R.T. Marshall

Microbiological Spoilage of Dairy Products

The wide array of available dairy foods challenges the microbiologist, engineer, and technologist to find the best ways to prevent the entry of microorganisms, destroy those that do get in along with their enzymes, and prevent the growth and activities of those that escape processing treatments. Troublesome spoilage microorganisms include aerobic psychrotrophic Gram-negative bacteria, yeasts, molds, heterofermentative lactobacilli, and spore-forming bacteria. Psychrotrophic bacteria can produce large amounts of extracellular hydrolytic enzymes, and the extent of recontamination of pasteurized fluid milk products with these bacteria is a major determinant of their shelf life. Fungal spoilage of dairy foods is manifested by the presence of a wide variety of metabolic by-products, causing off-odors and flavors, in addition to visible changes in color or texture.

Microbiological Quality of Raw Ground Chicken Processed at High Isostatic Pressure

Freshly ground raw chicken that had average aerobic and anaerobic bacterial populations of about 106 colony-forming units (CFU)/g was packaged and sealed in polyfilm pouches. Pouches were shipped on ice for high-pressure processing at ambient temperatures. Temperatures during the l0-min high-pressure treatment never exceeded 28°C. Samples were immediately cooled to <4°C and returned to the originating laboratory where counts of aerobic and anaerobic bacteria were made during storage at 4°C. Samples were considered spoiled at the time counts by either of the two bacterial enumeration methods reached 107 CFU/g, based on the upper limit of the 95% confidence interval of the three trials. Applications of 408 MPa, 616 MPa and 888 MPa to the samples resulted in estimated microbial spoilage times of 27 days, 70 days, and >98 days, respectively. The type of bacteria that survived best in the pressure-treated samples were the facultatively anaerobic psychrotrophic bacteria, Carnobacterium divergens and Serratia liquefaciens .

Evaluation of an Automated Beef Carcass Washing and Sanitizing System under Production Conditions1,2

Beef half carcasses were hand- or machine-washed and then machine-sanitized with 1.5% acetic acid. Sanitizer was applied at 14.4 or 52°C. Counts of Escherichia coli , Enterobacteriaceae and aerobic bacteria, made on samples collected by excision of tissues before and after treatments, demonstrated that machine washing and sanitizing reduced counts more than did hand washing. Counts were reduced more by hot than cool acetic acid. Percentages of samples with counts of log10 5.0/200 cm2 or higher after treatment were 26 and 46 for samples from carcasses sanitized with 1.5% acetic acid at 52 and 14.4°C, respectively. After hand washings 65% of the samples had these high counts.

Counts of Six Types of Bacteria on Lamb Carcasses Dipped or Sprayed with Acetic Acid at 25° or 55°C and Stored Vacuum Packaged at 0°C

This study evaluates the sanitizing effectiveness of applying, by spraying or dipping, 1.5 or 3.0% acetic acid at either 25°C or 55°C to freshly slaughtered lamb carcasses. After vacuum packaging and during extended storage at 0°C, samples were examined for microbial populations. Each treatment reduced counts significantly compared with counts of untreated controls. Numbers of gram negative bacteria and lactobacilli were affected significantly by the temperature of acid solutions. Other variables caused insignificant differences in the amounts that counts were reduced by sanitization. Overall, dipping in 3% acetic acid at 55°C was the most effective treatment.

A model system for studying the penetration of microorganisms into meat

This study assessed the effect of line pressure used in spraying meat surfaces on removal of inoculated bacteria and on penetration of Blue Lake, an insoluble dye, into sprayed meat. Pressures used in this analysis were 690, 2070, 4140, and 6200 kPa. The highest pressure, 6200 kPa, removed significantly less aerobic bacteria than the other three lower pressures. No significant differences in removal of Enterobacteriaceae were noted due to pressure. Nozzle type was also a variable in the study on penetration. Type of nozzle had a significant effect on the penetration of the Blue Lake into the meat at a pressure of 6200 kPa. An equation describing penetration of the Blue Lake into the meat is given.

Combined and Individual Effects of Washing and Sanitizing on Bacterial Counts of Meat - A Model System1,2

Strips of plate meat were sprayed with acetic acid, sodium hypochlorite, or tap water after they were washed with 0, 12.7, or 25.4 liters of tap water/min. Washing before sanitizing lowered bacteria counts significantly only when the higher volume of water, 1.4 ml/cm2, was applied, and this difference existed for samples taken immediately but not 48 h after treatment. Reductions in counts exceeded 99.9% when samples washed with 25.4 liters/min (1.4 ml/cm2) were sanitized with 3% acetic acid. This sanitizer was sprayed at the rate of 6.8 liters/min (1.9 ml/cm2) at a pressure of 14.0 kg/cm2 from a distance of 40 cm as the meat moved at 2 cm/sec through the spray. Under comparable conditions of application, both sodium hypochlorite (200 to 250 mg/liter) and tap water reduced counts by about 90%. Acetic acid had a much greater residual effect on numbers of viable bacteria than did hypochlorite. No effect of air drying was observed.

Microbial Growth on Plate Beef During Extended Storage After Washing and Sanitizing

Plate beef was washed and/or sanitized with cold water, hot water, steam, sodium hypochlorite, or acetic acid before being stored for up to 28 days at 3.3 C and 90% relative humidity. Microbial counts initially and at regular intervals thereafter disclosed that, compared with untreated controls, time to reach counts of 108 bacteria per cm2 were (a) 1 day less with steam- or water-treated samples, (b) 2 to 3 days more with hypochlorite-treated samples, (c) 5 days more with hot-water-treated samples, and (d) 16 to 17 days more with acetic acid-treated samples. Re-sanitization with acetic acid extended time to reach equivalent counts by 7 additional days.

Experiments in Sanitizing Beef With Sodium Hypochlorite1,2

Beef plate meat was sprayed with sodium hypochlorite (pH 6.0) from two sources, commercial and electroytically generated. Variables studied in two experiments were rate of flow of sanitizer, line pressure, speed of movement of meat through the sprays, and method and time of sanitization. Hypochlorite sprays reduced microbial counts significantly more than did water applied under the same conditions, but type of hypochlorite was unimportant. Maximum reductions in counts made immediately after sanitization approximated 97 and 93% as measured by swab and core sampling methods, respectively. Sprays were most effective when delivered in a single passage over meat at a rate of 2 rather than 10 cm/sec or in about seven successive passages at 10 cm/sec. Samples collected by coring and swabbing estimated microbial populations different from each other when the samples were taken after sanitized meat had been stored at 3 C for 48 h. Based on our findings we recommend the coring method.

In-Plant Evaluation of a Prototype Carcass Cleaning and Sanitizing Unit

An experimental cleaning and sanitizing unit was used in cleaning and sanitizing (3.0% acetic acid) beef carcasses. It cleaned the carcasses sufficiently that they would pass the Acceptable Quality Level test of the Food Safety and Quality Service, U.S. Department of Agriculture. The sanitizing unit reduced the microbial population on the surface of the meat by an initial 1.49 logs; the difference between washed and washed and sanitized carcasses after 1 week (168 h) was 0.92 log. A slight gray cast developed within the top 1 mm of fat almost immediately after the acid was applied. Sensory panel members detected no adverse effects on the lean portion of steaks from sanitized carcasses. However, they detected a slight off-flavor in treated fat.

Formulas and Recipes

This chapter provides guidance in the formulation of a wide variety of frozen desserts. Basic mix formulas vary as dictated by ingredients available, consumer buying preferences, costs, and finished product quality expected.