Public Health Aspects of Cheese | Industrial Microbiology

When pathogen-free milk, an active lactic starter culture, and good hy­gienic practices are used to produce cheese, a safe food results. If the cheese is then handled and distributed in a sanitary manner, this food will be safe for consumption. When one considers the many millions of pounds of cheese that are produced annually worldwide and the few cases of re­ported illness attributed to this food, one must conclude that cheese has a remarkable record as a safe food.

In spite of this record, sometimes mistakes have been made in the steps leading from cow to milk to cheese to the consumer, and these mistakes have caused outbreaks of foodborne illness. Problems of greatest concern include staphylococcal food poisoning, salmonellosis, aflatoxicosis, gastro­enteritis caused by enteropathogenic strains of E. coli, and presence in cheese of biologically active amines.

All of these will be discussed in some detail, not because they are major problems, but because making the infor­mation available here may help interested persons in taking the steps needed to keep the problems under control. Furthermore, most discussions of cheese either ignore this information or treat it superficially, and hence the information may not be readily available to the person who is working with cheese.

1. Staphylococcal Food Poisoning:

Staphylococcus aureus, a Gram-positive, coccus-shaped bacterium, can produce an enterotoxin (a protein with a molecular weight of approximately 28,000 to 34,000—there are several enterotoxins). This enterotoxin, when ingested, can cause such symptoms as nausea, vomiting, retching, and often diarrhea. Recovery often follows in 24 hr, but several days may be required.

The extent to which these symptoms may appear and the severity of illness is determined chiefly by the amount of toxin that was ingested and the susceptibility of the individual who be­comes a victim of the disease. Outbreaks of this disease have been caused by toxic cheese.

Staphylococci in Cheese:

Although outbreaks of staphylococcal food poisoning have been associated with rennet-type cheese, this problem can be largely avoided if- (1) cheese is made from milk that was stored to preclude staphylococcal growth and then was given a heat treatment, such as pasteurization, that was adequate to inactivate staphylococci; (2) heated milk is not contaminated with staphylococci; and (3) an active starter culture that, produces sufficient acid during cheese-making is used. Since illness has been associated with cheese, several investigators have studied the behavior of S. aureus during the manufacture of different kinds of cheese.

It is evident that some growth of staphylococci is possible even when cheese is made by a normal process; hence the need to inactivate staphylococci before the cheese-making process begins. The initial increase (after 2 hr) is largely attributable to the concen­tration effect when curd is formed.

Some growth continued during the manufacturing process except when the curd was salted. The detrimental effect of salt was limited, as might be expected, and growth of staphylococci again was evident by the time cheese was taken from press. Takahashi and Johns (1959) and Walker et al. (1961) made similar observations when they studied Cheddar and Colby cheese, respectively.

Additional work was done by Tatini et al. (1971). Their data indicate the importance of both the starter culture and the initial concen­tration of staphylococci in determining whether or not a cheese becomes toxic. These authors concluded that Colby cheese made normally would be toxic if it contained at least 15 million staphylococci/g.

The value for Ched­dar cheese was 28 million /g. If the starter culture failed to perform proper­ly, then toxic cheese could result if 3 to 5 million staphylococci/g were present. Finally, it should be mentioned that the number of staphylococci declines as the cheese ripens. However, if they are present initially, it is not usual for them to persist in well-ripened cheese.

Aged Cheddar cheese is sometimes added to another food product to impart a desired flavor. Procedures to accelerate ripening of the cheese have been sought so that relatively fresh, and less expensive, cheese could be used for this purpose instead of the more costly aged product made by conventional processes.

One method to prepare a liquid cheese product with a Cheddar-like flavor in fewer than 7 days has been described by Kristof fersen et al (1967). Basically, the process involves making slurry from 2 parts of 24 hr-old salted, un-pressed Cheddar curd and 1 part of a solution of 5.2% sodium chloride. The slurry then is held at 30°C. Addition of 10-100 ppm of reduced glutathione enhances development of the Cheddar flavor. Gandhi and Richardson (1973) inoculated similar slurries with S. aureus and found that the bacterium could grow and produce entero­toxin in 24 to 48 hr at 32°C when the slurries contained 45 to 60% moisture.

Treatment of the inoculated slurry with 0.5% hydrogen peroxide did not eliminate the staphylococci but addition of 0.2 or 0.3% sodium sorbate inhibited growth and thus prevented production of enterotoxin. The work of Gandhi and Richardson (1973) emphasizes the need to evaluate new processes for potential health hazards.

Tatini et al. (1973) also studied the potential for enterotoxin production by added staphylococci in the manufacture of brick, Swiss, Mozzarella, and blue cheese. No enterotoxin appeared in their experimental lots of Mozza­rella or blue cheese. Environmental conditions in these two types of cheese were such that the staphylococci, even though some growth occurred, were unable to produce enterotoxin.

In contrast to these observations, the inves­tigators noted that staphylococcal growth and enterotoxin production did occur during the manufacture of brick and Swiss cheese. The kind of starter culture used, the initial population of staphylococci, and the population achieved by 7 hr after cheese was made appear to have determined whether or not the cheese was toxic.

These authors evaluated several toxigenic strains of S. aureus when they experimented with Swiss cheese. As might be expected, when some strains were used, the cheese became toxic, whereas others failed to produce detectable enterotoxin even though growth was evident during the cheese-making process. The authors concluded that staphylococcal populations of 3-8 x 10⁶/g could result in toxic Cheddar, Colby, brick, or Swiss cheese, depending on the kind and activity of the starter culture that was used and the strain of toxigenic S. aureus that was present.

2. Salmonellosis:

This form of foodborne illness has an incubation period of from 3 to 72 hr, with most outbreaks occurring within 12 to 24 hr after food contaminated with the salmonellae has been ingested. The principal symp­toms of salmonellosis are nausea, vomiting, abdominal pain, and diarrhea that usually appear suddenly.

Occurrence of these symptoms may be preceded by a headache and chills, Additional symptoms often associated with the disease include watery, greenish, foul-smelling stools; prostration; muscular weakness; faintness; a moderate fever; restlessness; twitching; and drowsiness.

Severity and duration of the disease vary with the amount of food (and hence salmonellae) consumed, the kind of Salmonella, and the resistance of the individual. Intensity varies from slight discomfort and diarrhea to death in 2 to 6 days. Usually, symptoms persist for 2 to 3 days, followed by an uncomplicated recovery. In some instances, symptoms may linger for weeks or months. Some patients (0.2 to 5%) become carriers of the Salmonella organism which caused their infection. Mortality from gastro­enteritis caused by salmonellosis is generally less than 1%.

Outbreaks of salmonellosis have been associated with consumption of different cheeses. The earlier literature is primarily concerned with typhoid fever (a form of salmonellosis), whereas more recent investigations indicate that salmonellae other than Salmonella typhi may be associated with cheese-induced illness.

Cheddar Cheese:

Gauthier and Foley (1943) described an epidemic of typhoid fever in Canada during the autumn of 1941. Forty cases were involved and 6 deaths resulted. The only food common to all the patients was Cheddar cheese, made locally from raw milk and consumed when it was 10 days old Although the factory where the cheese was made lacked adequate sanitation, the source of the epidemic was found to be a known typhoid carrier who, against orders from public health authorities, milked cows whose milk was used by the factory to produce Cheddar cheese.

Another outbreak of typhoid fever attributable to Cheddar cheese oc­curred in Quebec during February, 1944, and was described by Foley and Poisson (1945). The original source of infection was never traced, although it was found that the cheese-maker’s wife had an active case. Nevertheless, the authors believe she was not responsible for infecting the cheese. Foley and Poisson (1945) also recommended use of a 3-month ripening period when Cheddar cheese is made from raw milk.

Out of 507 cases of typhoid fever in Alberta between 1936 and 1944, 111 were caused by Cheddar cheese, according to Menzies (1944). Samples of cheese from the last 3 outbreaks in 1944 were recovered and tested tor S typhi. The organism was found in 30-day-old cheese, but could not be recovered from 48- and 63-day-old cheese. As a consequence of this out­break, Alberta halted sale of cheese made from raw milk unless the cheese was ripened for at least 3 months.

Survival of S. typhi in Cheddar cheese was studied by Ranta and Dolman (1941) and Campbell and Gibbard (1944). The former authors mixed S typhi with Cheddar cheese and found that the organism survived for 1 month at 20°C. Inoculation of S. typhi onto the surface of cheese was accompanied by a similar survival at room temperature, a longer survival period at refrigerator temperature, and penetration of the organism to a depth of 4 to 5 cm into the cheese after 17 days.

Campbell and Gibbard (1944) inoculated milk with S. typhi and used it to make Cheddar cheese. All cheeses were ripened for 2 weeks at 14.4° to 15.6°C after which 1 cheese from each duplicate set was transferred to storage at 4.4° to 5.6°C. At the lower temperature, 7 of 10 cheeses contained viable S typhi cells for more than 10 months, whereas at the higher temperature the organism generally disappeared after 3 months of ripen­ing. Size of inoculum and acidity of the cheese did not appear to affect the longevity of S. typhi.

Goepfert et al. (1968) investigated the behavior of Salmonella typhimur­ium during the manufacture and ripening of Cheddar cheese. Pasteurized milk was inoculated with S. typhimurium when the lactic starter culture was added. A slight increase in number of salmonellae occurred during the time between inoculation and cutting of the curd, followed by a rapid increase during the interval between cutting the curd and draining the whey.

After accounting for concentration of cells through coagulation, an average of 3.5 generations of salmonellae developed during this period. Salting of the curd was associated with a reduction in the growth rate, and ripening of the cheese was accompanied by a decrease in the salmonellae population. Survival of S. typhimurium exceeded 12 weeks at a ripening temperature of 12.8°C and 16 weeks at 7.2°C. Limited tests demonstrated that acetate accumulating in ripening cheese may contribute to the demise of salmonellae.

Park et al. (1970B) made Cheddar cheese using a slow acid-producing strain of S. lactis to determine what might happen if salmonellae were present in cheese produced with an abnormal fermentation. Salmonellae grew rapidly during manufacture of cheese and limited additional growth occurred in cheese during the first week of ripening at 13°C, after which there was a gradual decrease in population of S. typhimurium. S. typhimuri­um survived during ripening of the low-acid cheese for up to 7 months at 13°C and 10 months at 7°C.

Park et al. (1970A) also made cold-pack cheese food that was contami­nated with S. typhimurium and stored it at 4.4° and 12.8°C. Rapid decrease in number of salmonellae occurred during the first week of storage regard­less of temperature or composition of the product. Viable salmonellae could not be recovered, after 3 weeks at 12.8°C or 5 weeks at 4.4°C, from cheese food adjusted to pH 5.0 with lactic acid and containing 0.24% potassium sorbate.

Substituting sodium propionate for sorbate resulted in 14 and 16 weeks of survival by salmonellae when cheese food was held at 12.8° and 4.4°C, respectively. Partial or complete replacement of lactic acid by acetic acid was accompanied by somewhat longer survival of S. typhimurium than when only lactic acid was used.

Elimiination of added acid from the cheese food resulted in survival of S. typhimurium for 6 to 7 weeks when potassium sorbate was present, for 16 and 19 weeks when sodium propionate was used, and in excess of 27 weeks without any preservative. The ability of potas­sium sorbate to inactivate S. typhimurium was confirmed in another study by Park and Marth (1972B).

Effects of lactic acid bacteria on S. typhimurium were determined by Park and Marth (1972A). They observed that S. cremoris, S. lactis, and mixtures of the two, when added to milk in the amount of 0.25%, repressed growth but did not inactivate S. typhimurium during 18 hr of incubation at 21° or 30°C. Increasing the inoculum of the mixed lactic cultures to 1% resulted in inactivation of S. typhimurium at 30°C.

When added at the level of 1%, S. thermophilus was more detrimental to S. typhimurium at 42°C than was L. bulgarieus. Mixtures of L. bulgaricus and S. thermophi­lus, when added to contaminated milks at levels of 1 and 5%, caused virtually complete inactivation of S. typhimurium during the interval be­tween 8 and 18 hr of incubation at 42°C.

Colby Cheese:

A typhoid fever epidemic started in January 1944 in the northern part of Indiana and covered 18 to 20 counties. Approx­imately 250 cases and 13 deaths were recorded in this outbreak. Thomasson (1944) noted that the carrier was never traced but illness was associated with consumption of Colby cheese. The cheese, made by a single dairy in the area, was produced from raw milk preheated to 32.2° to 37.8°C and was not allowed to ripen before sale.

Tucker et al. (1946) recorded an incident in which 384 cases of illness in Kentucky were caused by consumption of 12- to 14-day-old Colby cheese infected by S. typhimurium. Investigation revealed that a dead mouse had been removed from a 1000 gal. vat of milk used to produce the cheese. Tests on infected cheese demonstrated that S. typhimurium survived for 302 days during storage at 6.1° to 8.9°C.

Mold-Ripened Cheeses:

Mocquot et al. (1963) made blue cheese from milk inoculated with 10⁴ and 10⁶ Salmonella sp. cells per ml and observed the behavior of the organisms during manufacture and ripening. The death rate of Salmonella was related to the pH value of 24-hr-old cheese and increased with a decrease in the pH. Survival and death of the organism were similar in the inner and outer portions of the cheese. The percentage survival of salmonellae in 6-day-old cheese was less than 0.01%.

Camembert cheese was responsible for an extensive outbreak of illness in Germany, with more than 6000 cases involved. According to Bonitz (1953) the cheese was infected with S. bareilly, and rennet was thought to be the original source of the contaminant. After conducting additional experi­ments, Bonitz (1957) changed his mind and postulated that infection proba­bly entered the cheese via the glue used to fasten labels to the individual cheeses.

Other Cheese:

Occurrence of Salmonella in a variety of other ripened cheeses has been reported. Many of these cheeses are not common in the United States, but the information may be useful. In some instances inves­tigators simply stated that salmonellae were recovered from cheese, with­out specifying the type of cheese being studied.

Bruhn et al. (1960) studied the survival of salmonellae in Samsoe cheese. This cheese has a pH value of 5.15 to 5.20 after 24 hr and contains 44 to 46% moisture. Samsoe cheese was ripened at 16° to 20°C for 5 to 6 weeks, after which it was held at 10° to 12°C for an additional 7 to 10 weeks. A 60 day period was necessary to achieve a 10,000-fold reduction in number of viable salmonellae. The death rate was less rapid at 10° to 12°C storage than at 16° to 20°C.

In May of 1944, an unusually high number of typhoid fever cases were reported in 4 counties of California. Investigation, according to Halverson (1944), showed that the sources of infection were Romano Dolee, Teleme, and high-moisture Jack cheeses, all made from unpasteurized milk. Over 90% of the affected persons had consumed one or several of the cheeses just mentioned. This outbreak provided the impetus for the state of California to pass laws controlling the manufacture and sale of cheese in that state.

Wahby and Roushdy (1955) studied the survival of Salmonella enteritidis, S. typhi, and Salmonella paratyphi B in Domiatti cheese, an Egyptian dairy product. When the cheese was held at 20° to 25°C, S. enteritidis survived for 17 days, S. typhi for 12 days, and S. paratyphi B for 27 days.

Forty samples of Kareish cheese (an Arabian product) were examined chemically and bacteriologically by Moutsy and Nasr (1964). They observed that the cheese contained from 2.33 to 11.38% salt, 0.75 to 2.7% titratable acidity, 68 million to 6.3 billion bacteria per g, and 1000 to 100,000,000 coliforms per g. One sample yielded S. typhimurium.

The behavior of S. typhimurium and S. enteritidis in Kachkaval cheese (a hard cheese) was studied by Todorov (1966). He added 5 million to 350 million salmoneilae per ml of milk, which was then made into Kachkaval cheese. Heating of the curd in a water bath at 71° to 72°C for 70 to 90 sec did not destroy the salmoneilae. Their survival in the cheese ranged from 4 to 20 days, depending on the initial level of contamination.

Vizir (1940) studied the behavior of Salmonella breslau, a contaminant of Brynza cheese, in different media fortified with salt. At 12° to 16°C, the organism remained alive in milk with 5, 10, and 15% salt and in broth with 5 to 10% salt during the 45 days of the experiment. Lactic acid at a concentration of 0.9% or higher destroyed the organisms in 5 to 10 days, regardless of temperature. Zagaevskii (1963) noted that S. typhimurium and Salmonella dublin remained viable in Brynza cheese for up to 22 months.

An outbreak of typhoid fever attributable to a cheese made in a Norwe­gian home was reported by Hemmes (1942). A man ill with typhoid fever was a part of the household when the cheese in question was made. Studies on the aged cheese revealed that viable S. typhi were present after 40 but not 55 days.

3. Enteropathogenic Escherichia coli:

Enteropathogenic Escherichia coli can be defined as any strain of E. coli having the potential to cause diarrheal disease. Enteropathogenic E. coli strains (EEC) have been divided into 2 groups, based on the type of disease produced. Those causing a disease with cholera-like symptoms (watery diarrhea leading to dehydration and shock) also produce enterotoxins, and thus are called toxigenic EEC.

These strains have been implicated as the cause of “infantile diarrhea” and “traveler’s diarrhea”. Those strains causing a Shigella-like illness (diarrhea with stools containing blood and mucus) are called invasive EEC because of their ability to penetrate the epithelial cells of the colonic mucosa. These strains do not produce an enterotoxin. Invasive EEC is associated with dysentery-like disease in people of all ages.

Incidence of E. coli and Coliforms in Cheese:

Presence of coliforms in cheese has been the subject of research for over 80 years. Early investiga­tions were concerned with prevention of gassy defects in curd and cheese caused by coliform bacteria.

Since coliforms must reach numbers close to 10⁷/g to cause gassiness in Cheddar cheese, cheese of normal appearance can still have substantial numbers of E. coli present. Even with pasteur­ization, post-pasteurization contamination of milk with coliforms can be great enough to cause cheese to become gassy. Yale (1943) found that Cheddar cheese of high quality could contain up to 57,000 coliforms/g in the curd.

A general survey of coliform bacteria in Canadian pasteurized dairy products by Jones et al. (1967) showed that 18.7% of the coliforms isolated from these products were of intestinal origin. Three serotypes or 2% of the E. coli isolates were enteropathogenic serotypes. Light body (1962) found that 97% of Queensland Cheddar cheese contained coliforms after 2 to 3 weeks of aging. E. coli biotype I was found in 70% of the samples.

Cheese samples with more than 10⁶ coliforms/g were of poor quality. Some high grade cheese had more than 1000 coliforms/g. In further studies of Queens­land Cheddar cheese, Dommet (1970) reported that improper pasteuriza­tion of milk, unsanitary equipment, and contaminated starter cultures were all responsible for coliform contamination of the cheese.

In recent surveys of Canadian cheese varieties, Elliott and Millard (1976) noted that 15% of retail cheese samples contained over 1500 coliforms/g, and Collins- Thompson et al. (1977) found 18.1% of soft cheeses and 13.6% of semisoft cheeses exceeded 1600 total coliforms/g.

Recently Frank and Marth (1978) examined 106 samples of commercial cheese for the presence of fecal coliforms and EEC. Included in their survey were samples of Camembert, Brie, and brick, Muenster, and Colby cheese. Of the samples tested, 58% contained fewer than 100 fecal coliforms/g, but 17% contained more than 10,000/g. No EEC serotypes were found in any of the cheese. A similar survey by Glatz and Brudvig (1980) also demon­strated the absence of EEC from commercial cheese.

Inhibition of E. coli by Lactic Acid Bacteria:

Frank and Marth examined the effects of lactic acid bacteria on E. coli when both organisms were in skim milk. With no lactic acid bacteria present, the generation times of pathogenic and nonpathogenic strains of E. coli ranged from 28 to 35 min when incubation was at 32°C and from 66 to 109 min at 21°C.

Addition of 0.25 or 2.0% of a commercial starter together with E. coli served to completely inhibit growth of E. coli in 6-9 hr of incubation at 32°C. At 21°C, E. coli often had difficulty initiating growth in the presence of the lactic acid bacteria. S. cremoris and S. lactis were equally inhibitory to E. coli at 32°C. At 21°C, S. cremoris was more inhibitory to E. coli than was S. lactis, but a commercial mixed-strain lactic starter culture was more inhibitory than was either of the pure cultures.

Behavior of EEC in Cheese:

During November and December 1971 at least 227 persons in 96 separate outbreaks in several states in the United States became ill with acute gastroenteritis about 24 hr after consuming imported French Camembert or Brie cheese.

E. coli of serogroup 0.124:B17 was isolated from stools of several patients and from samples of cheese believed to have caused the illness. This episode of foodborne illness prompted Park et al. (1973) and Frank et al. (1977) to study the fate of EEC during the manufac­ture and ripening of Camembert cheese.

They observed the following- (1) growth of E. coli sometimes was minimal until after curd was cut and hooped, (2) populations of approximately 10⁴ E. coli/g appeared in some cheeses 5-6 hr after the cheese-making process began when milk initially contained about 10² E. coli/ml, (3) there was a demise of E. coli during ripening with some strains disappearing from cheese during the first 2 weeks and others surviving for 4 to 6 weeks, and (4) no growth of E. coli was observed in ripe cheese at a pH of 6.7 but rapid growth of E. coli occurred on the surface of the cheese.

The fate of EEC during the manufacture and ripening of brick cheese also was determined by Frank et al. (1978). Results differed from those obtained with Camembert cheese in that (1) somewhat larger populations of EEC developed initially during manufacture of brick cheese, (2) inactivation of EEC was slower in brick cheese with 10³-10⁴/g remaining after 7 weeks, and (3) growth of EEC on the surface of brick cheese was more limited.

4. Botulism:

This serious and often fatal disease results after ingestion of a toxin produced by the anaerobic spore-forming Clostridium botulinum. In the United States, there have not been any known cases of botulism attribut­able to consumption of natural or process cheese. One outbreak of botulism in the United States in 1951 and another in Argentina in 1974 were associated with consumption of process cheese spread.

This product also has an excellent safety record since there have been no known problems caused by it in nearly 30 years. Process cheese spread can meet legal standards if it has as much as 60% water and has a pH value of up to 5.2. At these limits, the product has less inhibitory potential than if the amount of water present and the pH were reduced.

Under the Good Manufacturing Practices for canned foods in the United States, process cheese spreads would be classified as a law-acid food requiring a heat process sufficient to destroy spores of C. botulinum unless it can be demonstrated that the spores cannot grow in the cheese spread. Since the heat treatment currently used is insufficient to destroy the spores of C. botulinum, the situation just described has prompted re­newed interest in the behavior of C. botulinum in cheese spreads.

Kautter et al. (1979) inoculated jars of several varieties of process cheese spread (pH 5.05-6.32, water activity 0.930-0.953) with 24,000 spores of C. botulinum per jar. One variety, cheese-with-bacon, also received 460 spores per jar. When stored at 35°C, 46 of 50 jars of Limburger and 48 of 50 jars of cheese-with-bacon spread became toxic after 83 and 50 days, respectively.

One jar of cheese-with-bacon spread that received the smaller inoculum became toxic during 6 months of storage at 35°C. No jars of 3 other varieties of cheese spread (Cheez Whiz, Old English, and Roka Blue) became toxic when subjected to the same treatment.

Another series of experiments was done by Tanaka et al. (1979) in which spores of C. botulinum (1000/g) were added to process cheese spread when it was manufactured. Moisture content and pH of various batches ranged from 49.7 to 59.2% and 5.80 to 6.28, respectively. Cheeses were incubated at 30°C for up to 48 weeks.

No samples became toxic under these conditions when the product contained 52 or 54% moisture and was made with sodium phosphate as the emulsifier. Use of sodium citrate as the emulsifier resulted in a product that remained toxin-free at 52% moisture and one that became toxic at 54% moisture. Products with 58% moisture became gassy and toxic regardless of the emulsifier that was used.

5. Aflatoxin:

Aflatoxin is a collective term that refers to a group of highly toxic and carcinogenic substances produced by the common molds Aspergillus flavus and Aspergillus parasiticus during their growth on foods or feeds. Our concern with aflatoxin and the toxigenic aspergilli is twofold. First, there is the potential hazard to health of consuming even small amounts of afla­toxin. Second, the toxigenic aspergilli are widely distributed in nature, and hence the likelihood is great that they will contaminate foods and feeds.

Aflatoxin sometimes can appear in milk, cheese, and other dairy prod­ucts. That aflatoxin can appear in milk has been recognized since 1962. 1ft spite of this, little attention has been given to this specific aspect of the aflatoxin problem in the United States. Considerably more research on this subject has been done in West Europe, particularly in the Federal Republic of Germany.

Two events have occurred recently in the United States which has served to direct attention to the problem of aflatoxin in milk and milk products. The first of these occurred in the southeastern states where corn harvested in the fall of 1977 often contained appreciable amounts of afla­toxin.

Feeding of such corn to dairy cattle resulted in aflatoxin in the milk produced by the cows. This prompted the Food and Drug Administration to establish a maximum of 0.5 part per billion (ppb) of aflatoxin M₁ as allowable in fluid milk that enters interstate commerce. The second event occurred in the summer of 1978 when dairy cows in Arizona were fed contaminated cottonseed, and milk produced by the cows contained aflatoxin M₁.

A considerable amount of cheese also was thought to have been made from the contaminated milk, but much of it was later cleared through an intensive testing program by the firm that owned the cheese. Some of the cottonseed that contained aflatoxin was shipped to several other western states and this served to further aggravate the problem. More extensive contamination of corn than in 1977 occurred in 1980 in the southeastern United States.

The Major Aflatoxins:

Aflatoxins B₁, G₁, B₂, and G₂ are the major forms produced by the molds, with B₁ and G₁ usually synthesized in largest amounts. The B-aflatoxins are given this designation because they have a bluish color under long-wave ultraviolet light, and the G-toxins are given their designation because they have a greenish-yellow color under the same type of light.

The form of aflatoxin that is produced from B₁ by the cow and is excreted in milk is called M₁. Chemically, aflatoxins are substituted coumanns that are relatively small molecules; Aflatoxins are not proteins as are many of the toxins produced by bacteria.

Of the aflatoxins, B₁ is most toxic and most carcinogenic. Aflatoxin M₁ is about as toxic but is considerably less carcinogenic than B₁. Aflatoxins G₁, B₂, and G₂ are less toxic and less carcinogenic than is B₁.

The aflatoxins are rather heat-stable but can be degraded by strong acidic or alkaline solutions, oxidizing agents, some molds, and a few bacteria. Recently Doyle and Marth demonstrated that bisulfite as well as the molds that produced them can degrade aflatoxins.

Routes by Which Aflatoxin Gets into Milk and Cheese:

Aflatoxin can get into milk in 1 way and into milk products in 2 ways. Milk becomes contaminated only when cows consume feed that contains aflatoxin B₁, usually the major and always the most toxic form of aflatoxin. Some of the ingested aflatoxin B₁ is converted to M₁ by the liver of the cow and this form of aflatoxin is excreted in the milk. Products made from such milk will also contain aflatoxin M₁.

Growth of a toxigenic aspergillus on a dairy product such as cheese also can result in contamination of that product with one or several of the aflatoxins that are synthesized by the mold. It is possible for cheese to contain M₁, if made from contaminated milk, and also Bi and other forms of aflatoxin if that same cheese subsequently supports growth of a toxigenic aspergillus. The routes of contamination as just described were first pre­sented by Kiermeier et al. (1975).

Aflatoxin in Commercial Milk and Cheese:

Does aflatoxin M₁ really ever appear in commercial milk products? The answer is an unqualified “yes,” although the incidence nationwide in the United States is likely to be small.

Tests on milk marketed late in 1977 in 4 southeastern states having contaminated corn showed that 4 to 8% of the samples contained 0.5 or more ppb of aflatoxin M₁. Lesser amounts appeared in an additional 38 to 72% of the samples.

Recently a subcommittee of the Dairy Farm Methods Committee of the International Association of Milk, Food and Environmental Sanitarians addressed a series of questions about aflatoxin in milk to regulatory agen­cies in the various states in the United States.

Of the regulatory agencies in 47 states that responded, (1) 13 indicated aflatoxin in milk is a problem in their states, (2) 18 indicated they test raw Grade A milk for aflatoxin, (3) 8 reported they found aflatoxin in retail milk, and (4) 1 each reported they had found aflatoxin in cottage cheese, cheese, butter- milk, yogurt, and ice cream.

Some data obtained in the Federal Republic of Germany (Table 4.6) indicate that aflatoxin M₁ was found in 34 to 82% of samples of milk, dried milk, yogurt, un-ripened cheese, Camembert cheese, hard cheese, and pro­cess cheese. It is apparent from these data and from those for the United States that aflatoxin M₁ does, in fact, sometimes occur in processed dairy products available to the consumer. The true incidence of aflatoxin M₁ in U.S. dairy products is probably low, and appearance of the toxin in finished products is likely to be limited largely to certain regions of the United States where cows are most likely to consume aflatoxin-contaminated feed.

Effects of Processing on Aflatoxin in Milk and Cheese:

Aflatoxin M₁ appears to be rather stable when exposed to the processing techniques that are used to manufacture dairy products. Hence, a substantial amount of the aflatoxin in milk would be expected to appear in products made from milk. Our information on what happens during processing is not complete, but some of the available data point to what can be expected.

Milk is commonly given some sort of heat treatment during conversion to a food for sale. Kiermeier and Mashaley (1977) exposed naturally and artificially contaminated milk to a series of heat treatments. In general, more aflatoxin M₁ was lost when milk was heated for minutes (15 or 30) than seconds (40). The loss of M₁ ranged from 6 to 41%. Although the results are somewhat variable, it is evident that heating of milk re­sulted in loss of some aflatoxin M₁.

Distribution of aflatoxin M₁ between cheese and whey was studied by Stubblefield and Shannon (1974). According to their results, approximately 50% of the M₁ in milk appeared in most varieties of cheese. An exception was Ricotta with only 30% of the M₁ in milk appearing in the cheese.

Aflatoxin in Moldy Cheese:

Growth of toxigenic aspergilli on a dairy product such as cheese could lead to the presence of aflatoxin in the food. Bullerman (1976) and Bullerman and Olivigni (1974) isolated numerous molds from Swiss and Cheddar cheese. Although most of their isolates were molds in the genus Pemcilhum, nevertheless a few isolates from each type of cheese were aspergilli capable of producing aflatoxin.

What happens if toxigenic aspergilli are afforded the opportunity to grow? Lie and Marth (1967) explored this question by inoculating Cheddar cheese with spores of toxigenic aspergilli, allowing the mold to grow at room temperature and testing cheese for presence of aflatoxin.

They found that both A. flauus and A. parasiticus could produce aflatoxin on cheese and that the aflatoxin penetrated into cheese to a distance of about 1.3 cm. Later Shih and Marth (1972) observed that the toxigenic aspergilli also could produce aflatoxin on brick cheese and that the toxin penetrated into this cheese to a depth of nearly 2 cm.

Cheese containing aflatoxin B₁ was used by Kiermeier and Rumpf (1975) to manufacture pasteurized process cheese. They noted that only about 5% of the B₁ was lost during the manufacturing process. A similar observation was made by Polzhofer (1977B) when he made pasteurized process cheese from cheese that contained aflatoxin M₁.

Finally, it should be mentioned that ordinarily cheese is not well suited for production of aflatoxin by the toxigenic aspergilli. This is true for several reasons- (1) cheese lacks the carbohydrate needed by the mold for maximum production of aflatoxin, (2) cheese is commonly stored at tempera­tures below the minimum temperatures (11°-13°C) for aflatoxin produc­tion, and (3) other molds on cheese can easily outgrow the aspergilli, which often do poorly in a competitive environment. However, care must be exercised in handling cheese to prevent development of conditions which allow growth of the toxigenic aspergilli.

6. Amines:

Ingestion of foods containing tyramine together with drugs of the mono­amine oxidase inhibitor (MAOI) type can lead to hypertension attacks in some persons. Drugs of the MAOI type include iproniazid, nialamide, tranylcypromine, tranylcypromine sulfate, isocarboxazid, and phenelzine sulfate. Some of these continue to be used as antidepressants.

Tyramine acts by releasing norepinephrine from tissue stores, which in turn causes an increase in blood pressure. Tyramine has 1/20 to 1/50 of the ability of epinephrine to increase blood pressure. MAOI- type drugs increase the tissue stores of norepinephrine and thus potentiate the action of tyramine. Symptoms of a hypertensive crisis prompted by tyramine include high blood pressure, headache, fever, and sometimes perspiration and vomiting.

Histamine has been implicated in several outbreaks of food poisoning, including at least one that resulted from eating 2-year-old Gouda cheese. Symptoms of histamine poisoning include nausea, vomiting, facial flushing, intense headache, epigastric pain, burning sensa­tion in the throat, dysphagia, thirst, swelling of the lips, and urticarial.

Tyramine and histamine are commonly found in fermented foods, includ­ing cheese. Some lactic acid bacteria, as well as other bacteria likely to be in cheese, possess the enzymes needed to decarboxylate tyrosine or histidine and form tyramine and histamine, respectively. This could account for the presence of these compounds in cheese.