Three separate batches of ground beef containing 25% fat were extended to 0, 10, 20 and 30% by weight with rehydrated defatted textured soy protein (TSP). Five samples of each soy level were wrapped in a polyethylene oxygen-permeable film and stored at 5°C for 4 days. Samples were analyzed daily for changes in bacterial numbers (total plate counts and coliforms), pH, titratable acidity, proximate analysis, ninhydrin reactive material, carbohydrate content, and gas liquid chromatographic (GLC) analysis of individual amino acids to assess spoilage in TSP extended ground beef. Total bacterial numbers increased significantly as days of storage progressed and in proportion to added TSP. Coliforms increased during storage, but not proportional to increased levels of soy protein. Spoilage in soy extended samples, as determined by off-odors and bacterial numbers, was evident by day 4 as total bacterial counts approached 10" bacteria/g. The differences of pH were significant among soy levels, but did not change appreciably during storage. Titratable acidities were not significantly changed with increases in soy protein. As soy level increased, percentages of moisture and ash increased, but percent fat decreased. No differences were noted for percent protein. Total carbohydrate as determined by proximate analysis increased with TSP levels. Concentration values for carbohydrates were proportionately larger with added amounts of TSP, but varied little during 4 days of storage. Ninhydrin reactive material differed significantly among samples, but not proportionate to added TSP, and decreased in the 30% soy level between days 3 and 4. Decreases in individual amino acids could not be shown from the qualitative results obtained by GLC analysis. Further investigation is suggested using more definitive measuring techniques to determine the factor(s) causing accelerated spoilage in soy extended ground beef.

Ground beef and ground beef extended with 20% textured soy I protein (TSP) were examined to determine the predominant bacterial flora present during storage at 4'C for 6 days. Bacterial numbers in the extended ground beef increased with time and at a faster rate than the nonextended product. Spoilage odors were also detected earlier in the TSP containing ground beef. For both extended and nonextended ground beef, Pseudomonas species were the predominant bacteria of fresh and stored samples. By the sixth day of storage at 4"C, the only psychrotrophs recovered were species of Pseudomonas. Considering these similarities, it appears that differences in the spoilage rate of extended and nonextended ground beef cannot be attributed to differences in the composition of their respective bacterial flora.

Bacterial isolates from soy extended ground beef were used to test the effect of pH on their growth in trypticase soy broth (TSB), ground beef, and ground beef + 20% textured soy protein QSP). Growth was measured in TSB acidified with hydrochloric, citric, lactic, or acetic acids at pH levels from 5.4-6.2. After incubation at 4°C for 7 days, acetic acid was determined to inhibit growth to the greatest extent with no growth at or below pH 5 8. Growth response was studied in meat blends at pH levels of 5.4, 5.7, 6.0, and 6.3. Growth was suppressed in ground beef initially at pH 5.4 and in ground beef + 20% TSP initially at pH 5.4 and 5.7. This resulted in an increased shelf life of 2 days in acidified blends.

The objective of this study was to determine the influence of five separate levels of textured soy protein (TSP) on growth of psychrotrophs, mesophiles, coliforms, Staphylococcus aureus, and fecal streptococci in soy-extended ground beef stored at −16°, 0° and 6°C. Highly significant increases in psychrotroph and mesophile counts accompanied increased levels of soy at 0° and 6°C, but not at −16°C. Soy-extended beef samples containing 20 and 40% TSP spoiled one day faster at 6°C and four days sooner at 0°C than non-extended ground beef. No significant differences in coliform, fecal streptococci or S. aureus counts could be attributed to increasing levels of TSP in extended ground beef at −16°, 0° or 6°C. Protein content did not vary significantly with TSP concentration; however, fat decreased as soy level increased. Moisture and carbohydrate-ash content increased significantly as soy level increased, as did pH which reached a maximum of 6.5.