Bulk Raw Milk Research Articles

Effects of chromatographically pure bovine chymosin and pepsin A on cheese curd firmness

SummaryThe effects of chromatographically pure bovine chymosin and pepsin A on cheese curd firmness have been investigated. The enzymes were studied in parallel experiments, using a Formagraph, under which the following parameters were tested and varied: reconstituted skim milk powder, bulk raw milk, CaCl2 addition, enzyme concentration and temperature. Chymosin had a slightly better rate of curd firming (P ≤ 0·001) and better curd firmness 30 min after gelling point compared to pepsin under all of the conditions investigated. However, as regards curd firmness in cheese manufacture, this difference is only a question of time, since pepsin will give the same degree of curd firmness of the cheesemilk as chymosin after some minutes. The biggest disadvantage with pepsin is its strong pH dependence, which can lead to very long gelling times and weak curd if the pH is too high and the rennet is dominated by pepsin.

Use of plasmid profiles and restriction endonuclease digest in environmental studies of Listeria spp. from raw milk

Forty-eight isolates of Listeria spp. (19 L. monocytogenes. 27 L. innocua, 2 L. welshimeri) from bulk raw milk were screened for plasmid DNA. These isolates were collected over a period of 1 year. Only L. innocua harboured plasmids ( 8 27 strains) which ranged in size 10–44 megadaltons. The plasmid bearing strains could be arranged into three groups 10 Md, 44 Md and 44+10 Md. In two farms where Listeria innocua were persistant in bulk raw milk different plasmids profiles were found (farm 1 and 3). In species carrying similar plasmid profiles (44 Md) isolated from the same bulk tank raw milk samples over a period of 2–4 months (farm 2) no similarities in restriction endonuclease digest patterns of the plasmids were observed. This study suggests there may be a constant influx of Listeria spp. into raw milk supplies on the farm.

Rapid estimation of psychrotrophic and proteolytic bacterial counts from total bacterial counts in cooled raw milk

SummaryCooled raw bulk milk samples were examined for total bacterial count after 3 days at 30°C, and the numbers of psychrotrophs and proteolytic psychrotrophs after 10 days at 7°C. Positive correlations were found between the total bacterial count and the numbers of psychrotrophs, enumerated on standard plate count (SPC) agar (r = 0.88, n = 65), and between the numbers of psychrotrophs counted on SPC agar and on milk agar, respectively (r = 0.89, n = 59). Only 30.4% of the bacteria were psychrotrophs but the regression shows that the percentage of psychrotrophs increases when the total bacterial count rises. On average only small differences were found between the counts of psychrotrophs on SPC agar and milk agar, suggesting that in most samples the same organisms were probably enumerated on both media. The numbers of proteolytic bacteria were less well correlated with the numbers of psychrotrophs (r = 0.65, n = 56), as wide variations occurred between the samples. It is suggested that the total bacterial count at 30°C can be a useful estimate of the number of psychrotrophs.

Heat-stable proteases of psychrotrophic bacteria isolated from cooled raw milk

143 psychrotrophic bacteria were isolated and identified from 53 cooled raw bulk milk samples. The most encountered isolates were Pseudomonas fluorescens and P. fluorescens-like organisms, Achromobacter xylosoxidans, Acinetobacter lwoffii, Aeromonas hydrophila and enterobacteria. The strongest activity was displayed by the pseudomonads, especially the P. fluorescens-like organisms. All the excreted proteases examined were heat-stable: the best inactivation was obtained after a heat-treatment of 1 h at 55°C and 2 min at 140°C. At temperatures of 80, 90 and 100°C often increasing heat-resistance was observed.

Somatic Cell Reference Samples for Calibration of Milk Somatic Cell Counting Methods

Somatic cell count samples (SCCS) for use in calibration of milk somatic cell counting methods were prepared from raw bulk milk preserved with potassium dichromate. Somatic cells were separated by centrifugation, then appropriate cell dilutions were prepared in the dichromate-preserved skim milk. Somatic cell counts from SCCS stored at 4°C were stable over a 23-wk period. No bacterial contamination was detected in these samples. In a collaborative study among eight laboratories, SCCS were not affected by usual conditions by shipping. The SCCS can be used as reference standards for the direct microscopic somatic cell count and the Fossomatic and Coulter Counter somatic cell counting methods.

The Effects of Season and Fortification on Variations in the Vitamin A Activity of Saskatoon Market Milk

The effect of seasonal variation on vitamin A and β-carotene content of raw bulk milk was investigated using commercial milk produced in Saskatoon over the period of June 1977 to June 1978. Pronounced seasonal variations were observed, with spring milk having the highest and winter milk the lowest vitamin A activity. Average values for spring, summer, fall and winter milk were 40.2, 37.0, 33.6 and 31.5 retinol equivalent of vitamin A activity (vitamin A plus β-carotene) per 100mL, respectively. Overall, spring and summer milk contained significantly more vitamin A than winter and fall milk.Vitamin A levels in unfortified skim, 2% butterfat, and 3.25% butterfat milk and fortified skim and 2% butterfat milk were investigated simultaneously throughout the same period. The levels of vitamin A in the unfortified milk products followed the same seasonal trend as in the raw bulk milk samples, being highest in summer and lowest in winter. Fortification, in the processing plant, of the skim and 2% butterfat milk products resulted in highly variable vitamin A levels in the milk samples. The results of the study indicate that, in order to obtain uniform levels of vitamin A in skimmed and partly skimmed milk, periodic on-line monitoring and periodic alterations in the level of fortification at the processing plant will be necessary to make up for seasonal fluctuations in the vitamin A levels of milk.

Inhibition of Milk Lipolysis by Lambda Carrageenan

Lambda carrageenan inhibited lipolysis in both raw and homogenized milk. Bulk raw milk samples with and without carrageenan were subjected to both thermal and Waring Blendor activations. Acid degree values revealed that .01 to .05% carrageenan reduced thermally activated lipolysis by 55 to 100% and Waring Blendor activated lipolysis by 36 to 85%. Flavor evaluations by a trained taste panel confirmed the inhibition. Carrageenan (.05%) also reduced the extent of lipolysis produced by the addition of 2% raw skim milk to pasteurized homogenized milk. Carrageenan even inhibited the lipolysis induced by the homogenization of raw milk. Regardless of activation procedure, carrageenan was more effective in inhibiting lipolysis when added before rather than after activation. Addition of carrageenan after lipolysis had occurred did not reduce acid degree values. The inhibitory effect of carrageenan is at least partially from an interaction with lipase. However, it is conceivable that carrageenan may protect the substrate to some extent by partially encapsulating it.

The Automated Pyruvate Method as a Quality Test for Grade A Milk

Concentrations of pyruvate in Grade A raw and pasteurized milks were determined by an automated procedure. The method was sensitive to 0.1 mg of pyruvate per liter and the coefficient of variation of the method was 1.6% for a sample of milk containing 3 mg/1. Since about twice as much pyruvate was produced in samples incubated 24 h at 20 C compared with 15 C, the higher temperature is suggested for the pyruvate difference test (ΔP); however, psychrotrophs would more likely be detected with an incubation temperature of 15 C. Pyruvate difference tests of pure cultures in steamed milk distinguished between concentrations approximating 102 and 103Pseudomonas fragi, 103 and 104Pseudomonas fluorescens, and between 104 and 105Escherichia coli, Streptococcus faecalis, Lactobacillus acidophilus and Micrococcus luteus, but did not distinguish between 104 and 105 initial concentrations of six other gram-positive bacteria and Salmonella typhi. Both the initial pyruvate (IP) concentration and the ΔP in raw milk were correlated with the logarithm of the Wisconsin Mastitis Test score, suggesting that somatic cells contribute to pyruvate content. Correlations of IP and ΔP with Standard Plate Counts of raw and pasteurized milks were low or insignificant and only with raw milk from normal quarters was there a significant correlation of IP and ΔP with initial Psychrotrophic Plate Counts (PPC). Rates of pyruvate production by pure cultures in steamed milk and by the mixed natural flora of pasteurized milk were consistent with rates of growth. However, certain psychrotrophic bacteria reduced the concentration of pyruvate to undetectable levels after producing 10 or more mg/1. Suggestions are made regarding applicability of the test in controlling keeping quality of pasteurized milk and in receiving of bulk raw milk.

Xanthine Oxidase Activity in Dairy Products

Xanthine oxidase activities were measured in a variety of milk and dairy products. Bulk raw milk, as received at a commercial plant, had an average xanthine oxidase activity of 110mU/ml. Homogenization inactivated more than half of the xanthine oxidase, and when packaged, these milks average 34mU/ml. Twenty-five varieties of cheese exhibited xanthine oxidase activities ranging from 0 to 340mU/g. Data are presented for enzyme activities in a variety of other dairy products.

AUTOMATED MODIFIED LOWRY METHOD FOR PROTEIN ANALYSIS OF MILK

ABSTRACTProtein concentrations in milk were compared using a modified Lowry and the micro‐Kjeldahl methods. Correlation coefficients for absorbance (Lowry) vs protein (Kjeldahl) were 0.925, 0.904 and 0.899 for individual Holstein, individual Guernsey and bulk raw milk samples, respectively, whereas, the R2 was 0.9998 for samples of 2.5‐4.0% protein prepared from three brands of nonfat dry milk. Added lactose, up to 16 mg/ml milk, did not interfere in the test. Storage at 4°C for up to 6 days was without effect. However, whey proteins had a lower specific absorbance than did casein. The standard deviation of differences among triplicate Lowry tests was about one‐third of that for duplicate micro‐Kjeldahl tests.

SUBJECT: BITTY CREAM AND RELATED PROBLEMS:PROBLEMS ASSOCIATED WITH BACTERIAL SPORES IN HEAT‐TREATED MILK AND DAIRY PRODUCTS

Spore spoilage in pasteurized milk and UHT and sterilized milk is reviewed with particular reference to cream. The problem of the incidence of bitty cream and the effectiveness of refrigeration in checking this is discussed. Results of studies on the bacterial quality of bulk raw milk and tanker reload milk using the thermoduric count are given in detail. Current methods for pasteurizing cream are described and studies on the effect of storage temperature on the shelf life of pasteurized cream are referred to in detail with particular reference to the lag in bacterial multiplication and the use of the Methylene Blue Test.

AN ECOLOGICAL INVESTIGATION ON THE USEFULNESS OF TWO SPECIFIC MODIFICATIONS OF EIJKMAN'S TEST AS ONE OF THE METHODS USED FOR THE DETECTION OF FAECAL CONTAMINATION OF FOODS

SummaryAn investigation has been carried out on the suitability of two modifications of Eijkman's heat tolerance test which had previously been found useful for the detection of Escherichia coli strains of faecal origin in foods. These were: plating on Chapman's lactose tergitol‐triphenyltetrazolium agar incubated at 44°, and enrichment in brilliant green‐bile fluid incubated at 44°, followed by subculture in the same medium and in tryptone water to test indole formation. Two materials having quite different histories of faecal contamination were examined. These were ‘coliform positive’ ice cream samples and bulk raw human milk. The reliability of both methods was found to be very satisfactory.In spite of this, such Eijkman tests appeared to have only limited usefulness because E. coli was found to be readily outgrown by other bacteria under extraenteral conditions; E. coli was therefore absent relatively often even from 1 ml samples of human milk which contained quite high numbers (i.e. > 103/ml) of other organisms of faecal origin. It is recommended, therefore, that too much reliance should not be placed on the outcome of Eijkman tests, especially when they give a negative result. The hygienic evaluation of foods should rather be based on the results of the simultaneous enumeration of three groups of organisms, namely E. coli, faecal types of Klebsiella (i.e. those which are urease positive) and Lancefield group D streptococci.A striking result of this investigation was the occurrence in 69% of the human milk samples of 44° positive Achr. anitratus (B5W) in numbers from 10–5.6 times 103/ml. These bacteria were almost invariably found in samples which were E. coli negative; klebsiellae, many of which were Kb. oxytoca, were also found in such samples. The properties of these organisms and their ecological significance are discussed.

THE INCIDENCE AND THERMAL RESISTANCE OF MESOPHILIC SPORES FOUND IN MILK AND RELATED ENVIRONMENTS

SUMMARY: A spore ‘spectrum’ is described of aerobic mesophiles capable of resisting different heat treatments. It is shown that B. licheniformis is the most common spore former found in bulk milk but since its spores are rapidly destroyed at 100°, the more heat resistant B. subtilis is the dominant surviving spore former in commercial sterilized milk. The thermal resistance of strains of B. subtilis and B. licheniformis isolated from different sources has been investigated and the strains of B. subtilis typed according to the behaviour of their spores when heated at 100°. All strains of B. licheniformis were destroyed more rapidly by boiling for 2 min than strains of B. subtilis but only those strains of the latter which showed some degree of heat activation were more resistant than B. licheniformis. The ‘resistant’ and heat activated strains of B. subtilis appear to be sparsely distributed in nature and were only isolated from sterilized milk where the heat treatment applied would tend to eliminate other strains. The spore content of bovine faeces was similar to that in bulk milk and the total spore content varied seasonally, the spore content of faeces being on the average a hundred times greater during indoor feeding than during the period when the cattle were fed outside. A faecal infection of the milk in the ratio of 1:104 would infect the milk with spores at about the same concentration as they are found in bulk raw milk, and it is suggested that bovine faeces could be a primary source of spore formers in milk supplies.

535. Reactivation of milk phosphatase following heat treatment. III

1. Of a number of farm milks examined, two have been found to develop a positive phosphatase reaction following pasteurization at 161° F. in a small H.T.S.T. plant. This phosphatase was not of bacterial origin but was reactivated milk phosphatase.2. Considerable variation in reactivation following pasteurization at 161° F. was found to occur using individual churns of the above farm milk from any one milking. Individual cow samples also showed similar variation when tested by the capillary method. No explanation of this variation is offered.3. Commercial bulk raw milk from road or rail tanks did not develop a positive phosphatase reaction after pasteurization at 161° F. on this plant.4. Cold storage of pasteurized milk susceptible to reactivation results in less reactivation, and some evidence was obtained that cold storage of the corresponding raw milk also results in less reactivation. No such effect has been observed either using the capillary method or with samples of Uperized milk.5. Pasteurizing at 170 and 180° F. usually resulted in increased reactivation as compared with that obtained after pasteurization at 161° F. Shortening the holding time at these two higher temperatures sometimes led to a further increase in reactivation.6. It is shown that no clear relationship exists between the results obtained by plant pasteurization and the capillary method.7. It is suggested that many of the reported cases of pasteurized cream developing a positive phosphatase reaction on storage can be attributed to reactivation of the enzyme.8. Practical implications of reactivation in relation to pasteurized milk are discussed.

426. Some bacteriological aspects of the deterioration of pasteurized milk

1. Examinations have been made of milk pasteurized by the L.T. ‘Holder’ and H.T.S.T. processes using the same original bulk raw milk. In the preparation of samples care was taken to eliminate the possibility of post-pasteurization contamination. It was hoped that any differences found would be due solely to the effects of the pasteurizing processes above.2. Samples were examined by (a) colony counts and coliform tests when fresh and after ageing at 18°C. for 24 and 48 hr., (b) methylene-blue and resazurin reduction tests when fresh and after ageing at 18°C. for 45½hr., (c) keeping quality at 18°C. to end-point of 68% alcohol and clot-on-boiling tests. All bacteriological and dye reduction tests were carried out at incubation temperatures of 37, 30 and 22°C.3. In seven trials the experiments were repeated after storage of the original raw milk for 24 hr. at 40–50°F. (4·4–10·0°C.).4. Some detailed examinations were made of the bacterial floras of the raw and corresponding pasteurized milks, both when fresh and after ageing.

The pathogenesis of tuberculosis in the calf

The pathogenesis of tuberculosis in young calves was studied in a series of twenty-four animals from a tuberculosis-free herd, given, when seven days old, an infective dose in the form of either naturally contaminated milk (ordinary raw bulk milk or undiluted milk from cows with tuberculous mastitis) or a virulent culture of bovine tubercle bacilli. The infective dose was given by various routes: ordinary and teat bucket, stomach tube, directly into the duodenum, and intra-tracheally. The pathogenesis was studied at autopsy, supplemented by histological examination and guinea-pig inoculation. The results are discussed with especial reference to the pathogenesis of tuberculosis in young calves naturally infected by contaminated milk either by suckling or by bucket feeding.