Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth
53
fact, along with the usual low pH and positive Eh of fruits, helps to explain the usual spoilage of these
products by molds rather than bacteria.
Antimicrobial Constituents
The stability of some foods against attack by microorganisms is due to the presence of certain
naturally occurring substances that possess and express antimicrobial activity. Some plant species are
known to contain essential oils that possess antimicrobial activity. Among these are eugenol in cloves,
allicin in garlic, cinnamic aldehyde and eugenol in cinnamon, allyl isothiocyanate in mustard, eugenol
and thymol in sage, and carvacrol (isothymol) and thymol in oregano.
47 Cow’s milk contains several
antimicrobial substances, including lactoferrin (see below), conglutinin, and the lactoperoxidase system (see below). Raw milk has been reported to contain a rotavirus inhibitor that can inhibit up to 10
6
pfu (plaqueforming units)/ml. It is destroyed by pasteurization. Milk casein as well as some free fatty
acids have been shown to be antimicrobial under certain conditions.
Eggs contain lysozyme, as does milk, and this enzyme, along with conalbumin, provides fresh
eggs with a fairly efficient antimicrobial system. The hydroxycinnamic acid derivatives ( p-coumaric,
ferulic, caffeic, and chlorogenic acids) found in fruits, vegetables, tea, molasses, and other plant sources
all show antibacterial and some antifungal activity. Lactoferrin is an iron-binding glycoprotein that
is inhibitory to a number of foodborne bacteria and its use as a microbial blocking agent on beef
carcasses is discussed in Chapter 13. Ovotransferrin appears to be the inhibitory substance in raw egg
white that inhibits Salmonella enteritidis.
4
Cell vacuoles of cruciferous plants (cabbage, Brussels sprouts, broccoli, turnips, etc.) contain glucosinolates, which upon injury or mechanical disruption, yield isothiocyanates. Some of the latter
possess antifungal as well as antibacterial activity. More on antimicrobials in foods can be found in
Chapter 13.
Lactoperoxidase System
This is an inhibitory system that occurs naturally in bovine milk, and it consists of three components:
lactoperoxidase, thiocyanate, and H 2 O 2 . All three components are required for antimicrobial effects,
and Gram-negative psychrotrophs such as the pseudomonads are quite sensitive. The quantity of
lactoperoxidase needed is 0.5–1.0 ppm, whereas bovine milk normally contains about 30 ppm.
6
Although both thiocyanate and H 2 O 2 occur normally in milk, the quantities vary. For H 2 O 2 , about 100
U/ml are required in the inhibitory system, whereas only 1–2 U/ml normally occur in milk. An effective
level of thiocyanate is around 0.25 mM, whereas in milk the quantity varies between 0.02 and 0.25 mM.
6
When the lactoperoxidase system in raw milk was activated by adding thiocyanate to 0.25 mM along
with an equimolar amount of H 2 O 2 , the shelf life was extended to 5 days compared to 48 hours for
controls.
6 The system was more effective at 30
◦ C than at 4
◦ C. The antibacterial effect increases with
acidity, and the cytoplasmic membrane appears to be the cell target. In addition to the direct addition
of H 2 O 2 , an exogenous source can be provided by the addition of glucose and glucose oxidase. To
avoid the direct addition of glucose oxidase, this enzyme has been immobilized on glass beads so that
glucose is generated only in the amounts needed by the use of immobilized β-galactosidase.
7 This
system was effective in goat’s milk against P. fluorescens and E. coli where the growth of the former
was controlled for 3 days and the latter for 2 days at 8
◦ C.
55
The lactoperoxidase system can be used to preserve raw milk in countries where refrigeration is
uncommon. The addition of about 12 ppm of SCN
− and 8 ppm of H 2 O 2 should be harmless to the
consumer.
44 An interesting aspect of this system is the effect it has on thermal properties. In one study,
53
fact, along with the usual low pH and positive Eh of fruits, helps to explain the usual spoilage of these
products by molds rather than bacteria.
Antimicrobial Constituents
The stability of some foods against attack by microorganisms is due to the presence of certain
naturally occurring substances that possess and express antimicrobial activity. Some plant species are
known to contain essential oils that possess antimicrobial activity. Among these are eugenol in cloves,
allicin in garlic, cinnamic aldehyde and eugenol in cinnamon, allyl isothiocyanate in mustard, eugenol
and thymol in sage, and carvacrol (isothymol) and thymol in oregano.
47 Cow’s milk contains several
antimicrobial substances, including lactoferrin (see below), conglutinin, and the lactoperoxidase system (see below). Raw milk has been reported to contain a rotavirus inhibitor that can inhibit up to 10
6
pfu (plaqueforming units)/ml. It is destroyed by pasteurization. Milk casein as well as some free fatty
acids have been shown to be antimicrobial under certain conditions.
Eggs contain lysozyme, as does milk, and this enzyme, along with conalbumin, provides fresh
eggs with a fairly efficient antimicrobial system. The hydroxycinnamic acid derivatives ( p-coumaric,
ferulic, caffeic, and chlorogenic acids) found in fruits, vegetables, tea, molasses, and other plant sources
all show antibacterial and some antifungal activity. Lactoferrin is an iron-binding glycoprotein that
is inhibitory to a number of foodborne bacteria and its use as a microbial blocking agent on beef
carcasses is discussed in Chapter 13. Ovotransferrin appears to be the inhibitory substance in raw egg
white that inhibits Salmonella enteritidis.
4
Cell vacuoles of cruciferous plants (cabbage, Brussels sprouts, broccoli, turnips, etc.) contain glucosinolates, which upon injury or mechanical disruption, yield isothiocyanates. Some of the latter
possess antifungal as well as antibacterial activity. More on antimicrobials in foods can be found in
Chapter 13.
Lactoperoxidase System
This is an inhibitory system that occurs naturally in bovine milk, and it consists of three components:
lactoperoxidase, thiocyanate, and H 2 O 2 . All three components are required for antimicrobial effects,
and Gram-negative psychrotrophs such as the pseudomonads are quite sensitive. The quantity of
lactoperoxidase needed is 0.5–1.0 ppm, whereas bovine milk normally contains about 30 ppm.
6
Although both thiocyanate and H 2 O 2 occur normally in milk, the quantities vary. For H 2 O 2 , about 100
U/ml are required in the inhibitory system, whereas only 1–2 U/ml normally occur in milk. An effective
level of thiocyanate is around 0.25 mM, whereas in milk the quantity varies between 0.02 and 0.25 mM.
6
When the lactoperoxidase system in raw milk was activated by adding thiocyanate to 0.25 mM along
with an equimolar amount of H 2 O 2 , the shelf life was extended to 5 days compared to 48 hours for
controls.
6 The system was more effective at 30
◦ C than at 4
◦ C. The antibacterial effect increases with
acidity, and the cytoplasmic membrane appears to be the cell target. In addition to the direct addition
of H 2 O 2 , an exogenous source can be provided by the addition of glucose and glucose oxidase. To
avoid the direct addition of glucose oxidase, this enzyme has been immobilized on glass beads so that
glucose is generated only in the amounts needed by the use of immobilized β-galactosidase.
7 This
system was effective in goat’s milk against P. fluorescens and E. coli where the growth of the former
was controlled for 3 days and the latter for 2 days at 8
◦ C.
55
The lactoperoxidase system can be used to preserve raw milk in countries where refrigeration is
uncommon. The addition of about 12 ppm of SCN
− and 8 ppm of H 2 O 2 should be harmless to the
consumer.
44 An interesting aspect of this system is the effect it has on thermal properties. In one study,