Natural exopolysaccharides enhance survival of lactic acid bacteria in frozen dairy desserts

Viable lactic acid-producing bacteria in frozen dairy desserts can be a source of beta-galactosidase for persons who absorb lactose insufficiently. However, freezing kills many of the cells, causing loss of enzymatic activity. Cultures selected for high beta-galactosidase activities and high survival rates in the presence of bile were examined for survivability during freezing in reduced-fat ice cream. Encapsulated S. thermophilus strains survived better than their nonencapsulated mutants in reduced-fat ice cream after freezing and frozen storage at -29 degrees C for 16 d (28 vs. 19%). However, a small nonencapsulated strain of Lactobacillus delbrueckii sp. bulgaricus survived better than the large encapsulated strain in reduced-fat ice cream. Factors that improved survival of encapsulated S. thermophilus 1068 in ice cream were 1) harvest of cells in the late-log phase of growth at 37 degrees C rather than at 40, 42.5, or 45 degrees C; 2) overrun at 50% rather than 100%; and 3) storage at -17 degrees C rather than -23 or -29 degrees C. Survival of strain ST1068 was unaffected by 1) neutralization of acid during growth or 2) substitution of nitrogen for air in building overrun.

Effects of milk fat, cocoa butter, or selected fat replacers on flavor volatiles of chocolate ice cream

Selected volatile compounds of chocolate ice creams containing 0.6, 4.0, 6.0, or 9.0% milk fat or containing 2.5% milk fat, cocoa butter, or one of three fat replacers (Simplesse, Dairy Lo, or Oatrim) were analyzed by gas chromatography and gas chromatography-mass spectrometry using headspace solid-phase microextraction. The headspace concentration of most of the selected volatile compounds increased with decreasing milk fat concentration. Fat replacers generally increased the concentration of volatiles found in the headspace compared with milk fat or cocoa butter. Few differences in flavor volatiles were found between the ice cream containing milk fat and the ice cream containing cocoa butter. Among the selected volatiles, the concentration of 2,5-dimethyl-3(2-methyl propyl) pyrazine was the most highly correlated (negatively) with the concentration of milk fat, and it best discriminated among ice creams containing milk fat, cocoa butter, or one of the fat replacers.

Effect of milk fat, cocoa butter, and whey protein fat replacers on the sensory properties of lowfat and nonfat chocolate ice cream

Lowfat and nonfat chocolate ice creams were made with 2.5% of milk fat, cocoa butter, or one of two whey protein-based fat replacers, Dairy Lo or Simplesse. Polydextrose was added as required so that all formulations contained the same amount of total solids. Ice cream was stored at a control temperature of-30 degrees C. Hardness, viscosity, and melting rate were measured by physical methods. Trained panelists conducted descriptive sensory analyses of the samples at 0, 6, and 12 wk. Attribute ratings were analyzed by analysis o variance with least significant difference mean separation and orthogonal contrasting. Data were also analyzed by multivariate analysis of variance with canonical variate analysis. Consumer acceptance (n = 50) did not differ among the fresh ice creams (wk 0). Ice cream containing milk fat had less intense cocoa flavor and was more resistant to textural changes over time compared with the other ice creams. Simplesse was more similar to milk fat than was Dairy Lo in its effect on brown color, cocoa flavor, cocoa character, and textural stability but was less similar in terms of thickness and mouthcoating.

Evaluation of ruminally protected methionine and lysine or blood meal and fish meal as protein sources for lactating Holsteins

Forty lactating Holstein cows averaging 55 days in milk were used in a randomized block designed experiment to evaluate the effectiveness of ruminally protected Met and Lys compared with that of ruminally undegradable protein for supporting lactation. Cows were fed total mixed diets for 15 wk. Diets were formulated to be isonitrogenous with the same base ingredients resulting in base crude protein percentage of 15.5. Supplemental crude protein supplied by urea, soybean meal, or a 50:50 (wt/wt) mixture of fish and blood meal increased total dietary nitrogen to 18.0% of diet DM. Two additional diets consisted of the basal diets soybean meal and urea, which were supplemented with ruminally protected DL-Met and Lys-HCL at 10 and 25 g/d, respectively (soybean meal + amino acids (AA), urea + AA). Mean measures of dry matter intake, milk yield, milk protein percentage, and milk fat percentage were not affected by protein supplement. Milk protein yield, milk fat yield, casein yield, and casein percentage also were not affected by source of supplemental protein. Results indicate that at the level of crude protein intake relative to milk production in this experiment, the source of protein did not affect lactational performance.

Sensory and physical properties of ice creams containing milk fat or fat replacers

The purpose of this research was to determine the relative effects of milk fat, nonfat milk solids, or each of three whey protein type fat replacers on the flavor and texture attributes of vanillin-flavored ice cream. Descriptive sensory analyses disclosed that ice creams containing 4.8% of any of the fat replacers in place of milk fat had no demonstrable effect on vanillin flavor but increased the intensities of whey flavor, syrup flavor, and cooked milk flavor. Compared with each substitute, milk fat significantly reduced the syrup, whey, and cooked milk flavors and increased the fresh milk and cream flavors of the ice cream. Results emphasized the importance of fat as a flavor modifier and the importance of certain fat replacers as aids in improving texture.

Antiproliferative effects of yogurt fractions obtained by membrane dialysis on cultured mammalian intestinal cells

The consumption of yogurt has been associated with a reduced incidence of colon cancer in population groups. Bioactive peptides produced during bacterial fermentation may alter the risk of colon cancer via modification of cell proliferation in the colon. Using our previously described cell culture model system, we have isolated a yogurt fraction that decreases cell proliferation. Yogurt was fractionated using 10,000- and 500-Da membrane dialysis. When the yogurt fraction was incubated with IEC-6 or Caco-2 cells, cell division was decreased compared with control treatments, as determined by thymidine incorporation. Cell division was not inhibited in response to a similarly produced milk fraction or in response to solutions of lactic acid. The determination of cell kinetics by flow cytometry revealed a decrease in the number of cells in the initial growth phase in response to the yogurt fraction for the IEC-6 cells, but not the Caco-2 cells. Alpha-Lactalbumin inhibited cell division of both cell lines, but beta-casein did not.

Reduced oxidation of fresh pork in the presence of exogenous hydrolases and bacteria at 2 degrees C

Exogenous lipase and phospholipase A2 added to ground pork released free fatty acids (FFA) and reduced lipid oxidation as indicated by TBA values during storage at 2 degrees C. Bovine serum albumin (BSA) added to ground pork to bind FFA accelerated lipid oxidation. Both lipase and phospholipase-producing bacteria increased in numbers, and pseudomonads that produced lipase and phospholipase became the predominant bacteria growing in pork stored at 2 degrees C. The TBA values of ground pork, which were exposed to the growth of natural microbial flora, were reduced up to 84% during storage at 2 degrees C for 16 d. Pseudomonasfragi K1 inoculated into sterilized ground pork reduced TBA values and oxidation volatiles including saturated aldehydes, unsaturated aldehydes, alcohols and ketones.

A cell culture model to identify biologically active peptides generated by bacterial hydrolysis of casein

Consumption of fermented dairy foods has been linked to reduced incidence of colon cancer in population groups. Recently, biologically active compounds have been isolated from these products. Bacterial proteinases, produced by dairy starter cultures, generate a variety of peptides from casein. Some of these casein-derived peptides are likely to alter intestinal cell kinetics. Effects on colon cell kinetics because of the presence of casein-derived peptides may be a mechanism through which fermented dairy foods reduce the risk of colon cancer. We have used two intestinal cell lines (IEC-6 cells, derived from normal rat intestine, and Caco-2 cells, derived from human colon adenocarcinoma) to identify casein peptides that affect intestinal cell kinetics. Cell culture media containing casein were inoculated with three commercial starter cultures and incubated for 4, 8, or 24 h. The bacteria-conditioned media were then filter-sterilized and incubated with the intestinal cells for 6 or 24 h. Rates of [3H]thymidine incorporation and cell cycle kinetics determined by flow cytometry were affected by the culture-modified media in both cell lines. The IEC-6 cells tended to reduce, and Caco-2 cells to increase, rates of cell division after exposure to the media. Intestinal cell response varied among the starter cultures. The results support the use of intestinal cell cultures to identify casein peptides generated by dairy starter cultures, which affect intestinal cell kinetics.

Improving survival of culture bacteria in frozen desserts by microentrapment

Lactobacillus bulgaricus cells were entrapped in beads of calcium alginate and evaluated for their ability to survive freezing processes. Cells survived freezing (without agitation) in ice milk mix much better than in distilled water, and more entrapped cells survived than did cells that were not entrapped. Glycerol and mannitol were cryoprotective, but glucose was not, when each was added (6%) separately to the beads. Entrapment protected the lactobacilli in batch frozen and continuously frozen ice milk mixes. The percentage of survival for entrapped and unentrapped cells in continuously frozen ice milk approximated 90 and 40%, respectively. Lactobacilli survived better in beads with mean diameters > 30 microns than in those averaging 15 microns. Addition of entrapped lactobacilli had no measurable effect on the sensory characteristics of the ice milk.

Abnormal Freezing of Milk Caused by Ice-Nucleating Bacteria

Cultures of ice nucleating bacteria isolated from raw producer bulk milk were gram negative rods, mostly Pseudomonas fluorescens . Milk samples from which the cultures were isolated prefroze during supercooling in the cryoscopic test of freezing point. Grown under conditions of restricted nutrition, cells transferred to normally freezing samples imparted the ice nucleating factor.

Interaction of Concentration and Temperature of Acetic Acid Solution on Reduction of Various Species of Microorganisms on Beef Surfaces

Cylindrical samples of beef semitendinosus muscle were dipped in inocula of diluted (1:8) fresh manure or suspensions of Escherichia coli or Salmonella typhimurium . After air drying 15 min, samples were sanitized by dipping in acetic acid (0,1,2, or 3%) for 15 sec at 25,40,55, or 70°C. Tests for survivors showed the most effective sanitizing treatment to be 3% acetic acid applied at 70°C. In general, numbers of survivors were inversely related to temperature of application. Concentration, an insignificant variable at the higher temperatures, caused significant variation at the lower temperatures. Treatment had the greatest effect on total aerobic count followed by Enterobacteriaceae count, and E. coli was least affected. S. typhimurium counts were affected least by temperature.

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.

Evaluation of Swab and Tissue Excision Methods for Recovering Microorganisms from Washed and Sanitized Beef Carcasses 1, 2

Samples taken from dressed beef carcasses by swabbing surfaces or excising tissues were plated to produce counts of aerobic bacteria, Enterobacteriaceae , and Escherichia coli . Counts were compared to determine efficiencies of the two sampling methods. Excised tissues produced much higher counts than did swabs, especially when washed and sanitized carcasses were sampled. Percentages recovered by swabbing appeared to be influenced by characteristics of the area sampled on the carcass. Numbers of Enterobacteriaceae and E. coli recovered by swabbing washed and sanitized carcasses were usually too few to be countable, whereas excision usually produced countable plates. The excision sampling method permitted differentiation among treatment effects of washing and sanitizing.

A Simple Qualitative Method for Detecting Cleanliness of Food Contact Surfaces

Two tests were developed for detecting protein on food contact surfaces. Wetted swabs rubbed over surfaces on which 3 μg of protein was dried became visibly blue when reacted with Folin's test reagents. Wetted Chemstrips, available to test for protein in urine, became visibly colored on rubbing on surfaces with 0.25 μg of protein. Sensitivity of the tests varied with type of protein.

Microbiological and Sensory Tests of Beef Treated with Acetic and Formic Acids 1, 2

One-centimeter cubes of the semimembranosus and adductor muscles of beef were inoculated with 5.2 × 10⁶ of Salmonella typhimurium , Shigella sonnei , Yersinia enterocolitica , Escherichia coli , Pseudomonas aeruginosa or Streptococcus faecalis . Exposure of the meat by dipping in 1.2% acetic acid for 10 s reduced averge numbers recoverable of these bacteria by 65%. E. coli was the most resistant, losing 46% of its viable cells. One-half of the acetic acid was replaced with 0.046% formic acid without loss in effectiveness. The rate of increase in antimicrobial effects of the treatment declined with time. Discoloration of the meat occurred after dipping in both 1.2% acetic acid, and 0.6% acetic plus 0.046% formic acids for 10 s. In triangle tests of flavor, panelists failed to differentiate samples of baked ground beef treated (before grinding) with 0.6% acetic acid and 0.046% formic acid from controls dipped in water (P<0.05). However, the same type of test showed a significant flavor difference between meat dipped in 1.2% acetic acid or distilled water.

Removal of Animal Fat From Food Grade Belting as Affected by Pressure and Temperature of Sprayed Water

This study was designed to determine the extent of removal of pork, beef and chicken fat from food grade belting with water when temperature and pressure were the experimental variables. For pork fat, no significant difference was observed between water temperature of 40 and 55°C, whereas at 25°C less fat (P<0.05) was removed regardless of pressure. Significantly more beef fat was removed at 40°C and the intermediate pressure than at 40°C and either the higher or lower pressure. For chicken fat, pressure was the only significant variable, with no interaction between pressure and temperature. The higher the pressure, the more chicken fat was removed. Overall, temperature was a more important variable than pressure. The data imply that temperature of the washing solution should be above the melting point of the type of fat to be removed.

Simple, Accurate Method for Evaluating the Amount of Fat and Protein Residue on a Food Contact Surface 1, 2

The objective of this research was to evaluate a method for quantitatively removing pork fat and blood plasma from different food contact surfaces - glass, stainless steel, plastic and food grade belting. Two studies were conducted. In the first study, a mass balance procedure was used to determine whether the developed method could remove virtually all the fat or protein placed on stainless steel and glass. In the second study, a gravimetric method was used to verify that the amount of fat on test strips could be harvested and quantified as residue. A recovery rate of from 98% or 100% was achieved for the different types of food contact surfaces.

Detachment of Pseudomonas fluorescens P26 from Beef Rinsed in Salt and Acid Solutions

The purpose of this study was to test the hypothesis that numbers of bacteria detached from meat could be increased by modification of the ionic environment surrounding both the meat and bacterial surfaces. Of five 0.1 M chloride salt solutions that were used to rinse cells of Pseudomonas fluorescens P26 from meat, KCl removed the highest average number of cells. These numbers were significantly greater than those for NH₄Cl and MgCl₂. Compared with water, the solution of 0.1 M KCl rinsed three times as many bacteria from cubes of inoculated meat shaken in solution for 1 min and there were one-half as many bacteria recovered from meat rinsed with KCl as from meat rinsed with water. The latter difference was significant, and we conclude that KCl assisted in detachment of the pseudomonads. However, no significant effect of 0.1 M KCl was observed when it was added to water that had been buffered to pH 4 and 5 with citrate-phosphate buffer. Instead, the buffered rinse caused a large loss in viability of the pseudomonads. This large loss in viability may have over-shadowed the smaller effect of KCl.

Characterization of glycosidases produced by Pseudomonas fluorescens 26 1

Six synthetic glycosides were used to characterize glycosidic activities of a cell-free filtrate of Pseudomonas fluorescens 26. The filtrate was prepared after growing the bacterium in glucose-enriched minimal growth broth. Temperature and pH optima for glycosidic activity were as follows, respectively, β-D-fucosidase: 15°C and 6.0, β-D-mannosidase: 25°C and 6.0, β-D-glucosidase: 25°C and 5.5, β-D-galactosidase: 30°C and 6.5, N-acetyl-β-D-glucosaminidase: 45°C and 7.0, and N-acetyl-β-D-galactosaminidase: 45°C and 6.0. Activation energies of B-D-galactosidase, β-D-mannosidase, β-D-glucosidase and N-acetyl-β-D-glucosaminidase were 25.4, 12.3, 9.8 and 6.0 Kcal/mol, respectively. β-D-fucosidase and N-acetyl-β-D-galactosaminidase appeared to have non-Arrhenius behavior, so activation energies were not calculated for them. All six glycosidases were heat-sensitive to conditions of pasteurization of milk.

Potassium Sorbate and Recovery of Pectinolytic Psychrotrophs from Vacuum-Packaged Pork

Pork loins were divided into small roasts. Thirty roasts were uninoculated and 30 were inoculated by dipping in 1 % peptone water containing 100 Colony-forming units (CFU)/ml of pectinolytic Yersinia enterocolitica . Twenty-four each of the uninoculated and inoculated roasts were sprayed with or dipped in 5 or 10% solutions of potassium sorbate. All roasts were then vacuum-packaged and stored at 5°C. After storage for 1 or 21 d, three roasts from each group were examined for psychrotrophic plate count (PPC) and the 3-tube most probable number of pectionolytic bacteria. Counts of psychrotrophs on lean surfaces of untreated controls increased by nearly 3.4 log₁₀. Numbers on sorbate-treated lean surfaces increased about 2.0 log₁₀. Growth and differences in counts on fatty surfaces were less. Numbers of pectinolytic bacteria on lean surfaces of controls increased by nearly 2.0 log₁₀, but numbers of pectinase producers did not change significantly during storage of sorbate-treated samples. Of 30 pectinolytic isolates identified from roasts stored 21 d at 5°C, 87% were Yersinia spp. and 13% were Klebsiella oxytoca . Since most of the pectinolytic isolates were psychrotrophs of public health significance, inhibition of their growth by sorbate is of particular importance.

Pyruvate in Producer and Commingled Manufacturing Grade Milk

The automated test for pyruvate concentration, [P], was evaluated as a substitute for the direct microscopic count (DMC) in determining quality of manufacturing grade milk. Samples of manufacturing grade milk from producers and tank trucks as well as skim milk were collected at a large milk drying plant. Each sample was tested immediately for standard plate count (SPC), psychrotrophic plate count (PPC) and DMC. Portions of milk were heated to 85°C for 10 min to stabilize [P] before being returned to the laboratory for analysis of initial pyruvate concentration, [IP]. Unheated samples stored at 4°C were analyzed for [P] and DMC daily for up to 120 h. [IP] was a good indicator of PPC in milk from individual producers (r=0.81). However, [IP] was not highly correlated (r=0.26) with PPC in skim milk samples which were characteristically high in SPC and homogeneous in PPC. With skim milk, [IP] was more highly and significantly correlated with initial DMC (r=0.61) than with initial SPC (r=0.31) or initial PPC (r=0.26). [IP] was a good indicator of [P] in stored fluid samples until counts exceeded about 10⁷/ml. Initial DMC and initial PPC were about equally correlated with [P] determined at 24-h intervals, and the initial DMC was a reasonably good indicator of DMC determined at 24-h intervals. Using 6.5 mg of pyruvate/L to represent bacterial counts of 3 × 10⁶/ml by the three methods tested, the pyruvate test correctly classified 91% of 57 samples of producer milk based on PPC, 88% based on SPC and 82% based on DMC. Most of the error was of the false-negative type in which samples with high counts had less than 6.5 mg of pyruvate/L. This was probably because some bacteria catabolize pyruvate once their numbers exceed 10⁷/ml.

Inexpensive Device for CIP Cleaning of the Inside of a Beef Carcass Spray Chamber 1, 2

An automated device for spray-type cleaning-in-place of a beef carcass washer constructed of linear polyethylene was evaluated by microbiological tests. Swabbing of seams at the welded joints, followed by plating of the swab rinse solution, disclosed that cleaning reduced counts by more than 99%. The average number of microorganisms recovered from seams after cleaning was 80/cm². However, it was necessary also to manually clean certain joints weekly. Flat surfaces were quite adequately cleaned according to results of tests with RODAC plates. With RODAC plates, numbers of yeasts and molds recovered on potato dextrose acid agar were better indicators of sanitary condition than numbers of coliforms on violet red bile agar or numbers of staphylococci on Baird-Parker agar.

Comparison of VRB and VRB-2 Agars for Recovery of Stressed Coliforms From Stored Acidified Half-and-Half 1

Half-and-half was acidified with delta-gluconolactone, inoculated with three species of coliform bacteria, stored for 31 days at 5 °C, and examined for numbers of viable coliforms on VRB and VRB-2 agars. Loss of culture viability was logarithmic with recovery of 50 and 10% of initial numbers on days 7 and 30, respectively. Escherichia coli had significantly more recoverable injured cells than did Enterobacter aerogenes or Klebsiella pneumoniae . As time of storage increased, the proportion of injured to noninjured cells also increased. However, the maximal number of injured cells was on the thirteenth day of storage of E. coli -inoculated product. VRB-2 agar averaged 20% higher in productivity than VRB agar.

Pyruvate as an Indicator of Quality in Grading Nonfat Dry Milk 1

Samples of skim milk and nonfat dry milk (NDM) made from it were collected, paired and tested for pyruvate concentration, [P], and Direct Microscopic count (DMC). The skim milk was tested for Standard Plate Count (SPC) and Psychrotrophic Plate Count (PPC). The geometric average DMC of skim milk was more than three times higher than that of the paired NDM samples. However, [P] of NDM was not significantly different from that of the skim milk. Although [P] of skim milk was poorly correlated with SPC and PPC, r = .31 and .26, respectively, it was relatively well correlated with DMC, r = .64. Data were widely dispersed around the regression line when [P] was ≤ 4.0 mg/L. However, [P] increased rapidly when DMCs were > 10⁶/ml. A limit of 10 mg/L of [P] in NDM reconstituted 1:9 was chosen to represent the current U.S. Department of Agriculture Standard for DMC in NDM. This limit failed to classify about 10% of the samples correctly, assuming that each geometric mean DMC was correct. However, the probability that samples meeting the DMC standard would be rejected by the pyruvate test was quite low and the probability was moderate that samples which would be acceptable by the pyruvate test would be rejected by the DMC. For the latter, 28% of the samples having DMCs of ≥ 10⁷/ml contained < 10 mg/L of pyruvate. No sample having ≥ 10 mg/L of pyruvate had a DMC of ≤ 10⁷/ml. Pyruvate concentration in NDM did not change during storage at 5 or 32°C for 90 days.

Soaking of Mustard Seeds to Release Microorganisms in Making Plate Counts 1

Experiments in plating mustard seed showed that soaking in buffered dilution water for 10 min and shaking 25 times resulted in dislodgement of most microorganisms attached to the seeds.

Moisture Loss from Agar Plates During Incubation 1

Composition of media, quantity of medium in petri plates, incubator relative humidity, placement of plates on upper or lower shelf and time in the incubator were important variables in evaporative losses of weight of agar in Petri plates during incubation. However, the most important variable was temperature.

Effect of Nozzle Configuration and Size on Water Absorbed by Meat During Cleaning 1, 2

Our previous studies suggested the need to study absorption and evaporation of water from meat during cleaning with water sprays. This article describes the effects of type of meat surface, line pressure, size and configuration of nozzle, and speed of meat travel past the nozzle on water absorption and percent shrinkage after 24 h. Cut meat surfaces had a higher percent gain and lower percent shrinkage than did uncut surfaces. Pressure had no significant effect on percent shrinkage. Solid stream nozzles caused a higher percent gain and a lower percent shrinkage than did flat fan nozzles. As the size of the nozzle was increased, the percent gain increased and the percent shrinkage decreased. The slowest speed of meat travel past the nozzles (5 cm/s) caused the highest percent gain in weight.

Evaluation of a Unique Chamber for a Beef Carcass Cleaning Unit 1, 2

Our objective was to develop basic design criteria for use in fabricating a functional chamber for a red meat carcass cleaning unit. Emphasis was placed on eliminating the doors. A model carcass cleaning chamber was constructed to test effects of selected design parameters on direction and velocity of airflow. Based on data from the tests using the model, a full-scale chamber with no doors was designed, fabricated, and installed in a commercial packing plant for testing. The air moves into the chamber at both the entrance and the exit. This movement of air into the chamber prevents water droplets entrained in the air from escaping into the slaughtering area and causing condensation on the walls and roof.

Isolation of Pectinolytic Aeromonas hydrophila and Yersinia enterocolitica from Vacuum-Packaged Pork 1

A survey was made of commercially available vacuum-packaged fresh pork held at 5 C for 7, 14, 21 and 28 days. Also, four vacuum-packaged leg roasts were stored for 21 days at 5 C then for 90 days at -18 C before sampling. Surface cores of meat were enriched in sorbitol bile broth 21 days at 5 C to enhance recovery of Yersinia enterocolitica on pectin agar. Of the 54 samples surveyed, 20% yielded highly pectinolytic colonies of Aeromonas hydrophila that were cytotoxic to Y1 and HeLa cells, 6% yielded Y. enterocolitica and 6% yielded Yersinia intermedia . Yersinia was recovered from both fresh and frozen samples. This is believed to be the first report of pectinolysis by A. hydrophila and recovery of cytotoxic A. hydrophila from vacuum-packaged pork.

Unique response to heat of extracellular protease of Pseudomonas fluorescens M5

Eight proteases selected from 38 cultures of psychrotrophic bacteria were subjected to heat stability tests. Cell-free filtrates of the broth in which the cultures were grown individually were heated to 40, 50, 60, and 70 degree C for 0, 15, 30, and 60 min. Six of the filtrates were less than 50% inactivated by heating at 40 degree C for 60 min, whereas the enzyme of Pseudomonas fluorescens M5 was inactivated completely by this treatment. Of the five most proteolytic cultures tested, including P. fluorescens M5, losses in protease activity ranged from 9 to 34% on heating at 70 degree C for 60 min. Purified M5 protease retained at least 75% of its activity over the pH range of 4.5 to 7.5. Electrophoresis of active M5 protease from four chromatographed fractions revealed two bands in each with approximate molecular weights of 35,000 and 45,000. Heating at 40 degree C did not alter mobility of either band. Reasons for lability at 40 degree C but stability at 50, 60, and 70 degree C are discussed. Complexation with casein, observed with another protease, was not a possible explanation for stability at 40 degree C.

Comparisons of Tests of Milk Samples taken Conventionally and with an Automatic In-Line Sampler 1

Samples of manufacturing bulk milk were collected from 102 producers by the conventional procedure and were randomly coded. At least 3 h later, samples were collected by haulers from the same tanks with in-line samplers (ISOLOK MS5) mounted on the truck. Overall weighted mean fat tests for producers on nine routes were 3.669% for conventional and 3.644% for in-line samples. The overall weighted mean of load samples taken at the plants was 3.655%. Unweighted means of the producer samples taken by the conventional and in-line methods were 3.781% and 3.752%, respectively. The difference between the means, 0.029%, was highly significant (P<0.001). There was a significant difference (P<0.05) in fat content of samples taken conventionally after 5 and 8 min of agitation. Carryover of bacteria from tanks with high counts to samples with low counts was not detectable by direct microscopic counts in field studies. However, in laboratory studies with an emptied hose and a few seconds of delay before sampling small carryover occurred. Significance of the carryover effect depends on volume of milk sampled and whether bacterial content is marginal.

Evaluation of a Prototype Beef Carcass Washer in a Commercial Plant 1, 2

Eighty half carcasses were either: (a) hand-washed with tap water (15 C) or (b) washed (15 C) with an experimental beef carcass cleaning unit. Overall logarithmic mean reduction in aerobic plate counts were 0.99 for hand-washed carcasses and 1.07 for those machine-washed. No significant difference (P ⩽ 0.05) was noted in percentage dirt and smear reductions due to type of wash (hand or machine). However, 12.5% of the hand-washed carcasses and 22.5% of the machine-washed carcasses contained defects caused by hair. The authors concluded that the machine was at least as effective as the careful hand washing in this experiment.

In-Plant Evaluation of a Prototype Carcass Cleaning and Sanitizing Unit 1, 2

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.

Short-Chain Fatty Acids as Sanitizers for Beef 1, 2, 3

Formic, acetic and propionic acids in various combinations and individually were screened for antimicrobial efficacy and effect on meat color. Microorganisms tested were two pseudomonads, three coliforms, a Streptococcus sp., a Micrococcus sp., a spore-forming bottom yeast and three film yeasts which reproduced by budding. Variables tested were microorganism, pH, concentration of sanitizer and exposure time. When the 11 cultures were exposed to individual sanitizers and mixtures of them in three replications, 2% formic acid and 1% formic plus 1% acetic acid were most effective, destroying 84 and 73%, respectively, of the test cultures. Three microorganisms that were refractory to 2% acetic acid were usually killed by 2% formic acid. Addition of ascorbic acid to the sanitizer to reduce oxidation of the meat pigments resulted in lowered microbial counts. Color was not affected by addition of 1% ascorbic acid. With added 5% ascorbic acid, discoloration was noticeable but not extensive. There was no effect of pH on microbial destruction at the concentration of acid used, However, more microorganisms were killed as time of exposure to the acid was increased.

Milk Films Exposed to High Humidity: Studies with Electron Microscopy and Electrophoresis 1, 2

Stainless steel plates, which are similar to milk contact surfaces, were dipped in fresh raw milk. The residual film was dried (37 C and 10% to 20% relative humidity) for 30 min. Treated plates were then exposed to 100% relative humidity for 30 min at 37 C. Scanning electron microscopy revealed splotches of fat on surfaces of dried films and the humidified films had a more aggregated and porous appearance than films that were dried only. The incidence of granulated lactose was greater among humidified samples than among nonhumidified samples. Discontinuous polyacrylamide gel electrophoresis revealed that α- and β-caseins resisted rinsing from plates on which dried films were exposed to 100% relative humidity but not from plates on which films had been dried only.

A Method for Decreasing Sampling Variance in Bacteriological Analyses of Meat Surfaces 1, 2

Viable counts of bacteria are often high in some areas and low in adjacent areas of the same surface of fresh meat. The present study indicated that rubbing meat surfaces together before sampling reduces variation among bacterial plate counts of pieces of beef plate meat. Counts before rubbing ranged from 2 to 6,187/cm², whereas counts after rubbing ranged from 15 to 2,043/cm². The reduced sample variance allowed for fewer samples to be taken in studies of cleaning and sanitizing of fresh beef.

Factors Influencing Tenacity of Dried Milk Films Exposed to High Humidity 1, 2

Rinsability of milk films on stainless steel was impaired by exposure to 100% relative humidity (RH). Rinsability was determined by automated Lowry protein tests of detergent used to remove films. Residue of milk films was 1% of the initial soil load when dried on stainless steel plates without humidification, but was 6.35% of the initial load after drying (30 min), humidification (15 min) and redrying (30 min) all at 37 C. Three successive exposures to 100% RH for 7.5 min at 37 C, each followed by 30 min of drying, yielded a residue of nearly 30% of the initial soil load. Exposure at 37 C produced the maximum amount of residue on plates. Experimental temperatures ranged from 0 to 75 C. Temperature of milk applied to plates was of little importance. Raw milk formed more tenacious film than skim milk or major components of milk. Milk produced during colder months yielded less soluble films than milk produced during warmer months. Lowering of milk pH to 5.7, adding soluble calcium, and aging milk at 0 C increased residues. Chelation of soluble calcium with EDTA or dissociation of milk protein with sodium dodecyl sulfate decreased soil residue levels. Exposure of instantized nonfat dry milk to the high humidity treatment decreased its solubility more than tenfold.

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.

Group Comparative Study of VRB-2 Agar in the Recovery of Coliforms from Raw Milk, Ice Cream and Cottage Cheese

The method of Hartman et al. for recovering stressed coliform bacteria was evaluated in a group study. The experimental medium, VRB-2, was more productive than VRB medium by 31% for raw milk, 70% for ice cream and 61% for cottage cheese. Differences in colony counts of coliforms caused by incubating for 48 vs. 24 h and by boiling vs. autoclaving were not statistically significant. Among 40 samples tested in two laboratories, counts obtained at 30 vs. 32 C were not significantly different.

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 10³ Pseudomonas fragi , 10³ and 10⁴ Pseudomonas fluorescens , and between 10⁴ and 10⁵ Escherichia 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.

Changes of Color of Aqueous Beef Extract Caused by Pseudomonas fragi 1

Growth of Pseudomonas fragi had a significant (P < .05) effect on the color of aqueous beef extract stored at 1 ± 1 C. As numbers of P. fragi increased from log 5 to 7.9/ml during 10 days of storage at 1 ± 1 C, the pH increased from 5.5 to 6.0. At the end of 10 days there was a 76% loss of oxymyoglobin in samples inoculated with P. fragi . There was a 45% loss of oxymyoglobin in sterile-control samples. A possible mechanism for conversion of oxymyoglobin to metmyoglobin is suggested.

Combined and Individual Effects of Washing and Sanitizing on Bacterial Counts of Meat - A Model System 1, 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.

Experiments in Sanitizing Beef With Sodium Hypochlorite 1, 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.

Evaluation of a Rapid Test for Antibiotic Residue in Milk Using Spores of Bacillus stearothermophilus var. calidolactis 1

Eleven analysts tested contaminated reconstituted (1:10) dry milk powders for penicillin residues using spores of Bacillus stearothermophilus var. calidolacris (Delvotest P method). Three types of responses were noted: positive, negative, and questionable. Prediction equations indicated that 95% of the time, analysts unfamiliar with the technique could detect positive results if penicillin concentrations in samples were 0.010 unit/ml or higher and positive and questionable results if the penicillin concentrations were 0.008 unit/ml. Increasing the reconstitution ratio from 1/11 to 1/4 increased the chances of detecting penicillin in milk powder. Penicillinase added to reconstituted penicillin-contaminated milks in all instances produced negative responses.

Incidence and identification of phospholipase C-producing bacteria in fresh and spoiled homogenized milk

Bacteria which produced phospholipase C were isolated from 13 of 34 fresh and 15 of 35 spoiled samples of homogenized milk. No single off flavor was assigned consistently to samples with phospholipase producers, but 75% of them were bitter. Pseudomonads constituted 62% of the isolates. Other phospholipase C-producing genera and their numbers were Acinetobacter, two; Alcaligenes, three; Bacillus, two; Citrobacter, one; Enterobacter, three; and Flavobacterium, two. Two unidentified yeasts also were isolated.

Lecithin agar for detection of microbial phospholipases

Lecithin agar was developed on which phospholipase C produced turbid zones and phospholipase A produced clear zones. Reactions on lecithin agar agreed 74% of the time with reactions in egg yolk broth. On lecithin agar, interpretation was easier, phospholipase A was detectable, and opaque zones were visible 1 or 2 days earlier than on egg yolk agar. All constituents of the medium can be autoclaved.