Biology 代写 不同类型牛奶发酵率

RESULTS:

Samples with starter culture

During the first hour of the experiment all the milk sample pH’s dropped significantly with the Unpasteurized milk having the highest drop in pH of 2.25 while the Pasteurized milk came in 2nd with a pH drop of 2.18 and the UHT milk came in 3rd with a pH drop of 2.16. So far, this is what I would expect considering the heat treatment (or lack of) that these milk samples have gone through.

However when the experiment goes on steadily with a decrease in pH, I noticed that some fluctuations in pH occurred between the 4th and 7th hour. No fluctuation in pH was seen in the UHT milk.

In the first 7 hours of the experiment the highest drop in pH came from the Unpasteurized milk with a pH drop of 2.59, then the UHT milk came in 2nd with a pH drop of 2.53 while the Pasteurized milk came in 3rd with a pH drop of 2.50.

In 24hours all the pH’s seemed to settle(as there was no evident change in pH) and the highest drop in pH came from the Unpasteurized milk with a pH drop of 3.03 settling at a pH of 4.12, the Pasteurized milk coming in 2nd with a pH drop of 2.77 settling at a pH of 4.23 and the UHT milk coming in 3rd with a pH drop of 2.66 settling at a pH of 4.27.

Samples without starter culture

During the first hour of the experiment, the pH does drop in the first 20 minutes but the drop in pH in the full hour is minimal. In the case of the Unpasteurized milk, the pH in the first 20 minutes does drop by a pH of 0.35 but there is no change till after the 1st hour. In the case of the Pasteurized and UHT milk there are some fluctuations in the pH of the milk in the 1st hour. In the 3 hours during the experiment, fluctuations are seen to occur in all the milk samples and after that the pH of the milk samples decrease as per normal.

In the first 7 hours the highest drop in pH came from both the Unpasteurized and the UHT milk with a pH drop of 1.37, the Pasteurized milk had a pH drop of 1.09.

After the 24hours, the highest drop in pH came from both the Unpasteurized and the UHT milk with a pH drop of 2.74 while the Pasteurized milk had a pH drop of 2.72.

DISCUSSION:

The addition of a starter culture, in theory, is meant to increase the rate at which the different types of milk samples ferment, because the enzymes in the bacteria of the starter culture are biological catalysts that are made up of proteins and are used to speed up chemical reactions. This in practice is true, using the results obtained during the experiment. The Thermophilic starter culture increases the rate at which the lactose in the milk is turned into lactic acid therefore decreasing the time it would take the different milk samples to ferment naturally.

To make yoghurt a Thermophilic starter culture is used that normally has a combination of both Lactobacillus bulgaricus and Streptococcus thermphilus. The growth of these two bacteria is independent but when combined and used together, the rate of production of lactic acid from the lactose in the milk is much higher than when they are used individually. The growth of Streptococcus thermphilus occurs much faster and produces both acid and carbon dioxide. The formate and carbon dioxide produced stimulates the growth of the Lactobacillus bulgaricus. The proteolytic activity of the Lactobacillus bulgaricus produces stimulatory peptides and amino acids used by the Streptococcus thermphilus. The lactic acid produced during milk fermentation causes proteins to coagulate leading to curds and a thickening of the milk texture. A further increase in the lactic acid in the milk causes a further drop in pH. The initial drop in pH is caused by the Streptococcus thermphilus to pH values of around 5.0. Using the results obtained this is true as all the milk samples had a drop in pH of values around 5.0. The bacterial growth of Lactobacillus bulgaricus is responsible for the further decrease in pH to values around 4.0. This information is consistent with the results I got from the experiment although there was no way I would have been able to know when each of the bacteria was growing.

The fermentation process of the milk samples without the Thermophilic starter culture occurred more slowly. Natural souring depends on the heat treatment or lack thereof of the milk. In theory it is said that milk contains bacteria and depending on the heat treatment of the milk, the souring/fermentation process will take place at different rates. For instance, the Unpasteurized milk hasn’t gone through any kind of heat treatment this would therefore increase the rate at which it sours/ferments as it contains more bacteria in it that will be used to convert more lactose into lactic acid. The HTST Pasteurized milk has undergone heat treatment but this heat treatment kills the bacteria that cause disease and not all the bacteria in the milk will be removed. This means that the milk will sour but at a slower rate than the Unpasteurized milk. In the case of the UHT milk, the heat treatment this milk undergone virtually kills all the bacteria in the milk so I wouldn’t expect it to sour, but it does. This is because as long as the UHT milk is kept sealed, it will not spoil but when it is left in open air it will spoil (a condition the manufacturers’ state on packages). This is because bacteria enter from the air and also through contamination by the probe. The bacteria from the air (and/from the probe) would cause a much slower souring process than in the Pasteurized milk. This means that I would expect the UHT milk to be the slowest fermentation/souring process in the experiment.

Fluctuations were recorded in the Unpasteurized and Pasteurized milk samples that contained the Thermophilic starter culture and in all the milk samples that didn’t have the Thermophilic starter culture. The fluctuations in these milk samples could be due to a possible increase and decrease in microbial activity. Fluctuations in pH could mean that the H+ ions are being used up in some other biochemical processes or just maybe that the probe was faulty!

Using the H+ concentration graphs, we note that there is barely any increase in H+ concentration in the milk samples without the culture. This is because the rate of fermentation is affected by the concentration of reactants and bacteria number (and hence enzyme concentration). A sharp rise occurs after 7 hours because bacteria got in and reproduced causing concentration to increase and speed up the reaction.

Biology 代写 生物论文代写