No abstract

SummaryThe combined effect of ultrasonic waves (20 kHz, 150 W) and heat treatment applied simultaneously (thermoultrasonication) on the survival of a strain ofStaphylococcus aureuswas studied in 0·05 m-phosphate buffer pH 6·8 and ultra-heat-treated whole milk. This combined process decreased by 63% the decimal reduction times for the heat treatment when the organism was suspended in buffer and by 43% when suspended in milk. These effects were much greater than the additive effect of the two agents considered independently.

Changes in bacterial numbers, metmyoglobin percentage and 2-thiobarbituric acid number during the chill storage of pork longissimus dorsi packed with air, carbon dioxide, carbon dioxide and oxygen or vacuum-packed in plastic bags of high (polyethylene) and low (Cryovac BB-1) permeability to gases were studied. The fast increase of viable counts in polyethylene bags showed that plastic films of high permeability are not suitable to extend the shelf-life of meat using modified atmospheres. In Cryovac bags both carbon dioxide-enriched atmospheres and vacuum-packaging inhibited aerobic organisms, keeping the bacterial flora (mainly lactobacilli and Brochothrix thermosphacta) below the level of 108/cm2 for about 3 weeks. Metmyoglobin formation was not affected by 20% carbon dioxide, whilst 80% oxygen significantly decreased its rate. Metmyoglobin accumulation in vacuum-packed samples was even slower than in the oxygen-enriched atmosphere, but meat color was less attractive in the former than in the latter. Lipid oxidation was not the limiting factor of shelf-life in either of these two atmospheres. Off-odors appeared in both at approximately 20–22 d of storage.

Restriction site analysis of polymerase chain reaction (PCR) products from a conserved region of the cytochrome b gene has been used for the identification of fresh and smoked samples of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Digestion of the 359-bp PCR product with the endonucleases EcoRV and TaqI yielded specific banding patterns for salmon and trout. This genetic marker can be very useful for detecting fraudulent substitution of the cheaper smoked trout for the more expensive smoked salmon.

The suitability of D(‐)lactic acid and/or acetoin/diacetyl as indicators of spoilage of vacuum‐packed meat and meat products has been studied. When pork was vacuum‐packed, these substances reached measurable amounts after storage for only about 10 days. Although microbial counts stabilized from the 20th day of storage, the D(‐)lactic acid and acetoin/diacetyl concentrations increased progressively. These substances could therefore be potential indicators of the storage time of vacuum‐packed pork. From a survey carried out with several vacuum‐packed meat products from the market, it was concluded that the D(‐)lactic acid content could be used as an indicator of the storage time of these products. No consistent results were obtained with acetoin/diacetyl.