The effect of reducing the fat content of low-moisture part-skim Mozzarella cheese from 19% to less than 5% on melted cheese properties, i.e., apparent viscosity, cheese melt, and cook color, was investigated. Functional properties of melt and stretch and cook color were evaluated at d 1, 7, 14, and 28. A rapid microwave oven method underestimated the moisture content of the low fat cheeses by approximately 10%. Low fat cheese did not melt as well as did the low-moisture part-skim Mozzarella cheese although the moisture content of the low fat cheese (moisture content ranged from 62.5% to 63.6%) was greater than the moisture content of the part-skim control (52.1 %). Storage for 28 d only marginally increased the meltability of low fat cheese. Lower fat content increased cook color. The amount of intact a αs-CN decreased by at least 48% in all cheeses as a result of proteolysis during 28 d of storage. The relative proportion of bound, entrapped, and expressible water was determined for a reduced-fat (8% fat) and control (19% fat) Mozzarella cheese on d 1, 7, 14, and 21 of refrigerated storage. Changes in the state of water were related to changes in cheese microstructure of a commercial Mozzarella cheese and to changes in cheese meltability of the control cheese. The amount of expressible water was proportional to fat content. Throughout storage, fat/serum channels became smaller and the protein matrix expanded into the areas between fat globules. The meltability of both cheeses increased during storage. Both cheeses contained 0.71g bound water/ g protein. Expressible water decreased in both cheeses until by d 21 no water was expressible. Entrapped moisture increased from approximately 10% to 60% for the control cheese and from approximately 33% to 50% for the reduced-fat cheese. An objective test was developed for measuring stretch, a characteristic of melted cheese. Three nonfat and four low-moisture part-skim cheeses were evaluated using the new test and the results compared with conventional test methods. Two new melted cheese parameters were defined: melt strength, the maximum load (g) obtained during the test, and stretch quality, the average load (g) as the cheese fibers stretched and elongated. Melt strength correlated with apparent viscosity. Stretch quality was determined for selected nonfat and low-moisture part-skim cheeses. A three-pronged probe was used to pull cheese vertically from a melted cheese pool. Use of this elongation stretch test, along with more traditional melted cheese tests, provides more complete information about the functional properties of Mozzarella cheese.

This study investigated the effect of cold, heat, or osmotic shock treatment on the resistance of L. lactis subsp. cremoris MM160 and MM310 and Lactococcus lactis subsp. lactis MM210 and FG2 cheese starter bacteria to freezing and freeze-drying. The ability to withstand freezing at -60°C for 24 h was variable among lactococci, but resistance to this treatment was significantly improved (P < 0.05) in most strains by a 2-h cold shock at l0°C or a 25-min heat shock at 39°C (L. lactis subsp. cremoris) or 42°C (L. lactis subsp. lactis). Stress treatments that improved lactococcal freeze resistance were also found to significantly (P < 0.05) enhance the resistance of most strains to lyophilization. Increased resistance to freezing or lyophilization was not detected when stress treatments were performed in broth that contained erythromycin, which indicated stress-inducible proteins were involved in cell protection. Membrane fatty acid analysis of stress-treated cells suggested that enhanced resistance to freezing and lyophilization may be related to heat or cold shock-induced changes in cell membrane composition. Heat-shocked cells had a higher 19:0 cyclopropane fatty acid content than did control cells, and cold-shocked cells contained a lower ratio of saturated to unsaturated fatty acids. Other factors must also be involved in cell protection, however, because similar changes in membrane composition were also detected in strains whose resistance to freezing and lyophilization was not improved by heat or cold shock.

Yogurts were made from intermediate-high temperature (100, 110, 120, and 130°C for 4 or 16 s), ultra-high temperature (140°C for 4 or 16 s), and vat heat (82°C for 20 min) treatments of skim milk fortified to 5% protein by either ultrafiltration or the addition of nonfat dry milk (NOM). Whey protein denaturation in heated milks increased with temperature and holding time from indirect plate heating and was highest in vat-heated milks. Whey protein denaturation and yogurt water-holding capacity increased with protein levels in the fortified milks compared to skim milk. Penetrometer gel strength and stirred viscosity in 21 day-old yogurt made from heated ultrafiltered skim milk exceeded those of yogurts made from NOM-fortified skim milk, even though the NOM yogurts contained more solids (13.0 vs 11.4%). Maximum gel strength and viscosity, and least syneresis of yogurts from ultrafiltered and NOM fortified yogurts occurred following intermediate-high temperature treatments of 1 00°C for 16 s, 110°C for 4 or 16 s, and 120°C for 4 s. There was significantly lower whey protein denaturation at these intermediate-high temperatures compared to UHT or vat heating. Gel strength and viscosity were lower and syneresis greater in yogurts from ultrafiltered or NOM-fortified skim milk following UHT treatment compared to yogurts made with intermediate-high temperature treatments or vat heating. The water-holding capacity of yogurts from fortified milks treated at intermediate-high temperatures was comparable to that of yogurts from vatheated milks. Fortification by ultrafiltration, to lower total solids (and without use of stabilizers) resulted in yogurt with higher gel strength and viscosity, and reduced syneresis compared to yogurt from NOM fortification. Yogurt prepared by intermediate-high temperature treatment had comparable or better gel strength and viscosity, and reduced syneresis compared to yogurt prepared by traditional vat heating.

Recent studies suggest aromatic amino acid catabolism by starter lactococci and flavor adjunct bacteria have a significant impact on off-flavor development during Cheddar cheese ripening. We hypothesized that a flavor adjunct bacterium, Brevibacterium linens BL2, produces off-flavor compounds from aromatic amino acid metabolism that will have a detrimental impact on cheese flavor. The mechanism of tryptophan (Trp) catabolism in Brevibacterium linens BL2, was investigated in a chemically defined medium during incubation in laboratory conditions (no carbohydrate, pH 6.50, 220 rpm, 25°C) and cheese-like conditions (no carbohydrate, 4% NaCl, static incubation, l5°C). In laboratory conditions, metabolic studies and enzyme assays confirmed that Trp was converted to kynurenine and anthranilic acid. However, cells incubated in cheese-like conditions did not utilize Trp, indicating that these enzymes are not likely to be involved in formation of Trp compounds associated with off-flavors in Cheddar cheese. In an attempt to verify the metabolic activity of the cells during incubation by monitoring the amino acid metabolism in chemically defined medium inoculated with B. linens BL2, a capillary electrophoresis-laser-induced fluorescence method was developed that could separate, detect, and quantitate 18 amino acids within 38 min. The data indicated that B. linens BL2 was metabolically active. Presumably, the cells will be metabolically active and metabolize amino acids in cheese as well. The ability to determine the Trp metabolic activity of B. linens BL2 in cheese, and to quantify Trp catabolic compounds in cheese during ripening, requires a quantitative extraction procedure. An analytical method was developed to extract and quantify aromatic amino acids and Trp catabolites from cheese using capillary electrophoresis. Methanol was used to extract Cheddar cheese made with Lactococcus lactis S3 alone and in combination with B. linens BL2 to quantitatively determine the influence of BL2 on the occurrence of aromatic catabolites. All cheeses contained aromatic amino acids, indole acetic acid, and indole. The concentration and time taken for development of these compounds were significantly decreased or delayed by the addition of B. linens BL2. After 6 months of aging, the concentrations of Trp catabolites were significantly lower in cheese made with B. linens BL2. Addition of BL2 did not directly contribute to off-flavors derived from Trp catabolism in Cheddar cheese. Therefore, the hypothesis was rejected.

Pasteurized skimmilk at 4°C was acidified to pH 5.8 with 85.5% phosphoric acid (136g H3Po4;100 kg skimmilk), then warmed to 54°C and ultrafiltered to a protein concentration 9.1 ± 0.2%. The retentate was heated to 76.5°C for 16 s then cooled to 2°C. Phosphoric acid (85.5%) was added at a rate of 3.41g per kg retentate. The acidified retentate was slowly warmed to 29.5 °C (3°C/5 min) when the pH was checked. The pH at this point was no lower than 5.4. Heating was continued until a temperature of 32.2°C was reached. Glucono delta lactone was added to the retentate (17.6 g/kg retentate) and left undisturbed for approximately 80 min. The curd was cut at pH 4.7 with 0.64 cm curd knives and allowed 10 min for syneresis. Permeate obtained from the same lot of milk was acidified to pH 4.8 (66 g H3Po4;100 kg permeate), then added to the curd at 32.2°C (three parts permeate to four parts retentate) and used as a cooking vehicle. The curd was cooked to 59°C in 90 min. The curd was held at 59°C for 10 min, drained and washed once with ice water. Cream dressing containing 12.5% fat and 3% salt was used at the rate of two parts curd to one part dressing. Control cottage cheese was produced by a direct acid method from the same skimmilk used to produce ultrafiltered curd. Use of ultrafiltered skimmilk retentate for cottage cheese making resulted in 2.24% more curd (corrected to 20% solids) and 2.24% more curd per kg original milk protein than the control. However, satisfactory firmness in UF curd required slightly more than 20% solids in the final product. Sensory evaluations indicated that creamed cottage cheese was not significantly different (p

The purpose of this research was to investigate the effect of ultra-high temperature process parameters on age gelation of milk concentrate. Skim milk was concentrated to 2X (volume reduction) using reverse osmosis. The milk concentrate was preheated at 75 or 90°C for 20 or 50 s and UHT-processed at 138 or 145°C for 4 or 16 s. Sterilizing methods used were direct steam injection and indirect plate heat exchanger. The samples were aseptically collected in presterilized plastic containers and stored at 15 or 35°C. At 15°C storage temperature, the steam-injected samples gelled in 5 months when 4 s UHT time was used. When UHT time was increased to 16 s, the samples gelled in 6 months. Of the samples that were UHT processed by indirect plate heat exchanger for 4 s and stored at 15°C, all gelled after 7 months. When UHT time was increased to 16 s, all the 138°C samples gelled after 7 months as did the samples that were preheated for 50 s and UHT-processed at 138°C. The samples preheated at 75°C for 50 sand UHT-processed at 145°C gelled after 8 months, whereas at 90°C preheat temperature the samples gelled after 9 months. The samples stored at 35°C did not gel but showed different sedimentation levels. The sediment depth in the container was always greater for the steam-injected samples. The samples that received higher heat treatments by the two processing methods had a higher sedimentation depth. The pH decreased during storage and the extent of reduction was higher at 35°C storage temperature . Maillard browning occurred at both storage temperatures. Browning was greater in samples stored at 35°C and processed by indirect plate heat exchanger.

Commercial microbial milk clotting enzyme preparations were purified by immunoaffinity chromatography using purified antibody covalently coupled to porous glass beads as the column matrix. Commercial enzyme preparation diluted in 1 mM sodium acetate buffer at pH 5.0 was then biospecifically adsorbed to the column matrix by end-over-end mixing of the glass-antibody complex in the enzyme solution for 12 h at 5°C. The antibody bound enzyme adsorbed glass beads were soaked in .2 M glycine or ethanolarnine at pH 7.0 to block uncoupled reactive sites on the matrix. Following this, the column was washed with 1 mM sodium acetate buffer at pH 7 .0, followed by additional wash with .5 M NaCl, until absorbance at 280 nm returned to baseline. Elution of adsorbed enzyme was achieved with .2 M sodium acetate at pH 3.0, .2 M acetate at pH 3.5, containing .15 M NaCl and .5 M acetate at pH 4.0 containing .5 M NaCL At the same protein concentration, immunoaffinity chromatography purified enzymes had higher clotting activity than the commercial enzyme preparations. Amino acid analysis and OPA proteolysis tests of TCA soluble peptides liberated from casein hydrolysis showed purified enzymes to exhibit lower general proteolytic activity. Immunological reactivity of Mucor enzymes with calf rennet was determined with antibodies produced by intramuscular injections of Mucor miehei protease, Mucor pusillus protease and calf rennet emulsified in Freund's adjuvant into three New Zealand White rabbits . Harvested antisera were heated at 56°C for 30 min to inactivate complement factors and contaminating proteins then centrifuged at 1700 x g for 30 min. Ouchterlony double immunodiffusion method was used to test for presence of antibodies in the antisera, and for cross immunoreactivity. Antibodies against M. miehei were cross reactive with M. pusillus antigen and M. pusillus antibodies cross reacted with M. miehei antigen. Immunodiffusion assay did not show cross reactivity of calf rennet antibodies with either M. miehei antigen or M. pusillus antigen. Antibodies against the Mucor enzymes did not show cross reactivity with calf rennet antigen. Although their actions in milk differ, proteolytic enzyme preparations from M. miehei and M. pusillus are both used as calf rennet substitutes in cheese manufacture. Differences in the characteristics of the two Mucor enzyme preparations exist, even though they exhibit some immunological homology . From our results, at least one antigenic factor is common to both enzyme preparations.

This study was undertaken to determine the effect of oxygen absorber packets (OAP) (AGELESS Z.300 E) in improving the shelf-life of selected dried food products, including dried potato pearls, dried sliced apple, dried carrot, white rice, nonfat dry milk, all purpose flour, rolled oats, and wheat. Items were stored at four different temperatures (-20, 10, 21, and 29.5°C) over a period of 24 months with observations at 6-month intervals. Hunter colorimeter, thiobarbituric acid reactive substance (TBARS) germination rate, gluten weight, and volume were parameters used as quality indicators of various dried foods. Statistical analysis indicated a significant time and temperature effect for almost all the stored products. Storage at a high temperature (29.5°C) renders OAP ineffective in reducing browning during storage. Potato pearls, dried sliced apple, and dried carrot exhibit darkening by the first 6 months of storage at 29.5°C regardless of oxygen status of the container. White rice showed yellowing under the same conditions. Most of the dried food products that were stored at 10 and 21°C with or without oxygen absorber packets maintained their color as indicted by a relatively constant lightness reading (L*) over time. TBARS values rose in all stored food with increasing storage time, especially when the dried products were stored at higher temperature (29.5°C). Statistical analysis indicated a significant OAP effect in reducing TBARS concentration over time. Germination of wheat kernel was reduced to less than 80% after 24 months of storage at 29.5°C. However, when stored with oxygen absorber packets, germination was still 86%. Storage length and temperature are the primary factors that determine the quality of home-stored dried food items.

Clostridium sporogenes PA 3679 spores were inoculated into a meat-vegetable mixture before extrusion, cooking, and vacuum packaging into "stewsticks" to simulate Clostridium botulinum growth. The experiment was a 3 x 5 x 2 x 3 factorial which determined the influence of pH, water activity, initial spore load, and storage period on spore survival. Spore levels decreased throughout storage for all treatments. Spore levels decreased linearly (P = 0.02) as water activity increased, in samples that were heated to kill vegetative cells and activate spores. Other significant interactions of heat-treated samples were observed with inoculum level (P < 0.01) and storage time (P < 0.01). Spore levels in stored products were also significantly affected by water activity* inoculum level (P = 0.03), pH * time, water activity* time (P = 0.01), inoculum level * time (P < 0.01), and water activity * inoculum levels * time (P < 0.01). The interaction between pH * water activity * time tended towards significance (P = 0.06). Most probable number estimates in nonheated samples accounted for naturally occurring viable cells and spores, and added spores and were significantly affected by the main effects of inoculum level (P < 0.01) and time (P < 0.01). The two-way interactions of water activity * inoculum level (P = 0.04), pH * inoculum level (P < 0.01),water activity * time (P < 0.01), and three-way interaction of pH * inoculum level * time (P = 0.03) were significant. Spore levels approached 102, or less (compared to an inoculum level of 106 spores per gram) due to the effects of many treatments. Some stewstick packages were observed to become "gassy" or "loose" during storage. Subsequently the stewstick packages were used to isolate microorganisms that were able to grow at water activities of 0.96-0.86, in glycerol-adjusted Rogosa agar, and were acid tolerant to pH 4.4-4.2. One produced gas in pure culture, and some produced indole. These bacteria were not destroyed by heating to 74°C for at least 30 minutes, and lowered the pH in the stewstick during storage. In conclusion, in all stewstick samples, regardless of pH or Aw, inoculated clostridial spore levels decreased during storage, apparently because spores germinated and vegetative cells subsequently died. Thus, if stewsticks are cooked to 74°C throughout, have a Aw ≤ 0.86 and pH ≤ 4.8, they appear to be safe.

This research project was divided into three parts. In the first part, heme, nonheme, and total iron methodologies for meats were evaluated. The accuracy, precision, and specificity of each method were determined by spike recoveries of heme and nonheme iron, and by analysis of National Institute of Science and Technology standard reference materials. The most reliable and practical methods were then used to determine the total, nonheme, and heme iron contents of various meats before and after cooking. The meats analyzed were beef, pork, lamb, chicken, and turkey. The wet-ashing technique was a novel procedure in which nitric acid was used to digest most of the solids followed by peroxy-monosulfuric acid to complete the digestion. Total iron values of the meats were consistent with those previously reported, but the percentage of heme iron in red meats was much greater than commonly assumed, both before and after cooking. In the second part, the distribution of heme and total iron in heat-processed poultry products was investigated using light and dark chicken meat in the form of deep-flied chicken breasts and legs purchased from fast food restaurants and grocery stores in a ready-to-eat condition. Heme and total iron values were 1.7 ± 0.5 and 6.5 ± 2.0 μg Fe/g meat for light chicken meat and 7.6 ± 1.6 and 19.3 ± 2.2 μg Fe/g for dark chicken meat. Percent heme iron values averaged 29 and 40% for light and dark chicken meat, respectively. In the third and final part, an application for the heme and nonheme iron data assembled above was developed to give dieticians an important tool in dietary formulations designed to maintain iron homeostasis. From the data it is evident that cooked light chicken meat, taken from the breast, would provide the lowest quantity of absorbable iron among the meats investigated and that ground beef, highest in heme iron, would provide the greatest quantity of bioavailable iron. Additional research was performed on processed beef products. Cooked ground beef, frankfurters, beef steak, and roast beef were analyzed for heme and total iron. The different beef products contained similar amounts of total iron, 31.4 to 34.2 μg/g, but the heme iron content ranged from 6.2 μg/g in frankfurters to 36.3 μg/g in beef steak. Percent heme iron ranged from 33.0 to 63.8% in all meats. Total iron, heme iron, and percent heme iron varied significantly (P < 0.01) among meats, sources, and preparations. This research was published and has been reproduced in Appendix F.