Microflora Of Raw Milk Research Articles

Use of PCR-based methods and PFGE for typing and monitoring homofermentative lactobacilli during Comté cheese ripening

This study investigated the genotypic characteristics of selected and wild homofermentative thermophilic lactobacilli strains during ripening of Comté cheeses, made into two cheese plants. Both amplification and restriction analysis of the 16S rRNA gene (PCR-ARDRA) and classical biochemical tests were used to identify isolates. Diversity within homofermentative lactobacilli was not found in their species composition since the same two species Lactobacillus helveticus and L. delbrueckii susbp. lactis were isolated from cheeses. In cheeses made with natural whey starter, it appeared that the most likely sources of L. helveticus and L. delbrueckii susbp. lactis were the starter and raw milk, respectively. The examination of RAPD profiles of lactobacilli strains revealed 19 RAPD groups among 50 isolates, which were different from selected starter strains. Using RAPD, REP-PCR, and PFGE to identify selected starter strains during cheese ripening, we showed that L. helveticus decreased quickly while L. delbrueckii susbp. lactis sustained high viability during ripening. The use of selected L. delbrueckii susbp. lactis strains diminished the genetic diversity among strains isolated from cheese, probably in preventing the raw milk microflora from growing.

Evolution of free amino acid content during ripening of Mahon cheese

Evolution of free amino acids (FAA) during ripening (10–300 days) of five batches of Mahon cheese was studied; three batches were made from raw milk and two from pasteurized milk. Major FAA were GLU, VAL, LEU, LYS and PHE (43–67% of total FAA). Relative THR, SER, GLU and GLY contents significantly increased during ripening whereas LEU, PHE and ORN decreased. GLU, ILE, SER, THR, GLY and PHE varied during ripening according to a zero order reaction ( r 2 > 0.99). Principal component analysis showed that the first principal component could be considered representative of the ripening time. The most ripened cheeses showed high GLU, GLY, SER and THR contents and the less ripened cheeses presented high ORN, LEU, PHE and ALA contents. The second component distinguished between cheeses made with pasteurized milk (with high ASN and GLN contents) and raw milk. Indigenous microflora of raw milk showed a strong influence on the proteolytic activity in Mahon cheese.

Influence of raw milk microflora on the characteristics of Swiss-type cheeses : II. Biochemical and sensory characteristics

Summary - Using the same microfiltered milk, three 'raw milk' Swiss-type minicheeses were made after addition of three different retentates; one, remaining bacteria-free, was used as reference. The experiment was repeated three times at two different seasons. Factorial discriminant analyses were made on biochemical, physico-chemical and microbiological data at 12 weeks (end of warm storage, 18 C) and 24 weeks (end of cold storage, 7 C), At each ripening period, 'raw milk' cheeses were easily and significantly separated from the reference cheese. At 12 weeks, the three 'raw milk' cheeses were separated on the basis of propionic fermentation products, and to a lesser extent on proteolysis and levels of facultatively heterofermentative lactobacilli and starter lactic acid bacteria. At 24 weeks, differentiation was mainly based on proteolysis, since volatile fatty acids and lactate concentrations were nearly the same, irrespective of the cheeses. The same conclusions were drawn for win ter and autumn chee ses. Based on sensory evaluations made at the end of ripening, it was also possible to differentiate between the different 'raw milk' cheeses in winter and in autumn. Reference cheeses were always clearly separated from the other cheeses. The qualitative and quantitative importance of the indigenous microf1ora of milk on the biochemical and sensory characteristics of Swiss-type cheeses has been discussed.

Influence of raw milk microflora on the characteristics of Swiss-type cheeses. / Evolution of microflora during ripening and characterization of facultatively heterofermentative lactobacilli

Summary - During the ripening of raw milk 8wiss-type minicheeses, the microbial ecosystem changed, and the quantitative and qualitative levels of microflora evolved constantly. The starter lactic acid bacteria population, ie, thermophilic streptococci and thermophilic lactobacilli, decreased regularly from 108 to 105 cfu/g. The propionibacteria and the facultatively heterofermentative lactobacilli, which were in very low numbers before ripening, increased rapidly to reach their final level after 6 weeks (approximately 108 cfu/g). The other microflora (Micrococcaceae and enterococci) remained at a subdominant level, or were even undetectable during ripening (coliforms, spores of butyric acid bacteria). Indigenous microflora enumerated in microfiltered milk cheeses evolved identically, except that they started at a lower level. Moreover, the starter bacteria decreased faster. Facultatively heterofermentative lactobacilli seemed to be the most important non-starter lactic acid bacteria. Eighty-eight colonies were isolated on FH agar du ring ripening, and characterized by means of the API system. The species identified evolved from a great diversity in the beginning to Iwo major subspecies: L paracasei subsp paracasei 1 and 3. However, the method used to detect this microflora did not allow us to evaluate the nature of the subdominant lactobacilli, although certainly many other species could be isolated. Swiss-type minicheese / microflora / facultatively heterofermentative lactobacilli / bacterial growth

Rapid Detection of Psychrotrophic Bacteria In Manufacturing Grade Raw Milks

Methods to rapidly assess the bacteriological quality of raw milk were investigated. Whereas direct microscopic count, modified psychrotrophic plate count, and direct epifluorescent filter technique (DEFT) did not correlate well with initial psychrotrophic bacterial count of raw milk, improvements were obtained after preincubation of the milk samples. The best preincubation conditions were identified as 30°C for 6 h, 21°C for 10 h, 13°C for 15 h, 13°C for 20 h, or 7°C for 37 h.The “square root” equation was applied to the data, and a model was produced for predicting growth of the native microflora of raw milk. Using this equation, a DEFT count after preincubation of the milk at 21°C for 10 h could accurately predict the initial psychrotroph count and the count after storage of the milk at 6°C for 48 h.

Influence of Milk Aeration on Growth of Psychrotrophic Pseudomonads

The psychrotrophic microflora of raw milk from a Cornell University herd was examined and the three most frequently occurring isolates (Pseudomonas species) were subjected to oxygen concentrations of 1 to 12 ppm and temperatures of 3 to 9°C in growth studies in raw milk. At 3°C, a reduction in oxygen level from 9–12 to 1–3 ppm resulted in a 63% increase in generation time for Pseudomonas fluorescens. However, the reduction in growth temperature from 9 to 3°C at 9–12 ppm oxygen produced a 280% generation time increase for P. fluorescens. Similar observations were made for the other isolates. An analysis of variance revealed a significant interaction between the effects of oxygen and temperature on growth of the isolates.

The microflora of raw bulk tank milk.

Milk samples from 141 farms were plated on standard plate count agar and the colony count determined after incubation for 48 hours at 32 °C. Twenty colonies were picked at random from plates containing between 30 and 300 colonies. The isolates were inoculated into litmus milk and subsequently characterized on the basis of shape, Gram-staining reaction, catalase production, lactose fermentation, and the ability to form spores.Certain general trends in the flora were observed. In milk of a colony count less than 2 × 104 per ml, micrococci were the dominant group of organisms, and as the colony count of the milk increased the percentage of micrococci decreased and the percentage of Gram-negative rods and streptococci usually increased. In spite of these general trends a study of the flora of individual samples showed that quite marked variations did occur.

Effect of Preliminary Incubation (55 F/18 HR) on Microflora of Raw Bulk Tank Milk, with Some Observations on Microflora Milking Equipment

Effect of Preliminary Incubation (55 F/18 HR) on Microflora of Raw Bulk Tank Milk, with Some Observations on Microflora Milking Equipment

THE DETERMINATION OF THE COLI‐AEROGENES CONTENT OF MILK AND DAIRY EQUIPMENT BY PLATING ON VIOLET RED BILE AGAR INCUBATED AT 30°

SUMMARY: Violet red bile agar (VRB) incubated at 30° for 20–24 hr was as good an indicator of coli‐aerogenes bacteria in milk as MacConkey's broth. A high proportion (82%) of the large, deep red colonies considered to be formed by coli‐aerogenes bacteria were confirmed as such. A British brand of dehydrated VRB agar was as suitable as an American brand of this medium for determining the coli‐aerogenes content of milk. All the strains of typical coli‐aerogenes bacteria tested formed red colonies. In a small proportion of cases the diameter was less than 0·5 mm. The only other milk bacteria which formed colonies resembling those of coli‐aerogenes organisms were some acid forming strains of Gram‐negative rods. Coli‐aerogenes bacteria, determined on VRB agar at 30°, generally constituted only a small proportion of the microflora of fresh raw milk and of farm dairy equipment.