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Title: Effects of low temperature storage and thermisation on the quality of raw and heat treated milk
Author: Manap, Mohd. Yazid B. Hj. Abd.
ISNI:       0000 0001 3617 8618
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1988
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On-farm milk quality studies: 1.0 Three surveys were conducted approximately at three months apart to evaluate the quality of raw milk produced from dairy farms in the south-west of Scotland. The farms assigned to the study were within the scheduled route of the three road tankers supplied by the local bulk milk haulage contractor of the Scottish Milk Marketing Board. The sequence of milk collection by each road tanker during the first survey was noted and the sequence was repeated in the subsequent surveys. The number of farms taken to fill each road tanker to full capacity, limits the number of farms investigated. A total of 24 farms were included in the investigation. 1.1 The results of the investigations showed that the bacteriological quality of raw milk sampled from farms in the present study gave little cause for concern. The means of total plate count (9.8 x 10³ cfu/ml), psychrotrophic bacteria count (2.2 x 10³ cfu/ml) and thermoduric bacteria count (22 cfu/ml) compared favourably with earlier studies (Smillie et al., 1958). No seasonal variations in bacterial counts were noted, but slight variations between farms were observed. 1.2 The temperature of milk taken from the farm bulk tank immediately before pick-up varied highly between the farms and seasons studied. Farms milk temperature of more than 6°C was not uncommon and were frequently noted especially from farms with higher milk production. 1.3 The variations between farms and seasons in freezing point depression (-0.541 °C) and pH (6.72) of raw milk were slight, however, large seasonal variation in the percentage lactic acid content was observed. The percentage lactic acid was higher in December (0.154) than in March (0.137) or June (0.129). The mean free fatty acids content of milk sampled from the farms studied were 0.64 mEq/100 ml milk. No significant variations between farms and season were observed. Simulated bulk milk silo studies: 2.0 A simulated study was conducted to measure the effects of blending and subsequent storage of raw milk at low temperatures on the quality of pasteurised milk made from it. Raw milk collected by the three road tankers from farms in the south-west of Scotland were delivered to the West of Scotland College and a proportion from each tanker was blended in a 1200 litres storage tank. The percentage of milk used for blending from each road tanker (44, 28 and 28 per cent) was calculated based on the capacity of the road tanker and the final volume of the blended milk. The blended milk was then divided into 2 lots. Each lot was stored for 9 days either at 2°C or 5°C. The milks were subsequently pasteurised after 2, 4 and 7 days of raw storage. 2.1 The mean bacterial counts of milk sampled from the road tanker was slightly higher than the mean bacterial counts of the corresponding milk sampled on the farms. This was expected, the increase in handling, increased the surface area (potential source of contamination) that comes in contact with the milk. In addition, higher milk temperature during transport ( ≥ 8°C), the possible break-up of bacterial clumps through agitation during handling and the mixing of raw milks from various sources, all contributed to the higher counts of milk sampled from the road tanker. No seasonal variation was observed. 2.2 The mean total plate count and psychrotrophic bacteria count of the blended raw milk following storage at 2°C was consistently lower than that stored at 5°C. The differences in the bacterial counts were significant 2 days following storage and continued throughout the storage period. Taking the total plate count or psychrotrophic bacteria count of more than 10⁶ cfu/ml as the standard for deterioration of raw milk quality to occur, storage of raw milk at 2°C was found to increase its keeping quality by one day as compared to raw storage at 5°C (4 days vs 3 days). The thermoduric bacteria count was unchanged throughout both storage temperatures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Food technology & food microbiology