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The comparison of bacterial content in a range of milks.

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Yasmin White 1210 The comparison of bacterial content in a range of milks Introduction Milk is a nutritious food. It is an excellent source of calcium, phosphorus, riboflavin, and vitamin D and a good source of protein, vitamin A, potassium, and several B vitamins. These constituents differ widely in molecular size and solubility, therefore milk is a complex physiochemical system. The enzymes found in cow's milk are: protease, amylase, lactase, lipase, xanthine oxidase, phosphatase, aldolase, catalase, and peroxidase. It's the lipase here that can cause hydrolytic rancidity. The smallest molecules, those of salts, lactose, and water-soluble vitamins, are in 'true solution'. The proteins, including the enzymes, are in the colloidal state (suspension in the liquid) because of the large size of their molecules. Lactose (milk sugar) is a white crystalline disaccharide, it has the same molecular formula as sucrose (C12H22O11) but it differs in structure, making it an isomer. It contains the simple sugars, glucose and galactose. Galactose is catalysed by lactase. When milk sours, the lactose is converted by bacteria to lactic acid, this causes a change in the milk's consistency. Milk is often naturally contaminated with bacteria. Lactobacillus and Streptococcus Lactis (the two most common lactic acid bacteria) produce lactic acid during fermentation: Bacteria: Bacteria are very small organisms, each being a living cell. Most bacteria are between 0.0005mm to 0.002 mm long and they rarely exceed 0.01mm in length. Whether or not the bacteria move, all must respire; most use oxygen, respiring aerobically, while others, including several of the pathogenic or disease-causing bacteria, respire anaerobically. Anaerobic respiration: Anaerobic respiration is the release of energy from food material by a process of chemical breakdown that does not require oxygen. The food e.g. Carbohydrate, is not broken down completely to carbon dioxide and water but to intermediate compounds such as lactic acid or alcohol. C6H12O6 ? ?CO2 ? ?C2H3OH ? (118KJ) Some bacteria derive all their energy from anaerobic respiration, and the end products are frequently lactic acid; the processes know as fermentation. ...read more.


0.0351 0.0360 0.0585 55 0.0576 0.0396 0.0405 0.0630 75 0.0468 0.0927 0.0495 0.0801 80 0.0720 0.0450 0.0270 0.0540 100 0.0675 0.0603 0.0360 Unspecified 105 0.0630 0.0540 0.0450 Unspecified (Graph overleaf) Analysis of the results The results show a number of conclusions. To begin with, the raw milk seems to have proved my hypothesis accurately. The levels of lactic acid started at 0.0225cm� and rose in a relatively steady rate up to a level of 0.0270cm� after just five hours of exposure to room temperature. It then leveled out at this amount for a further 20 hours. These results suggest that the first stage of growth was happening- the lag phase-the cells are active but there is little growth as they are taking up water and starting to produce enzymes. The next phase of growth is outlined after 25 hours of exposure-the exponential (or log phase)-the population increases rapidly-from just 0.0207cm� of lactic acid present in the milk, to 0.0585cm�-a total increase of 0.0378cm� in just 25 hours. Surprisingly it seems the bacteria endure another lag phase-the cells are still active but again there is little growth. Without looking at the results following these, you might assume that the bacteria are progressing towards the stationary phase-bacterial cells are dying more or less at the same rate, as they were produced-however looking at the results at 75hours-80hours it seems that this stage is highlighted here. After 50 hours the levels of lactic acid, production seems to decrease-only a slight increase of lactic acid content-from 0.0585cm� - 0.0801cm� (an increase of 0.0216cm�). If you compare this to the increase of lactic acid content from a similar time difference there is an obvious decline in production, for example between 25 hours and 30 hours there is an increase of content of 0.00450cm�-almost double the rate of production. Finally, the levels of lactic acid after 55hours suggest that the bacteria are now progressing through the second exponential phase (log phase)-an increase from 0.0630cm� to 0.0801cm� in just 20 hours. ...read more.


0.0360 0.0360 0.0360 Raw Hours exposed to the air: 100 Amount of lactic acid present in the milk: g Type of milk Trial one Trial two Trial three Average Whole Pasteurised 0.0630 0.0630 0.0630 0.0630 Semi-skimmed Pasteurised 0.0540 0.0540 0.0540 0.0540 UHT 0.0450 0.0450 0.0450 0.0450 Raw Hours exposed to the air: 105 Results of 'experiment to investigate the relationship between bacterial content and pasteurization' Cooled for 1 minute Amount of NaOH needed to titrate the milk/ml Time the milk was heated for/mins Temperature the milk reached/ ?c Test 1 Test 2 Test 3 Average 0 24 2.70 2.70 2.70 2.70 1 32 2.50 2.50 2.50 2.50 2 50 2.30 2.30 2.30 2.30 3 57 2.20 2.30 2.30 2.30 4 61 2.00 2.00 2.00 2.00 5 80 1.50 1.50 1.50 1.50 6 105 1.50 1.40 1.40 1.40 Cooled for 1 minute Amount of lactic acid present in the milk/ml Time the milk was heated for/mins Temperature the milk reached/ ?c Test 1 Test 2 Test 3 Average 0 24 0.0243 0.0243 0.0243 0.0243 1 32 0.0225 0.0225 0.0225 0.0225 2 50 0.0207 0.0207 0.0207 0.0207 3 57 0.0198 0.0207 0.0207 0.0207 4 61 0.0180 0.0180 0.0180 0.0180 5 80 0.0135 0.0135 0.0135 0.0135 6 105 0.0135 0.0126 0.0126 0.0126 Cooled for 2 minutes Amount of NaOH needed to titrate the milk/ml Time the milk was heated for/mins Temperature the milk reached/ ?c Test 1 Test 2 Test 3 Average 0 24 2.70 2.70 2.70 2.70 2 55 2.30 2.30 2.30 2.30 4 75 1.90 1.90 1.90 1.90 6 90 1.50 1.40 1.40 1.40 8 110 0.90 0.90 0.90 0.90 Cooled for 2 minutes Amount of lactic acid present in the milk/ml Time the milk was heated for/mins Temperature the milk reached/ ?c Test 1 Test 2 Test 3 Average 0 24 0.0197 0.0197 0.0197 0.0261 2 55 0.0167 0.0167 0.0167 0.0167 4 75 0.0139 0.0139 0.0139 0.0139 6 90 0.0110 0.0102 0.0102 0.0102 8 110 0.00657 0.00657 0.00657 0.00657 ?? ?? ?? ?? 1 ...read more.

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