathogenic
bacteria that survive the early acidifi cation
phase may grow when acidity is neutralized

Table 3.2. Extrinsic and intrinsic parameters infl uencing cheese

safety and quality.

Extrinsic Intrinsic

Time/temperature of storage Moisture and salt content

Process controls in place pH and acidity

Product history Redox potential

Plant hygiene/sanitation Presence of competing microfl ora

Personal hygiene

Process temperature

Presence of antimicrobials

(natural or artifi cially added)

Nutrient availability