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). / Graduation date: 1987
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/27217 |
| Date | 03 June 1986 |
| Creators | Isaza, Liliana Urrea |
| Contributors | Bodyfelt, F. W. (Floyd W.), 1937- |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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