Graduate Thesis Or Dissertation
 

Rapid methods for the determination of post-pasteurization contamination of fluid milk and shelf-life prediction

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  • Several methods for estimation of the potential shelf-life of pasteurized fluid milk products were evaluated for their efficacy in this investigation. These methods were evaluated and compared to sensory, biochemical and bacteriological indices through a series of experiments conducted on different brands of commercially pasteurized fluid milk. The methods evaluated included: Standard Plate Count (SPC), Psychrotrophic Bacteria Count (PBC), Modified Psychrotrophic Bacteria Count (MPBC), the Moseley keeping quality test (MKQT), Parmelee tube test (PTT), tetrazolium salt-resazurin test (TRT), modified Parmelee tube test (MPTT), and p-iodonitrotetrazolium violet-phenazine methosulfate test (INT-PMS). Several different conditions of preliminary incubation (PI) were attempted in an effort to accelerate outgrowth of psychrotrophic bacteria and hence obtain sufficient numbers and metabolic activity to reduce the redox potential indicator dye. Correlation coefficients (r) and chi-square (χ²) values were obtained in an attempt to detect significant relationship between the parameters studied and the potential shelf-life of the product. Results suggested that the PTT, TRT and MPTT tests were not reliable predictors of the potential shelf-life of pasteurized milk (r values between -0.445 and 0.734, non-significant P>0.05). The INT-PMS Test at 21°C for 20 minutes following PI at 21°C for 25 hours provided the best estimate of the potential shelf-life of pasteurized whole milk (r= -0.840). This method shows some potential as a method for determining post-pasteurization contamination: it was accurate (92.3%), rapid ( <26 hours), simple, inexpensive (4.54 to 9.64 cents/sample), and sensitive (it was able to detect less than 1 PBC/ml and less than 5.0 x 10¹ total CFU/ml in fresh milk if bacteria were able to reach 1 PBC/ml and 1.0 x 10³ total CFU/ml during PI). However its accuracy could be significantly affected by the intensity of the pasteurization heat treatment given to the milk due to possible denaturation of the whey proteins and release of heat activated reducing substances (-SH groups).
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