Abstract
Rework is a common practice used in the dairy industry as a strategy to help minimize waste from processing steps or errors that might otherwise render the product unsaleable. Dairy processors may rework their high-temperature, short-time (HTST) fluid milk products up to code date (21 d) at a typical dilution rate of ≤20% rework into ≥80% fresh raw milk. Bacterial spores present in raw milk that can survive pasteurization and grow at refrigeration temperatures are often responsible for milk spoilage. However, the potential impact of growth and thermal resistance of organisms in reworked product has not been investigated. Our objective was to characterize growth, sporulation, and thermal resistance of Paenibacillus odorifer under conditions representative of extreme storage conditions (time and temperature) of reduced fat (2%) and chocolate milk to evaluate whether product containing rework would have a reduced shelf life. Commercial UHT-pasteurized 2% milk and chocolate milk were independently inoculated with 4 strains of P. odorifer at 1 to 2 log cfu/mL and stored at 4°C and 7°C for 30 d. Changes in P. odorifer cell densities were determined by standard serial dilution with spread plating on tryptic soy agar with yeast extract and incubation at 25°C for 48 h. Spore counts were determined following thermal treatment at 80°C for 12 min. Thermal resistance of a cocktail of P. odorifer in milk was determined after treatments at 63°C for 30 min and 72°C for 15 s. Strains of P. odorifer grew rapidly at 7°C and reached a maximum cell density of ~8 log cfu/g in both 2% and chocolate milk within 12 d. All strains grew more slowly at 4°C and had not reached maximum cell density by 21 d. With extreme temperature abuse (25°C, 24 h), P. odorifer will sporulate in milk; however, thermally resistant subpopulations, including spores, did not develop in milk at 4°C until after stationary phase was achieved (>24 d). Vegetative cells of P. odorifer were verified to be sensitive to pasteurization (>7 log reduction); therefore, P. odorifer would not be expected to contribute to reduced shelf life of fluid milk products containing rework, even with extended storage before rework.