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Cheese Production

( Originally Published 1939 )



TECHNOLOGY

Milk production. The cheese-manufacturing industry was formerly centered in the states of Pennsylvania and New York, but the in-creasing demands of the large metropolitan population for bottled milk have raised the price of milk beyond the level where the cheese manufacturer in this part of the country can compete with those who secure their milk from the cheaper supplies in more remote areas. This price factor is forcing cheese production into the northern central states especially `Wisconsin, the Pacific Coast, and more recently, the southeastern and southwestern states.

Good cheese cannot be made from dirty milk. Manurial contamination or other sediment renders milk unfit for cheese making. Contamination from dirty utensils, cows' udders, and unclean cans con-tributes to the undesirable bacterial population. Milk containing off-flavors from feed, dirty stables, or microbic growth must be excluded. The containers and utensils of all kinds that are used in connection with milking and the transportation of milk, as well as those used in the manufacture and storage of cheese, should be of such construction as to permit thorough cleansing. These utensils and the stored milk should be protected against the entry of dust, dirt, flies and other insects, rats, mice and other animals, and any foreign material. Cleansing should immediately follow the emptying of the container or the use of the equipment, and the empty vessels should be protected from contamination when idle. The milk should be immediately cooled and kept cool. When but one delivery per day is made, night and morning milk should be kept in separate containers. Milk from cows with mastitis should not be shipped because, aside from health considerations, cheese made from this kind of milk is weak in body, low in fat, and high in moisture, and its manufacture entails high losses in butterfat.

The type of microorganisms present may ruin a cheese fermentation. Hucker's work at the New York State Agricultural Experiment Station showed that the production of Cheddar cheese of proper quality was related not so much to the total numbers of bacteria as to the specific types of bacteria present. Marquardt and Hucker showed that milk of low bacteria count, produced from farms where good sanitary conditions prevailed and where adequate cooling facilities were utilized, made the best cheese.

Milk is inspected on the plant receiving platforms, and examined for flavor, freedom from sediment, and absence of the particular type of bacterial flora that causes a gassy fermentation in cheese. Combs and associates 2 showed that the clarification of milk may raise the score of the cheese more than 2 points.

Procurement. Although the large cheese-manufacturing companies have their own milk-receiving stations and cheese factories, they actually purchase large amounts of cheese from outside sources. Scattered throughout the cheese-producing territory, there are many small plants which utilize the milk that is available in the immediate vicinity. They may sell their entire output of cheese to whomsoever they can. Many of these businesses are too small to avail themselves of the most efficient machinery or the latest information concerning the best methods, particularly sanitary control. They are too numerous, scattered, and remote from local health control centers to be very highly sensitized as to what is the significance of sanitary regulation. Most of the work of educating these necessary accessory factors in the cheese industry has hitherto fallen on the larger cheese manufacturers who purchase their production. Since the inauguration of the clean milk and dairy products program by the U. S. Food and Drug Administration, the various states are now beginning to adopt quality improvement campaigns to educate the producer and manufacturer to make better cheese.

Manufacture. In general, two very characteristic types of cheese constitute the bulk of the industry, namely, one produced by treatment with rennet and pepsin, and the other produced by the action of lactic acid. Rennin and pepsin are enzymes which are normal constituents of the stomach. The commercial products are prepared from the digestive stomachs of calves, sheep, and pigs, and the mixture is called rennet. When milk is treated with such a preparation, the casein coagulates and forms a thickened liverish mass. This property is imparted by the formation of small particles of insoluble or coagulated casein. When these are gathered together, they form a semi-solid curd. The soluble constituents of the milk, consisting of some of the uncoagulated protein, the lactose, the soluble minerals, and the water-soluble vitamins, constitute the whey. The different varieties of cheese are determined by the composition and kind of milk used (cow, sheep, goat), the degree and type of acidification, the amount of moisture left in the curd, and the temperature and other conditions of subsequent enzymic or microbiological action on the curd, called ripening.

Rennet cheese may be made into hard or soft types, depending on the amount of moisture left in the curd, the kind of microorganism used to impart the characteristic flavor, and the conditions of ripening. Typical hard cheeses are Cheddar and Swiss. The soft types are rep-resented by Neufchâtel and Camembert. Intermediate types are brick and Limburger. Some types are ripened by bacterial action, as Cheddar and Swiss, others by molds, as Roquefort; and others by both mold and bacteria, as Camembert.

The lactic acid method is based on the property of acids of precipitating or coagulating casein, generally familiar in the natural souring of milk. The milk is inoculated with a certain culture of acid-producing bacteria called a starter, and then allowed to stand during an incubation period until the proper degree of acid formation has produced a curd of satisfactory property. This curd is chemically very much like the curd from rennet coagulation, but the flavor is different. Examples of this process are cottage and cream cheese.

The methods common in this country for the manufacture of the most popular kinds of cheese are illustrated in the main by the procedure followed in the manufacture of Cheddar, cream, and cottage cheeses. The process for making cottage cheese is very similar to the general procedure of cream-cheese manufacture but is given separate treatment because cottage cheese is made in many milk distribution plants and constitutes one of the items which a health officer en-counters in his supervision of milk-plant operations. More detailed descriptions of' the manufacture of the different cheeses are given by Doane and Lawson and by Thom and Fisk.

Fresh clean milk is run into large vats, each capable of holding 1000 gallons. It is warmed to about 87° F. by a hot-water jacket built into the walls and bottom of the vat. The pure culture of acid-producing bacteria is added to impart the correct acidity to the milk (about 0.17 percent) so that the rennet will react properly, and also to overgrow and counteract any putrefactive types of organisms which may have been originally present in the milk. Great care is necessary to control this acidity in order to insure proper rennet coagulation and curd texture. If the cheese is to be colored, the vegetable dye annatto is added at this point, and thoroughly stirred in. A solution of commercial rennet is now added, and again the batch is thoroughly agitated. The milk is left undisturbed so that the mass will coagulate to a smooth, properly textured product. In about half an hour or so, the mass "sets" to form a smooth clabber simulating junket. Operators then cut this curd into small cubes by drawing special curd knives through the batch, first in one direction, and then again crosswise. The purpose of this treatment is to facilitate uniform drainage of the whey from the curd. The small diced cubes of curd are now gently stirred, and the batch is slowly and uniformly heated to about 100° F. in order to facilitate the removal of the whey from the curd. The heating toughens the cubes and squeezes out some of the whey. The conditions under which this draining is effected constitute one of the factors which distinguish different types of cheese.

When the whey has drained off, the cheese curd is left in the bottom of the vat. Channels are made by "ditching" the curd, to facilitate drainage.

The characteristic treatment given to renneted milk in the manufacture of Cheddar cheese is the procedure of "cheddaring." After the ditched, cooked cubes of curd have been allowed to stand for about 15 minutes or until the whey has almost completely drained off, the curd is cut into large portions. Those are turned over from time to time, and piled on top of one another to press out more residual whey. During this treatment the temperature should not be allowed to fall below about 85°-90° F.

The cheddars are now ready to be milled in order to incorporate the salt thoroughly and to prepare them for subsequent treatment. Salt imparts proper seasoning, hardens the curd, and controls fermentation during the ensuing ripening treatment. Although the amount of salt varies, it is usual to add about 2 pounds of salt to the curd from each 1000 pounds of milk (yielding about 100 pounds of cheese).

The cheese is then placed in the hoops for pressing. The hoop is a circular, metallic band or cylinder for molding the cheese into cakes of the proper size. The hoop is lined with cheesecloth which remains on the cheese when it is marketed, comprising the outside of the rind. After about 18 hours in the press, the cheese is placed on shelves in the curing room. These rooms are kept at a uniform temperature of about 55°-60° F. The ripening process develops the final desired flavor and imparts the proper soft and mellow texture to the body of the cheese.

Pasteurization. The cheese industry has been slow to recognize the advantages of pasteurization of the milk used. Inasmuch as there are no official requirements, the progress can be made in this direction only as the cheese industry itself voluntarily introduces this valuable practice. The agricultural experiment stations and the government laboratories have done excellent research in this field, and their results are being increasingly applied. Just as with bottled milk, commercial expediency was the prime incentive for the introduction of pasteurization in the cheese industry. Increasingly longer hauls of the milk, the operation of larger factories, the demand for a uniform product, and increased scientific control of the actual cheese-making operations all have combined to convince the industry that, by modification of the age-old rule-of-thumb practice, it is possible to make excellent cheese from pasteurized milk.

It is generally recognized that the pasteurization of milk entails some changes in the mineral balance and in the colloidal condition or the character of the proteins, affects the enzymes to a greater or less degree, greatly changes the numbers and dominant types of the microbial flora, and in some cheeses causes too great a retention of water. Hochstrasser and Price report that several investigators have shown that cheese of superior quality can be manufactured from pasteurized milk when certain more or less minor changes are introduced into the operations. Phillips naïvely states 6 that, in his (large) state, cheese is the only dairy product which can be manufactured from milk produced from herds infected with tuberculosis without pasteurization of the milk, but proceeds to show that, when the milk is flash-pasteurized at 160°-168° F., the cheese is better and more uniform in quality and there is less loss of butterfat in the whey than when the milk is not heated. Price and Prickett showed that pasteurization gave cheese of higher scores and greater yields by 2.5 to 4 percent than identical milk not pasteurized. Sherwood states that, in New Zealand, pasteurized milk is commonly used for the manufacture of Cheddar cheese to produce a cheese of greater uniformity, although it ripens more slowly. Many cheese authorities hold that cheese made from pasteurized milk does not develop the fine flavor of that made from unpasteurized milk, especially Swiss cheese. Inasmuch as both the milk and the butter industries in the early days of pasteurization experienced these differences of opinion as to the effect of pasteurization on the quality of the finished product, it may be expected that cheese makers will find ways and means to overcome their difficulties.

Process cheese. A very great impetus to improved cheese sanitation has been the development of process cheese. This process was introduced to eliminate the waste of the rind, to give a perfectly pasteurized product, and to package it for sale in a sanitary manner. Cheeses from many plants are graded. The rinds and bandages are then removed, and batches are made up by blending the different cheeses to produce a product of uniform composition and flavor. These cheeses are then ground and heat-pasteurized in automatically con-trolled pasteurizers. In order to facilitate blending and to secure a smooth texture, about 3 percent of emulsifying agents is added to the batch. These may be finely divided sodium citrate or phosphate. Their use insures not only a smooth product but also an increased content of moisture. Process cheese is now made from Cheddar, Swiss, Limburger, and other standard types.

Wrapper. The wrapper on process cheese prevents the access of mold spores, retains the moisture, and serves as a sales package. When the cheese is wrapped in tinfoil, the adsorption of tin may be pre-vented by coating the foil on the cheese side with a thin layer of shellac. The cause of the darkening of foil is not clearly understood, but it is not believed to be lead because of its small amount in foil.

Canned cheese. Canned raw cheese has recently been developed in the U. S. Department of Agriculture. Cheddar cheese, made in the regular way from high-quality materials, is pressed in hoops to fit the cans, cut into 12-ounce sizes, loosely wrapped, and sealed in the cans. The lid has a venting valve which permits the escape of gases but prevents the entrance of air, thereby precluding the development of mold. There is no rind wastage on this cheese.

CREAM CHEESE

Cream cheese is a distinctly American development. Fresh milk is standardized to a butterfat content of about 14 percent by the addition of proper amounts of fresh cream. The batch is then pasteurized in large vats, cooled and homogenized at 3000 to 4000 pounds pressure, according to the so-called Geneva process, then inoculated with a, lactic starter, and incubated at about 70° F. for about 18 hours to an acidity of 0.37 percent. These cheeses can be made without starter by adding finely comminuted Roquefort or Cheddar cheese to the standardized cream before pasteurization and homogenization. Other flavors such as sweet pickle relish, olives, and nuts can be added to the cream. The cream-cheese curd is then poured onto sterilized, closely woven cheesecloth to drain off the whey. After several hours, the corners are drawn together and tied at the top, forming bags. These are pressed between pieces of ice overnight, and then pressed, salted, and worked the next day. Some brands are wrapped in foil by automatic packaging machinery. This cheese is not cured, and is very perishable. It must be kept under refrigeration and marketed while fresh.

COTTAGE CHEESE

Cottage cheese is made in many milk distribution plants from the surplus milk which is not sold bottled. This milk should be pasteurized at 142°-143° F. for 30 minutes and set with about 5 percent starter at 90° F. to develop an acidity of about 0.5-0.6 percent as lactic acid. In 4-5 hours the batch will have thickened and will be ready for cutting. If set at 70° F., it incubates overnight or longer. The curd is cut into small cubes of about 1/2 inch with curd knives. The temperature is slowly and carefully raised to 100°-120° F. over a period of 1-2 hours by adding hot water to the batch, or by means of a steam-hot-water jacket, with careful stirring to avoid breaking the curd cubes. After a brief cooking period, the whey is mostly drained off, and some cold water is added to bring the temperature down to 60° F. After complete draining, the curd is trenched by pushing it away from a channel down the center of the tank. Salt is shaken in at the rate of 1 pound for every 100 pounds of curd. About 151/2 pounds of curd are formed from 100 pounds of skim milk. Usually the cottage cheese is enriched by the addition of some cream. This cheese is not ripened and must be kept refrigerated and sold fresh. It is subject to spoil-age by molds which destroy acidity and thereby open the way for the action of putrefactive bacteria.

CHEESE SPREADS

Cheese spreads or cheese food compounds are a class of products which contain cheese in greater or less amount, often with the addition of different kinds of relish such as olives, pickles, pimentos, some-times with the addition of cream, condensed whey, whey powder, or skimmed milk powder. The product has a consistency which allows it to be spread easily on bread. The moisture or fat content must be high enough to give this desired body. In general, these products are made by adding to the ground cheese in the kettle whatever type of relish is desired, or some dairy product such as those mentioned above to give the desired consistency. The mixture is then heated .to about 160°-180° F. and treated with sodium citrate, di-sodium phosphate or Rochelle Salt, and possibly some ordinary salt. The hot mixture may be rim directly into the tinfoil-lined containers. There are no food standards nor uniform procedure of rating the quality of these products. However, large quantities are now marketed. Templeton and Sommer 12 discuss the properties of these products and give analyses of commercial samples.

RELATION TO THE PUBLIC HEALTH

Composition. It has been pointed out that there are about 18 distinct varieties of cheese to which have been given about 400 names, mostly of local origin. The U. S. Department of Agriculture has published 3 a list of more than 100 different kinds of cheese with descriptions of their production and composition. The proximate analyses of some of the cheeses listed in the official standards above are summarized in Table IX.

TABLE IX

COMPOSITION OF CHEESE (percentage)

Name Water Fat Protein Ash

Cheddar 35.56 34.48 25.80 4.20

Limburger, American. 40.99 29.04 25.08 4.97

Imported 54.79 19.61 21.27 5.17

Camembert 47.88 26.32 22.21 4.11

Cottage 69.82 1.03 23.26 1.91

Emmenthaler 33.00 30.50 30.44 4.17

Neufchâtel 52.05 23.51 19.33 4.97

Roquefort 38.69 32.31 21.39 6.14

Cream23 38-43 43-48 13-16 ....

Digestibility. There is a persistent popular belief that cheese is less readily digested than most other foods and that it is a cause of gastrointestinal disturbances. Langworthy and Hunt" report that many experiments were conducted by feeding healthy young men on a diet of bread, fruit, and American factory, full-cream cheese made with different amounts of rennet and with different stages of ripening. About 90 percent of the protein was assimilated, about 90 percent of the available energy was utilized, and there were no physiological disturbances or constipation. The cheese was thoroughly digested, and there were no important differences between Cheddar cheeses of different stages of ripeness. Similar results were obtained for the digestibility of skimmed-milk cheese, Swiss cheese, Roquefort cheese, and cottage cheese. These authors further stated that experiments with the respiration calorimeter did not indicate that cream cheese differed materially in ease of digestion from a comparable amount of meat.

It is pointed out that cheese protein is digested in the intestines rather than in the stomach, and that this has led to its reputation as a "hearty" food. Moreover, cheese protein is intimately associated with fat, and there is evidence that this condition has an effect upon the progress of digestion in the stomach. In addition, cheese that has been overheated in cooking may have undergone some near-burning of the ingredients with attendant formation of irritating substances. All these "distress" conditions can be remedied by the proper use and treatment of cheese. It should be masticated thoroughly so as to effect intimate admixture of the food and the digestive juices. It should not be eaten between meals or on an empty stomach or in large amount at the end of a hearty meal. When used in cooking, it should be grated, flaked, or shaven, distributed through the other ingredients of the cheese dish, and cooked at a very moderate temperature. Under these conditions the cheese has no opportunity to form the leathery mass which is so slow to digest. Some people have found from experience that cheese spreads may be eaten by children two years old with no harmful effects.

The calcium of American Cheddar cheese is utilized to about the same extent as the calcium of pasteurized milk.

Nutritional value. Cheese has been a staple article of food among the races of the world as far back as records are available. It contains more calorific value than an equal weight of most other food products. Its protein is completely adequate for all nutritive needs because of its content of all the essential amino acids. Quantitatively its percentage of protein is greater than that of meat and about twice that of eggs.

Cheese has value as an appetizer by virtue of its characteristic flavor. This flavor is imparted by the products formed during the culturing, ripening, and salting of the curd. The action of the microorganisms resulting in these products is not to be considered as a de-composition in the sense that usually is associated with that word. It is true that the proteins are broken down, sometimes all the way down to ammonia. The distinction lies in the fact that ordinary decomposition or putrefaction is an uncontrolled microbiological process whose agents may be numerous kinds of organisms which may produce desirable, indifferent, or harmful end-products. Cheese making requires the use of pure cultures of known microorganisms whose metabolism has produced the different cheeses found by the accumulated experience of the ages to be nutritious for the body and appealing to the appetite.

Epidemiology. Typhoid fever. An epidemic of typhoid fever involving 51 cases and 4 deaths was reported by Rich 15 to have been traced to cheese made from milk on a farm where a typhoid carrier was employed. This was a green cheese only 3 to 12 days old at the time of shipment, and probably from 9 to 30 days old at the time of retail. It was described as a full-cream cheese of short cure, pressed into cakes. An outbreak involving 39 cases at Lansing was traced to cheese from the same manufacturer. The incubation period seems to have been 14 to 37 days.

Wade and Shere carefully studied a typhoid outbreak of 29 cases and 4 deaths. The epidemiological data showed that Eberthella typhosa had lived in the cheese approximately 63 days. These investigators studied the longevity of typhoid organisms in cheese by inoculating milk and examining the cured Cheddar cheeses therefrom regularly until negative to these bacteria. The typhoid germs in 16 of these cheeses could not be recovered after the eighth day, but in 2 cheeses they were recovered after 34 and 36 days, respectively. The complete utilization of these two cheeses in the sampling precluded the possibility of ascertaining what the longevity of the organisms would have been. The different organic acids which develop in the curing were found to have greatly different bactericidal effectiveness.

Graham-Stewart and associates report an outbreak of 23 cases of mild infection among boys in a preparatory school, traced to an Italian cream cheese. No carrier among the school personnel could be found. Agglutination tests of the blood serum showed definitely that Salmonella schottmülleri was the etiological factor.

An extensive outbreak of typhoid fever in Canada was recently traced to fresh cheese 18 There were 627 cases of typhoid fever and 57 deaths. All the cases presented a common date of onset, and were distributed in 16 different municipalities. Carriers, and such vehicles as water, milk, butter, ice cream, oysters, and shellfish, were all eliminated. It was established that the single food commonly consumed by all the cases was a "fresh, sweet Canadian cheese manufactured in a municipality of the district." The manufacturer of the infected cheese had used the milk of several producers in whose families there had been, or still were, cases of typhoid fever.

Gastroenteritis. Macauley states that, from records available involving many hundreds of cases, he has been unable to find conclusive evidence of any fatal case in any outbreak which incriminated cheese, whereas, in other food-poisoning outbreaks, the case mortality rate was about 1.5 percent. He distinguishes a food poisoning caused by a bacterial infection from an intoxication caused by bacterial toxins by the latter's suddenness of onset, briefness of incubation period, and susceptibility of recovered patients to an immediate repetition of the illness. He describes an epidemic affecting 126 persons out of 135 in 43 families. The onset of the symptoms came in 31/2 to 11 hours, the majority being in 7 to 8 hours. These individuals did not know that the cheese was the incriminated agent, and after they recovered ate more of the same cheese and became ill again. The cheese was a red Canadian Cheddar. Its appearance, taste, and smell were normal except that it had a few small spherical holes like bubbles which are absent from other cheeses of the same batch. No metals or tyrotoxicon (see below) were found. This outbreak was attributed to toxin poisoning from a Salmonella-group infection.

Savage and White 2° report at length on their studies on cheese poisoning. They found 8 out of a total of 100 food-poisoning out-breaks. They were all characterized by an onset of symptoms within 4 to 12 hours. These consisted of abdominal pain, sickness, vomiting, and sometimes diarrhea, with more or less prostration. There were no deaths and sometimes not even any medical assistance. In one outbreak, the cheese tasted bitter, but in all the others it seemed to be perfectly good and wholesome. The cheeses were Gorgonzola, Cheddar, and a Wensleydale, none of which were of the kind which ripen by a secondary change of a decomposing type. All the cheeses were examined for metals and the compound tyrotoxicon with negative results.

In order to ascertain to what extent the manufacturing and ripening procedure may destroy any harmful organisms, Savage and White inoculated two batches of unpasteurized milk with cultures of S. aertrycke and S. enteritidis, respectively—types of the Salmonella which have been incriminated in many food-poisoning outbreaks. The cheeses were made by a practical cheese maker and had no abnormal properties. The cheese was fed to mice with negative results. Even after subcutaneous inoculation with broth from the first cheese 10 days after manufacture, no aertrycke were recovered. They found that aertrycke lived for 24 days but not for 30 days, and that enteritidis lived for 29 days but not for 34 days. They concluded that Salmonella strains will survive for several weeks but that animal experiments fail to show evidence of toxicity.

Among the several outbreaks studied by them, they report one which is of particular interest to all food-control officials because of the light it throws on a puzzling aspect of cheese epidemiology. A locally made cheese known as Wensleydale was supplied to some 6 or 8 families. It was known that 10 or more persons ate it without harm but 9 other persons became ill with the typical symptoms. The severity corresponded roughly in proportion to the amount eaten. The cheese was soft and friable and did not seem to be abnormal in any way. Numerous streptococci and "a few B. coli" were present, but no living Salmonella organisms and no non-lactose fermenting organ-ism of any type were isolated. Two separate samples were examined. Two mice were fed, and a rabbit was injected intraperitoneally, but they were not affected. No tyrotoxicon was found. Four samples of blood from patients all yielded positive agglutination with various Salmonella strains. S. suipestifer was clearly demonstrated. These investigations indicated that the outbreak was caused by the undestroyed toxins of a S. suipestifer strain which had died out in the maturing cheese. Particularly significant was the finding that only a part of this cheese was poisonous, the other portion of the same cheese being wholesome.

S. aertrycke has been incriminated in two outbreaks of 39 and 26 cases respectively from the consumption of soft cheese 21 The symptoms were sudden fever, headache, vomiting, diarrhea, pains in abdomen and back, and general malaise for several days. The epidemic was attributed to a previous epidemic of diarrhea in calves from which a strain of S. aertrycke was isolated, identical with that isolated from the cheese.

Tanner 22 reports an outbreak of 40 cases of acute gastroenteritis among two ship crews which ate the same cheese. No bacteriological investigation was made. The cheese appeared to be normal, although it had a strong flavor and crumbly body. In another outbreak re-ported on shipboard a cheese caused illness in all the messes which ate it; there was no illness in the mess which did not eat it. Bacteriological examination of the cheese failed to disclose any paratyphoidenteritidis (Salmonella) organisms. These cases are particularly interesting because they are representative of many such outbreaks that are never reported in the literature but are frequently encountered in public-health practice. The symptoms are explosive in onset, develop within a few hours, and are characterized by nausea, abdominal pain, fast pulse, diarrhea, and more or less prostration. Recovery is usually complete in about 24 hours. Seldom are any causal organisms found, and the cheese seems to be wholesome in appearance, taste, and flavor.

Cottage cheese, containing some unpasteurized milk and made under conditions allowing further contamination, caused the illness of 37 persons with typical symptoms of staphylococcus poisoning within a few hours after its consumption. No person was discovered who had been ill with these symptoms who had not eaten this cottage cheese. Staphylococcus albus was isolated from two specimens of the cheese. Another outbreak also was traced to a "quarter skimmed cheese" found to contain Staphylococcus aureus which showed hemolysis on blood agar, and wide zone on Stone's gelatin media.

Linden, Turner, and Thom 24 report an epidemic of gastroenteritis, involving 22 cases, traced to American Cheddar cheese, and an outbreak of 9 cases from an imported Albanian cheese. They re-mark that "such outbreaks are exceedingly frequent."

Tuberculosis. The presence of Mycobacterium tuberculosis in cheese is discussed at length by Tanner." He reports that several investigators found that cottage cheese and other soft cheeses, purchased in the open market and injected into guinea pigs, produced tuberculosis.

Mohler, Washburn, and Doane 26 quote previous work to show that viable tuberculosis organisms were found in cottage and other soft cheese from 14 days to more than 2 months, in Emmenthaler cheese up to about the fortieth day, and in Cheddar after 104 days. Their own experiments consisted in inoculating milk with M. tuberculosis, setting it with rennet at 88° F., cooking the cut curd at 100° F., drawing off the whey at 0.18 percent acidity, mellowing on rack, milling at acidity of 0.6 percent, salting, pressing, curing at 65° F. for 7 days, and storing at 33° F. The results showed that the organisms were very irregularly distributed. When the cheese was 23 days old, it was cut up, emulsified in physiological salt solution, and inoculated into guinea pigs. Each one of the 14 lots was similarly examined at successive intervals. Samples were also fed. Only 5 animals developed tuberculosis from the feeding tests, whereas 32 animals became positive from the inoculation. It happened that the sample which was 200 days old imparted generalized tuberculosis by both feeding and inoculation, and samples that were 261 days old were slightly infective. These authors quote previous work to show that cottage cheese bought in the open market was infected with tuberculosis. They conclude that cheese from raw, unpasteurized milk should be considered a possible carrier of tubercle bacilli.

The infectivity of cheese by tubercle bacilli was studied by Harrison, who, while in Europe, inoculated milk with these organ-isms, manufactured it into Swiss Emmenthaler and American Cheddar types, and inoculated samples into guinea pigs. The organisms were viable in the Swiss cheese on the twenty-eighth day but negative on the thirty-third day. In the Cheddar, they were positive on the one hundred and fourth day and negative from the one hundred and fourth to one hundred and thirty-second days. Three out of 5 samples of cream cheese bought on the Berne market were infective. He repeated his work by making a Cheddar according to American methods. When the cheese was 1 month old, the acidity of the water ex-tract was 0.95 percent that of lactic acid. The cheese was infective by subcutaneous injection when 62 days old but was negative from the seventieth to the one hundred and twelfth days.

The extent to which cheese on the open market was infective with tuberculosis was studied by Schroeder and Brett 28 All the samples were purchased under ordinary market conditions from retail dealers in Washington, D. C. The findings were as follows:

CHEESE SAMPLES INFECTIVE SAMPLES

Cheddar 59 0

Miscellaneous varieties.... 3 0

Neufchâtel 32 0

Cottage 31 1

Cream 131 18

Nine cream cheese samples marked "pasteurized" and all the long-ripened cheeses were negative. They conclude that although cottage and Neufchâtel cheese are much less affected than cream cheese, all varieties of fresh cheese should be pasteurized. Practically all cream and Neufchâtel cheese are now made from pasteurized milk. The rapid elimination of tuberculous cattle from dairy herds is reducing this health hazard.

Other infections. Carpenter and Boak 29 injected guinea pigs with saline suspensions of extracts from 82 samples of cheese, of which 72 were imported, and found no infections from Brucella abortus or Brucella melitensis in any case.

Much of the earlier work on cheese poisoning is reviewed by Levin, who points out that deaths through cheese poisoning make up a very small percentage of the total number of cheese poisoning cases. He reports that Lochte tabulated 11 fatal cases from cheese poisoning up to 1903, and that Berg mentions a fatal case occurring in 1908. Lewis isolated a bacillus of the colon group from a cheese which caused illness in 6 persons. The organism did not form tyrotoxicon but did form a thermostabile toxin. Poisoning could not be induced in animals by feeding with cultures of this organism. An incubation period of 6 hours indicated that the outbreak was caused by an infection, not an intoxication.

In order to ascertain the effect of the cheese manufacturing and ripening treatment on an infection of Streptococcus pyogenes, Hucker 31 inoculated a batch of milk with a culture of this organism which had been isolated from milk incriminated in an outbreak of septic sore throat, and made some Cheddar cheese by the usual process. In a batch cured at 40° F., the organisms had greatly increased in number at the one hundred and sixtieth day but in a batch cured at 60° F. only a relatively few organisms remained after 85 days. More-over, he isolated a streptococcus of the human type from cheese made commercially in a factory supplied with milk which had been suspected of having caused a septic-sore-throat outbreak.

Botulism has been caused by cheese. An outbreak involving 3 deaths was traced to some home-made cottage cheese from which Clostridium botulinum (type B) was isolated. Nevin made a protective serum for rabbits against the homologous toxin. Subcutaneous inoculation of 3 milliliters of the emulsion after incubation at 37° C. for 48 hours killed guinea pigs in 36 hours. She also inoculated fresh sterilized and unsterilized cottage cheese with 1 loopful of a 48-hour broth culture and injected 1-milliliter portions into guinea pigs with fatal results, whereas controls of market cheese were negative.

Ptomaines. Pursuant to an outbreak of food poisoning traced to cheese, Vaughan in 1883-1884 made an extensive investigation into the causes. This resulted in his isolation of a crystalline product from the incriminated cheese. He called this tyrotoxicon. Several out-breaks of illness traced to cheese have been attributed to this product. The epidemiology often has incriminated the cheese, but the laboratory examinations have usually been unsuccessful in isolating or identifying any toxic substance. Direct examination for tyrotoxicon is usu-

CONTROL MEASURES 199 ally negative. Tanner presents a good review of this subject. The early work of Vaughan is the basis for much of the belief that gastroenteritis is caused by the eating of food which contains decomposition products called ptomaines. Unfortunately for this theory, no investigations of food poisoning since the earlier work have shown that any such products are involved. Savage and White point out 20 that there is no evidence that tyrotoxicon is associated with cheese poisoning as now encountered, and they raise doubts as to the dependability of the early work in this field. Tanner sums up the situation by stating that the microbiology of the problem has not received much study.

Infestation. Simmons 34 points out that the fly pest, Piophila casei (L.), known under numerous common names such as cheese skipper, cheese mite, cheese fly, and many others, may contribute to the spread of enteric diseases and other maladies. Although there is no direct experimental evidence at hand to indict this insect, its haunts in decomposing filth render it an object of suspicion. Cases are re-corded of the viable larvae passing through human intestines and causing serious lesions. This is the insect that is often found in the intestines of man in military camps. Infestation with this pest is an-other reason for using screens, with 30 meshes to the inch, to protect food from exposure to flies. Shrader examined the cottage cheese sold in the open markets of Baltimore, and showed that infestation could be demonstrated by incubating farm-made cottage cheese under cheesecloth covering, whereas that made in the screened milk plants from pasteurized milk did not develop any maggots when similarly treated.

CONTROL MEASURES

Definition. Cheese is defined 35 by the federal regulatory officials as follows:

Cheese. The product made from the separated curd obtained by coagulating the casein of milk, skimmed milk, or milk enriched with cream. The coagulation is accomplished by means of rennet or other suitable enzyme, lactic fermentation, or by a combination of the two. The curd may be modified by heat, pressure, ripening ferments, special molds, or suitable seasoning.

Certain varieties of cheese are made from the milk of animals other than the cow.

The name "cheese" unqualified means Cheddar cheese (American cheese, American Cheddar cheese).

Definitions or standards are given for several of the more common cheeses such as Cheddar, Limburger, brick, Stilton, Gouda, Neufchâtel, cream, Roquefort, Gorgonzola, Edam, Emmenthaler, Camembert, Brie, Parmesan, and cottage. Process and pasteurized cheese are de-fined as follows:

Pasteurized Cheese, Pasteurized-Blended Cheese. The pasteurized product made by comminuting and mixing, with the aid of heat and water, one or more lots of cheese into a homogeneous, plastic mass. The unqualified name "pasteurized cheese," "pasteurized-blended cheese," is understood to mean pasteurized Cheddar cheese, pasteurized-blended Cheddar cheese, and applies to a product which conforms to the standard for Cheddar cheese. Pasteurized cheese, pasteurized-blended cheese, bearing a varietal name is made from cheese of the variety indicated by the name and conforms to the limits for fat and moisture for cheese of that variety.

Process Cheese. The modified cheese made by comminuting and mixing one or more lots of cheese into a homogeneous, plastic mass, with the aid of heat, with or without the addition of water, and with the incorporation of not more than 3 percent of a suitable emulsifying agent. The name "process cheese" unqualified is understood to mean process Cheddar cheese, and applies to a product which contains not more than 40 percent of water and, in the water-free substance, not less than 50 percent of milk fat. Process cheese qualified by a varietal name is made from cheese of the variety indicated by the name, and conforms to the limits for fat and moisture for cheese of that variety.

Filled cheese. The Federal act of June 6, 1896,36 defines filled cheese as comprising all substances made of milk or skimmed milk, with the admixture of butter, animal oils or fats, vegetable or any other oils, or compounds foreign to such milk, and made in semblance or imitation of cheese. The manufacturer is required to pay a tax of 1 cent per pound to the Commissioner of Internal Revenue. Filled cheese must be packed in wooden packages bearing the words "filled cheese" in black-faced letters, not less than 2 inches in height, in a circle. Wholesale and retail dealers must display conspicuously a sign bearing the words "Filled cheese sold here" in black-faced letters not less than 6 inches in height.

Types of adulteration encountered. A cheese may be suspected of being a filled cheese if it is offered for sale at a particularly low price. It may be sold in most states if its composition is declared. Examination for adulteration of cheese should include a determination of the moisture and fat content in accordance with the standards of the U., S. Department of Agriculture. The federal officials have enforced mostly the requirement that 50 percent of the water-free substance of the cheese must be milkfat. Different states have special laws governing the labeling of cheese of specified composition.

Lack of proper sanitation has been revealed by the presence of various types of extraneous matter. The phosphatase test has been used to indicate whether cheese was made from properly pasteurized cream.

Examination for adulteration. Chemical tests. SAMPLING. The official method 37 provides that, for hard cheeses, a wedge-shaped segment shall be ground three times through a sausage machine. Soft cheese is made into a homogeneous mass by grinding or by stirring.

MOISTURE. A sample of about 2 or 3 grams is weighed into a flat-bottom dish and dried to constant weight in a vacuum oven at a temperature of boiling water.

ACIDITY. Water is added to a 10-gram, finely divided sample of cheese at 40° C. until the solution equals 105 milliliters, which is then shaken, filtered, and titrated against sodium hydroxide solution with phenolphthalein as indicator.

COLORS. Finely divided samples of about 25 grams to 50 grams of the cheese are extracted with ether. This is evaporated off, and the residue is treated as in the determination of color in milk (see page 179) .

FAT. A sample of about 2 to 5 grams is weighed onto a filter mat in an extraction tube, extracted with ether, evaporated at the temperature of boiling water, cooled, and weighed. An alternate official method is to rub up 1 gram of the cheese with 9 milliliters of water 1 milliliter of NH4OH in a narrow beaker. After digestion, it is neutralized with hydrochloric acid, transferred to a Röhrig tube, and treated as for milk (see page 94).

FOREIGN FAT. The presence of foreign fats in filled cheese may be detected by the physical constants of the separated fat. Hydrogenation may vitiate the significance of the iodine number, but the saponification number, the Polenske number (insoluble volatile acids), and the Reichert-Meissl number (total volatile acids) are not significantly affected by this treatment. For detailed analytical directions, see Mojonnier and Troy.

THICKENERS. Sometimes foreign gums and other thickening agents have been added to cheese to give an appearance of greater body and to mask too great a moisture content. Several methods are employed 38 to detect these products, using selective solubility and precipitation with group reagents.

Fat, moisture, and salt in hard cheese are determined by slight modifications of the foregoing official methods, by procedures published by the Subcommittee for the Analysis of Cheese of the American Dairy Science Association.

PASTEURIZATION. Cheese made from pasteurized milk will react negative to the phosphatase test. Scharer directs that 20 grams of the cheese be triturated with 20 milliliters of the buffer and 20 milli-liters of water, and that 1 milliliter of this be used for the regular test.

Microbiological examination. SAMPLING. Samples for microbiological examination are collected by means of a sterilized cheese trier, or by cutting a triangle from the center of the cheese with a sterile knife and trimming off the outer surfaces.

BACTERIOLOGICAL PLATE COUNTS. NO systematic and official method has been worked out for the bacteriological examination of cheese. Common practice consists in weighing out a 1-gram sample, emulsifying it in sterile water by means of a sterile mortar and pestle, and diluting up to 100 milliliters. The diluting and plating technic is similar to that for milk analysis (see page 94).

MICROSCOPIC EXAMINATION. Rucker has outlined a method for the direct examination of cheese by bedding a sample of the cheese and cutting sections with a microtone. These are stained and examined in standardized fields under an oil-immersion lens of a high-power microscope.

EXTRANEOUS MATERIAL. Sediment in cheese can be determined by filtering the prepared sample through a filter-paper disc and examining it under a magnifying glass.

Control procedure. Inasmuch as the cheese industry is not under the control of regulatory officials through the operation of any permit system, and particularly since the cheese is manufactured in country remote from the metropolitan consuming centers, it is impossible for the local food-control officer to enforce the generally recognized provisions that would insure the production of a clean, safe product. He must leave the inspection of the sources of the milk sup-ply and the manufacturing operations to the manufacturing companies themselves. His inspection activities are restricted entirely to checking the conditions under which cheese is handled in his own particular district, and in collecting samples for organoleptic and laboratory examination.

In view of the economic and competitive necessity of producing a quality product, increasing care is being taken by the manufacturers to pasteurize and package a safe, clean product. Sometimes the cheese is not given as good care as possible in the retailer's establishment. Most cheese will spoil if not kept under refrigeration or not protected from fly contamination. The health officer should insist that the retailer have adequate facilities for handling cheese.

If a reported outbreak of illness is charged to the consumption of cheese, the food official should make an immediate investigation to work out the epidemiology and to collect samples of all the food. Usually illness is charged to cheese with little or no evidence that it is actually to blame, thereby failing to locate the true cause, and leaving the door open, so to speak, for a fresh outbreak from the same unrecognized source,

REFERENCES

1. J. C. MARQUARDT and G. J. HUCKER, N. Y. State Agr. Exp. Sta. But. 534, 1926.

2. W. B. Comm, W. H. MARTIN, and N. A. HUGGLAR, J. Dairy Sci., 7, 524 (1924).

3. C. F. DoANE and H. W. LAWSON, U. S. Dept. Agr. Bul. 608, 1932.

4. C. THOM and W. W. Fisx, The Book of Cheese, Macmillan Co., New York, 1918, p. 108.

5. W. HOCHSTRASSER and W. V. PRICE, J. Dairy Sci., 10, 448 (1927).

6. C. A. PHILLIPS, ibid., 11, 292 (1928).

7. W. V. PRICE and P. S. PRICKETT, ibid., 11, 69 (1928).

8. I. R. SHERWOOD, J. Dairy Research, 7, 271 (1936).

9. H. L. TEMPLETON and H. H. SOMMER, J. Dairy Sci., 20, 231 (1937).

10. H. L. WILSON, U. S. Dept. Agr. Circular 352, 1935.

11. A. C. DAHLBERG and J. C. MARQUARDT, N. Y. State Agr. Exp. Sta. Bul. 226, 1934.

12. H. L. TEMPLETON and H. H. SOMMER, J. Dairy Sci., 15, 155 (1932); 17, 373 (1934).

13. C. F. LANGWORTHY and C. L. HUNT, U. S. Dept. Agr. Farmer's Bud. 487, 1912. See also Bureau of Animal Industry Circular 166.

14. M. G. MALLON, L. M. JOHNSON, and C. R. DARBY, J. Nutrition, 5, 121 (1932).

15. E. D. RICH, Am. J. Pub. Health, 13, 210 (1923).

16. E. M. WADE and L. SHEER, ibid., 18, 1480 (1928).

17. A. GRAHAM-STEWART and associates, Brit. Med. J., I, 934 (1928).

18. A. R. FOLEY, 11th Ann. Rept. Prov. Bur. Health 1932-3, p. 25, Montreal, Canada.

19. H. M. C. MACAULEY, Lancet, II, 1012 (1922).

20. W. G. SAVAGE and P. B. WHITE, Med. Research Council, Spec. Rept. Series 92, '1925, p. 90.

21. F. W. TANNER, Food-borne Infections and Intoxications, Twin City Printing Co., Champaign, Ill., 1933, p. 286.

22. Ibid., p. 251.

23. Sanitation Section, U. S. Pub. Health Service, 1937.

24. B. A. LINDEN, W. R. TURNER, and C. THOM, Pub. Health Repts., 41, 1647 (1926).

25. Reference 21, p. 286.

26. J. R. MOHLER, H. J. WASHBURN, and C. F. DOANE, 26th Ann. Rept. Bur. Animal Industry, 1909, p. 187.

27. F. C. HARRISON, 19th Ann. Rept. Bur. Animal Industry, 1902, p. 217.

28. S. C. SCHROEDER and G. W. BRETT, J. Am. Vet. Med. Assoc., 52, 674 (1917).

29. C. M. CARPENTER and R. SOAK, Am. J. Pub. Health, 18, 743 (1928).

30. W. LEVIN, J. Lab. Clin. Med., 2, 761 (1916-17).

31. G. J. HUCKER and J. C. MARQUARDT, 10th Ann. Rept. N. Y. State Assoc. Dairy and Milk Inspectors, 1936, p. 171.

32. M. NEVIN, J. Infectious Diseases, 28, 226 (1921).

33. Reference 21, p. 318.

34. P. SIMMONS, U. S. Dept. Agr. Department Bul. 1453, p. 7, 1927.

35. Service and Regulatory Announcements, Food and Drug 2, fifth revision, November, 1936.

36. "Filled Cheese," U. S. Code, Title 26, Sec. 1000 et seq., June 6, 1896.

37. Methods of Analysis, Assoc. Official Agr. Chemists, 4th ed., 1935.

38. T. MoaoNNIER and H. C. TROY, The Technical Control of Dairy Products,

Mojonnier Bros. Co., Chicago, 1925, p. 637.

39. "Subcommittee for the Analysis of Cheese," J. Dairy Sci., 20, 27 (1937).

40. H. SCHARER, J. Milk Technol., 2, 16 (1939).

41. G. J. HUCKER, J. Agr. Research, 22, 93 (1921), quoted from The Microbiology of Foods, by F. W. TANNER, Twin City Printing Co., Champaign, Ill., 1932, p. 253.

42. W. S. GREENE, Mimeograph release from the Microanalytical Division, U. S. Food and Drug Administration, Nov. 29, 1935.

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