Improved microbiological assay procedures for dihydrostreptomycin residues in milk and dairy products.

Because dihydrostreptomycin can remain as a slowly removed antibiotic residue in dairy animals and because of the need for more sensitive procedures with which to provide information concerning antibiotic residues in food products, procedures were developed in more sensitive assays of dihydrostreptomycin in milk and some representative dairy products. The cleanup procedures that aided these improvements were (i) precipitation of milk proteins by acidification and (ii) centrifugation to extract cheeses and to remove physical barriers to diffusion in the cylinder plate assay. In addition, a thin, single seeded assay layer was used to maximize diffusion. Levels of dihydrostreptomycin as low as 0.06 microgram/ml in milk and 0.2 to 0.4 microgram/g in cheeses were measurable; these levels were some fourfold more sensitive than those presently recommended by the Food and Drug Administration.     

Membrane filtration of dairy products for microbiological analysis.

Incubation with protease or Tween 80 or both dramatically improved the membrane filterability of liquid milks, powdered skim milk, and frozen dairy products without reducing the viability of five common species of bacteria. The technique can permit isolation and enumeration of microorganisms from test samples of these foods as large as 5 g. Flow direction through the filter was an important factor in filterability of dairy products.     

Spore germination based assay for monitoring antibiotic residues in milk at dairy farm

Spore germination based assay involves the transformation of dormant spores of Bacillus stearothermophilus 953 into active vegetative cells. The inhibition of germination process specifically in presence of antibiotic residues was used as a novel approach for monitoring target contaminants in milk. The indicator organism i.e., B. stearothermophilus 953 was initially allowed to sporulate by seeding in sporulation medium and incubating at 55 °C for 18 ± 2 h. The spores exhibited a typical chain behavior as revealed through phase contrast microscopy. The minimal medium inoculated with activated spores was incubated at 64 °C for 2-3 h for germination and outgrowth in presence of specific germinant mixture containing dextrose, whey powder and skimmed milk powder added in specific ratio along with reconstituted milk as negative control and test milk samples. The change in color of the medium from purple to yellow was used as criteria for detection of antibiotic residues in milk. The efficiency of the developed assay was evaluated through a surveillance study on 228 samples of raw, pasteurized and dried milks and results were compared with AOAC approved microbial receptor assay. The presence of antibiotic level was 10.08 % at Codex maximum residual limit having false positive result only in 0.43 % of the samples. The results of the present investigation suggest that developed spore based assay can be a practical solution to dairy industry for its application at farm level, milk processing units, independent testing and R & D centres in order to comply with the legal requirements set by Codex.     

Membrane filtration of dairy products for microbiological analysis

Incubation with protease or Tween 80 or both dramatically improved the membrane filterability of liquid milks, powdered skim milk, and frozen dairy products without reducing the viability of five common species of bacteria. The technique can permit isolation and enumeration of microorganisms from test samples of these foods as large as 5 g. Flow direction through the filter was an important factor in filterability of dairy products.     

Situation of mycotoxins in milk, dairy products and human milk in Egypt

Abstact  Milk and dairy products purchased at Egyptian markets and breast milk from lactating mothers in Cairo and Giza governorates were analyzed for some mycotoxins. Three of 15 cows’ milk samples were found positive for Afl. M₁ with mean value 6.3 ppb. Only one sample of dried milk was positive (5 ppb). Two of 10 hard cheese samples contained detectable levels of Afl. M₁ (3and 6 ppb), whereas one sample containing Afl. B₁ and G₁ (10 and 4 ppb resp.). For soft cheese one sample of 10 was positive for Afl. M₁ (0.5 ppb). Blue veined cheeses were free of Afl. M₁ and PR-toxins. For breast milk two of 10 samples were positive for Afl. M₁ (20%) with mean value 2.75 ppb, while 3 of 10 samples were positive for Ochratoxin A (30 %).     

Occurrence of Listeria species in raw milk and dairy products produced in Northern Ireland.

The overall incidence of Listeria spp. in raw milk samples surveyed was found to be 25.0% (Listeria monocytogenes 15.3%), with the incidence in samples from processing centres 54.0% (L. monocytogenes 33.3%); this was higher than that in samples from dairy farms (Listeria spp. 8.8%; L. monocytogenes 5.3%). The FDA enrichment procedure was much more productive than cold enrichment and Oxford agar was superior to modified McBride agar for isolation of Listeria. Listeria monocytogenes was never isolated by direct plating of raw milk samples on Oxford agar at a detection level of 1.0 cfu/ml. Listeria spp. were isolated from 1 of 95 pasteurized milk samples (L. monocytogenes) and 1 of 33 soft cheese samples (L. seeligeri). Restriction fragment length polymorphism was more useful than sero- or phage-typing for typing of L. monocytogenes strains, and results suggest that specific L. monocytogenes strains may persist in both farm and processing environments.     

Communicable disease associated with milk and dairy products in England and Wales 1951-80.

In England and Wales between 1951 and 1980 233 reported outbreaks of communicable disease attributed to milk or dairy products affected nearly 10 000 people, of whom four died. Tuberculosis and brucellosis have been controlled, but milk-borne outbreaks of salmonellosis and campylobacter enteritis due to raw or defectively pasteurised milk are common and may be increasing in number. Universal heat treatment of milk is an effective preventive measure, and it is regrettable that the continued sale of untreated milk is to be permitted in England and Wales.     

Occurrence of Listeria species in raw milk and dairy products produced in Northern Ireland

J. HARVEY AND A. GILMOUR. 1992. The overall incidence of Listeria spp. in raw milk samples surveyed was found to be 25.0% ( Listeria monocytogenes 15.3%), with the incidence in samples from processing centres 54.0% ( L. monocytogenes 33.3%); this was higher than that in samples from dairy farms ( Listeria spp. 8.8% L. monocytogenes 5.3%). The FDA enrichment procedure was much more productive than cold enrichment and Oxford agar was superior to modified McBride agar for isolation of Listeria. Listeria monocytogenes was never isolated by direct plating of raw milk samples on Oxford agar at a detection level of 1.0 cfu/ml. Listeria spp. were isolated from 1 of 95 pasteurized milk samples ( L. monocytogenes ) and 1 of 33 soft cheese samples ( L. seeligeri ). Restriction fragment length polymorphism was more useful than sero- or phage-typing for typing of L. monocytogenes strains, and results suggest that specific L. monocytogenes strains may persist in both farm and processing environments.     

A microbiological assay for sodium azide

A reproducible and sensitive method is presented for quantitating sodium azide (NaN₃) that exploits the fact that NaN₃ inhibits Escherichia coli RNA synthesis. A linear correlation is observed between incorporation of [³H]uridine into a trichloroacetic acid-precipitable form and NaN₃ concentration over a 31- to 2000-μg range of azide. This technique was used to determine the azide content of a complex enzyme solution where established colorimetric azide determinations proved to be unworkable. This technique when properly controlled should be applicable to a variety of similar solutions.     

Culture and molecular method for detection of Mycobacterium tuberculosis complex and Mycobacterium avium subsp. paratuberculosis in milk and dairy products

A combined molecular and cultural method for the detection of the Mycobacterium tuberculosis complex (MTBC) and Mycobacterium avium subsp. paratuberculosis was developed and tested with artificially contaminated milk and dairy products. Results indicate that the method can be used for a reliable detection as a basis for first risk assessments.