TABLE 9.3. Approximate pH Values of Some Foods of Animal Origin"

Product pH


Butter 6.1-6.4

Buttermilk 4.5

Cream 6.5 Cheese

American mild 4.9

Cheddar 5.9 Meat and poultry

Veal 6.0

Chicken 6.2-6.4 Fish and shellfish

Fish, most species' 6.6-6.8

Clams 6.5

Crabs 7.0

Oysters 4.8-6.3

Tuna fish 5.2-6.1

Shrimp 6.8-7.0

Salmon 6.1-6.3

Whitefish 5.5

"Just after death.

microbial growth and are considered buffered. Meat and milk products are buffered by the various proteins they contain. In contrast, vegetables are low in protein and do not resist pH changes.

Acid has two significant effects on respiring microbial cells: It renders the food less optimal as an environment for key enzymatic reactions, and it influences the transport of nutrients into the cell. Metabolic functions such as the synthesis and utilization of deoxyribonucleic acid (DNA) and adenosine triphosphate (ATP) require a neutral pH. When microorganisms are grown below or above their optimal pH, an increase in the length of the lag time (the period just after inoculation or contamination when cells have not yet begun to grow exponentially) is observed. The lag time may be extended even more if the substrate on which the cells are growing is buffered at a low pH.

The transport of metabolites into bacterial cells can be affected by the environmental pH. Bacterial cells tend to have a residual negative charge. Therefore, nonionized (uncharged) compounds can enter the cell, but ionized (charged) compounds cannot. Specifically, organic acids in their ionized form

(at higher, i.e., neutral or alkaline, pH) do not enter microbial cells, whereas nonionized acids (at low pH) are capable of transport into microbial cells.

The other effect exerted on microorganisms by adverse pH is the interaction between the H"1 and the enzymes in the cytoplasmic membrane. Under the influence of acidity, the morphology of some microorganisms changes: For example, the hyphae of Penicillium chrysogenum are shortened when the organism is grown in medium whose pH is >6.0.

Other environmental factors, such as temperature and salt, may interact synergistically with pH. For example, the pH of the substrate becomes more acid as the temperature increases. Thus many microorganisms may have higher acid tolerance at lower temperatures. For most microorganisms, when salt concentrations exceed the optimal range, the pH range that permits growth is narrowed. Adverse pH also makes microorganisms more sensitive to a wide variety of toxic agents.

Because enteric pathogens must survive the acidity of the stomach before reaching the intestinal tract to cause illness, their acid survival properties are important to their pathogenicity. Certain strains of Yersinia enlerocolitica have shown low pH stability and survival in tartar sauce (Aldova et al., 1975), cheese (Moustafa et al., 1983), and yogurt (Ahmed et al., 1986). Listeria monocytogenes has shown the ability to survive the manufacture of fermented products including sauerkraut, cheese products (Papageorgiou and Marth, 1989; Ryser and Marth, 1987, 1988), and sausages (Junttila et al., 1989; Glass and Doyle, 1989). The waterborne pathogen Plesiomonas shigelloides, which is often associated with seafood, has been shown to be both acid- and salt tolerant, with some strains exhibiting growth at pH 4.0 (Miller and Koburger, 1986). Certain strains of Escherichia coli 0157:H7 have been shown to have exceptional tolerance for acid pH, surviving in apple cider (pH 3.7-4.1) stored for 14-21 days at 4°C (Miller and Kaspar, 1994). Further, survival of these E. coli Ol 57:H7 strains in acidic trypticase soy broth (pH 2, 3, and 4) was greater at 4°C than at 25°C.

Exposure to a moderately low pH can result in cells with enhanced acid survival properties. This phenomenon, known as acid adaptation, has been observed in E. coli and in species of Salmonella (Leyer and Johnson, 1993), Listeria (Kroll and Patchett, 1992), Streptococcus, and Enterococcus (Belli and Marquis, 1991). The most extensively studied acid adaptation is the acid tolerance response (ATR) of Salmonella typhimurium (Foster, 1993). Acid-adapted 5. typhimurium has been shown to have increased resistance to food processing and preservation treatments (i.e., heat, salt, hydrogen peroxide, and increased osmolarity; Leyer and Johnson, 1993).

Indigenous antimicrobial agents Certain naturally occurring substances indigenously found in some foods enhance their stability by killing or inhibiting microorganisms. Examples of such compounds in plants are essential oils such as eugenol in cloves, alliein in garlic, cinnamic aldehyde and eugenol in cinna mon, allyl isothiocyanate in mustard, eugenol and thymol in sage, and carva-crol, isothymol, and thymol in oregano.

Cow's milk contains several antimicrobial substances, such as lactoferrin, conglutinin, and the lactoperoxidase system. The lactoperoxidase system, the best-known of these agents, consists of three components—lactoperoxidase, thiocyanate, and peroxide—all of which are required for antimicrobial activity. Gram-negative bacteria such as pseudomonads are very sensitive to extremely small amounts (<1.0 ppm) of these compounds (Zapico et al., 1983). This system has been used to preserve milk in underdeveloped countries where refrigeration is rare. An interesting feature of the system is that it can alter the thermal properties of microorganisms in milk. For example, the thermal D values (decimal reduction: the time required at constant temperature to reduce the bacterial population by 1 log) of L. monocytogenes and Staphylococcus aureus may be reduced by >80% (Kamau et al., 1990). The underlying mechanism remains unclear. Among other components of milk, fatty acids and casein have been shown to have antimicrobial activity under certain conditions. Raw milk also contains a rotovirus inhibitor, but this is destroyed by pasteurization.

Eggs, milk, clams, and oysters contain lysozyme, which can act as an antimicrobial agent (Cheng and Rodrick, 1975). Fruits, vegetables, tea, molasses, and a number of plants show antibacterial and antifungal activities thought to arise from hydroxycinnamic acid derivatives, such as ferulic, caffeic, and chlorogenic acids. Cruciferous plants such as cabbage, brussels sprouts, broccoli, and turnips contain glucosinolates in their cell vacuoles. On rupture, these compounds release isothiocyanates, which possess antifungal and antibacterial activity.

Oxidation-reduction potential It is well known that microorganisms exhibit different sensitivities to the oxidation-reduction (O/R) potential of their growth media. The O/R potential of a substrate is the ease with which the substrate loses or gains electrons. When an atom or molecule loses electrons it is oxidized, and when it gains electrons it is reduced; therefore, a substrate that gives up electrons easily is a good reducing agent and one that readily takes up electrons is a good oxidizing agent. The transfer of electrons from one compound to another creates a potential difference (E) between them that can be measured with a potentiometer. E, expressed in millivolts (mV), may be positive (oxidation), negative (reduction), or zero.

The O/R potential of food systems or complex growth media (expressed as £h) is affected by the oxygen tension of the environment, the availability of the food system to that environment, the inherent O/R characteristics of the system, and the poising capacity (resistance to E^ change). Reducing conditions in food products are maintained by reducing components that include the sulf-hydryl (SH) groups in proteins and amino acids, ascorbic acid moieties, and/or reducing sugars. Oxidizing conditions are influenced by the presence of oxygen, oxidizing catalysts (e.g., iron and copper), and certain oxidation reactions (e.g., lipid oxidation). Because Eh measurement is dramatically influenced by pH, reported values should indicate the pH of the system. The Eh of foods varies widely. Plant foods and juice products tend to have positive Eh values ranging from 300 to 400 mV. Protein-based foods generally have negative Eh values (e.g., meat products -200 mV; cheese products -20 to -200 mV).

Generally, aerobic microorganisms require positive Eh values and anaerobes require negative Eh values for growth. The Eh requirements for the growth of strict anaerobes (such as Clostridium) are approximately —200 mV. Such low Eb values would be inhibitory to strict aerobes such as Bacillus. Other bacteria may be classified as microaerophilic—defined as aerobes that grow better at lower (reducing) E^ values—or as facultatively anaerobic (those that can grow either anaerobically or aerobically).

Moisture content One of the oldest methods of preserving food is drying or dehydration, accomplished by removing water and/or binding the water in the food so that microorganisms cannot grow. The water requirements for microorganisms are described in terms of the water activity (aw) in their environment. This value is defined by the ratio of the water vapor pressure of a food to the vapor pressure of pure water at the same temperature (thus pure water has an «v, of 1.00). For example, the aw of a saturated solution of sodium chloride in water is 0.75 (see Table 9.4). Water activity is related to relative humidity (RH; discussed below): RH = 100 x aw. Because all biochemical reactions require an aqueous environment, reducing water availability adversely affects enzyme activities and hence impairs biological processes. As a general rule, lowering aw lengthens microorganisms' lag phase of growth, decreases their growth rate, and reduces the final population size.

The aw of most fresh foods is >0.98; approximate minimal aw values for growth of important food microorganisms are shown in Table 9.5. In general,

TABLE 9.4. Relationship Between Water Activity (aw) and Concentration of Salt Solutions"

Water activity

Sodium chloride concentration


Percentage (w/v)

0 0

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