| Summary: | This research was undertaken because of the need to develop an objective method
for quality control of milk. A major problem in the dairy industry is off-flavour often
found in milk. Quality control of milk is heavily dependent upon sensory evaluations
supported by microbiological and chemical analyses. The chief purpose of this research
was to demonstrate a simple and economical system for quality control of milk using a gas
chromatogf aph and computer-aided data processing.
Two experiments were conducted: one using microbial off-flavours and another
using chemically induced off-flavours. First, Ultra High Temperature (UHT)-sterilized
milk was inoculated with Pseudomonas fragi, Psuedomonas fluorescens, Lactococcus
lactis, Enterobacter aerogenes, Bacillus subtilis and a mixed culture (L. lactis:
E. aerogenes: P. fragi = 1:1:1) with approximately 10⁴ CFU mL⁻¹. The samples were
stored at 4°C up to 10 days for P. fragi and P. fluorescens and at 30°C up to 24 hours for
L. lactis, E. aerogenes, B. subtilis and the mixed culture. Several multivariate analyses
were applied to the standaridized peak areas of GC data. A new multivariate analysis
technique, principal component similarity analysis (PCS), was capable of classifying milk
samples with regard to bacterial species and storage time. Artificial neural networks
(ANN), partial least squares regression analysis and principal component regression analysis were also applied. ANN provided the most accurate means of classification.
Secondly, pasteurized milk was treated to develop different off-flavours (lightinduced,
oxidized, cooked and heated) according to the procedures of the American Dairy
Science Association. The same pasteurized milk samples as those used for gas
chromatographic analysis were used for sensory evaluation. Gas chromatography (GC)
combined with PCS was more effective than sensory evaluation as a means of
distinguishing milk samples.
It was concluded that a combination of GC and chemometric methods may have
great potential in evaluating the chemical and microbial quality of milk.
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