Assessment of bacterial quality and shelf life pasteurized milk

Abstract

In the present study raw and pasteurized milk samples were collected from two processing plants viz., DP1 and DP2 and pasteurized milk from retail shops. A total of 254 samples were analyzed for the bacterial quality by estimating various bacterial counts and also assessed the presence of certain bacteria of public health importance. The bacterial, physical and organoleptic qualities of pasteurized milk samples from two dairies stored under refrigeration (4 ± 1oC) were evaluated. Raw milk revealed an inferior bacterial quality with 50 per cent samples graded as fair (based on total viable count) and 85.7 per cent as poor quality (based on coliform count). The total viable count from both dairies was obtained at the level of 7 log10 cfu/ml but coliform count was high in the samples obtained from DP1 (3.34 ± 0.05 log10 cfu/ml). The psychrotrophic count and faecal streptococcal count in the samples belonging to both sources were at the level of 7 and 3 log10 cfu/ml, respectively. Bacteria of public health significance like Escherichia coli, Staphylococcus aureus and Pseudomonas was detected from a few samples. Pasteurization reduced the level of total viable count, coliform count, psychrotrophic count and faecal streptococcal count to a highly significant (P<0.01) level. Pasteurized milk under refrigeration (4 ± 1oC) showed an increase in total viable count and psychrotrophic count throughout the storage period with a difference of more than 3 log with that of fresh sample. However, coliform count, Escherichia coli count, and faecal streptococcal count of samples belonging to DP1 initially showed increasing tendency up to six days and thereafter the counts decreased. The increase in total viable count, coliform count, Escherichia coli count, psychrotrophic count and faecal streptococcal count between zero and 10th day from DP2 was 4.8, 1.95, 2.08, 4.78 and 2.32 log10 cfu/ml, respectively. The increase in the counts during storage may lead to the reduction in shelf life due to bacterial deterioration of milk. Isolates of Escherichia coli was obtained from DP1 on all days except eighth and 10th day. A total of six isolates were obtained from DP2. The isolates belonged to O116 (3), O22, O46, O65 (2), O95 and the rest were rough variety. Staphylococcus aureus was also isolated from two samples stored on sixth day and three from the samples stored on zero, second and fourth day, respectively (DP1). From DP2, three isolates were obtained from the samples stored on 10th day and one from fresh samples. A total of 22.62 and 20.24 per cent Pseudomonas were isolated from DP1 and DP2, respectively and the isolates were identified as Pseudomonas putida, Pseudomonas aeruginosa and Pseudomonas flourescens. Sensory and physico-chemical (COB test) analyses of refrigerated milk samples showed an overall reduction in the score of colour and appearance, flavour, odour and body as the storage period increased. The mean total scores from DP1 revealed that the samples were of excellent quality for up to second day of storage. The sensory quality of the samples stored on fourth day was good and then the quality of milk remained fair till eighth day and on 10th day the quality became poor. In DP2 samples had excellent quality for upto second day of storage. The sensory quality of the sample stored up to sixth day was good and thereafter the quality of milk remained as fair till the end of storage period. COB test of samples from DP1 showed positive test on all samples stored on 10th day. However, one sample stored on day six was COB test positive. The samples belonging to DP2 showed that three samples stored on 10th day and one sample stored on eighth day was COB positive. The bacterial profile of the retail milk samples of the brands A, B, C, D, E and F was assessed and the samples belonging to the brand D had highest mean total viable count (5.94 ± 0.09 log10 cfu/ml), psychrotrophic count (5.09 ± 0.16 log10 cfu/ml) and faecal streptococcal count (2.87 ± 0.24 log10 cfu/ml). Highest coliform count was seen in the samples of brand A (2.40 ± 0.14 log10 cfu/ml) and Escherichia coli count (3.44 ± 0.72 log10 cfu/ml) in samples of the brand C. Low counts especially total viable count (4.89 ± 0.79 log10 cfu/ml) and coliform count (1.19 ± 0.42 log10 cfu/ml) were seen in the samples of the brand F. Escherichia coli were detected from 20.8 per cent samples and the isolates consisted of the serotypes O46, O65, O95, O116, O166 and O171. Out of 15 isolates obtained six showed a positive congo red reaction indicating their property of invasiveness. Staphylococcus aureus was isolated from only six samples (6.94 per cent). All retail milk samples were also tested for the isolation and identification of Pseudomonas and the organism was isolated from 16 (22.22 per cent) samples. The isolates were identified as Pseudomonas putida (7), Pseudomonas aeruginosa (6) and Pseudomonas flourescens (3). Polymerase Chain reaction was employed to identify and confirm the Escherichia coli isolates obtained from the milk samples and a 366 bp product was obtained.