Objective: To investigate the factors which affect the production of yoghurt
Title: Investigating The Factors Which Affect The Production Of Yoghurt
Objective: To investigate the factors which affect the production of yoghurt
Background summary:
Fermentations are a significant way of modifying raw fresh food. The fermented product has properties that are different from the original material. The term fermentation is used in two senses. In the narrower biochemical sense, fermentation is a form of anaerobic respiration and is a means by which organisms, or cells within organisms, obtain energy from an inorganic substrate in the absence of oxygen. In the broader sense, the term is used to describe a very wide range of process carried out by microorganisms.
Fermentation of milk into yoghurt is both a very ancient and a widespread practice. In Europe, they are most familiar with yoghurt from cow's milk, or from sheep, but milk from other mammals, including goats, buffalo and camels, is also used. Probably the first yoghurt was from the Middle East. Milk being carried out under warm conditions doubtless became sour, developed agreeable flavours and could be kept longer than fresh milk, with obvious advantage to nomadic people.
In this practical, we investigate some of the factors that affect the production of yoghurt. Bacteria in the starter culture ferment milk sugars to produce organic acids, such as methanoic and lactic acid, and consequently the pH will fall. The rate of change in pH can be used to indicate the rate of formation of yohgurt.
Materials:
UHT milk, natural yogurt to use as a starter culture, boiling tubes, pipettes, syringes, cling film, glass stirring rod, pH paper, water bath.
Method:
. 10 cm3 of fresh milk is added into a boiling tube.
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In this practical, we investigate some of the factors that affect the production of yoghurt. Bacteria in the starter culture ferment milk sugars to produce organic acids, such as methanoic and lactic acid, and consequently the pH will fall. The rate of change in pH can be used to indicate the rate of formation of yohgurt.
Materials:
UHT milk, natural yogurt to use as a starter culture, boiling tubes, pipettes, syringes, cling film, glass stirring rod, pH paper, water bath.
Method:
. 10 cm3 of fresh milk is added into a boiling tube.
2. After that, 1 cm3 natural yogurt is added into the boiling tube.
3. The initial pH of the mixture is obtained using pH papers and is recorded.
4. The boiling tube is covered with cling film.
5. Then, it is incubated in water bath at 43 ºC.
6. The pH and changes in the appearance of the yogurt at intervals of 30 minutes is recorded up to 4 hours.
Results:
Time (hours)
0
2
3
4
pH
6
6
6
6
6
6
5
5
5
Discussions:
The starter culture contains two species of bacteria which enhance each other's activities. They are Lactobacillus bulgaricus and Streptococcus thermophilus. Firsly, L. bulgaricus acts on milk proteins, converting it to small peptides and amino acids. These stimulate the growth of the second species, S. thermophilus, which in turn produces formic acid. This will stimulates the growth of L. bulgaricus. L. bulgaricus is mainly responsible for the conversion of lactose to lactic acid and production of some ethanal (acetaldehyde),which, with other compound, contributes to the flavour.
The culture is incubated at 40 to 45 ºC for 3 to 6 hours (or at 32 ºC for 12 hours), then cooled rapidly to prevent further bacterial fermentation. At the end of the fermentation, the lactic acid concentration should be about 1.4 per cent with pH around 4.4 to 4.6. The pH is weakly acidic because lactic acid is an organic acid and weaker than the mineral acids. The thickening of the yogurt is the result of the coagulation of proteins. In this experiment, it is found that even though the pH decreases over time, the mixture only reach the pH 5.
There are some factors that contribute to this result. One of them is the duration of the experiment. It seems that 4 hours of experiment is not enough to allow the lactic acid to accumulate and reach the right concentration. Besides that, the L. bulgaricus and S. thermophilus bacteria might be present in small amount in the starter culture. Thus, the rate of lactic acid accumulation will become slower. Another factor is that the pH paper used has a wide range pH. Therefore, it does not give an accurate reading of the pH of the mixture but only the estimation.
To improve the results and reduce the errors, some action could be taken. The first is to extend the duration of the experiment until 5 or 6 hours. This will allow sufficient lactic acid to accumulate and give the correct reading. Besides that, an electronic pH meter should be used to give an accurate result. If unavailable, narrow range pH papers could be used as an alternative. It is also important to ensure that the starter culture used has a considerably adequate amount of L. bulgaricus and S. thermophilus to increase the rate of lactic acid production.
In the modern preparation of yoghurt, whole milk may be blended with skimmed milk or skimmed milk solids. Starch or sugar may be added to give flavour or consistency. Sometimes, the milk is heated to allow evaporation and make a thicker yoghurt, though on a large scale the viscosity of the end-product is controlled by the initial mixture of milk and milk solids. The fat content can be adjusted by removing fat or by adding cream.
The milk is homogenised to disperse the fat as small globules, then heated at 88 to 95 ºC for between 15 and 30 minutes to pasteurise the milk. The high temperature and time used are necessary to kill bacteria which may be active at high temperature (thermophilic bacteria). Besides, milk inevitably carries a microflora from the udder, and these contaminants could act as competitors in the yoghurt-making process. The yoghurt is actually the protein casein in the milk that coagulates and form a soft gel.
Conclusion:
The pH of the sample mixture decreases over time until between pH 4.4 to 4.6. The lactic acid formed coagulates the milk proteins into yoghurt which is the soft gel formed.
Reference:
. John Adds, Erica Larkcom, Ruth Miller. 2003. Nelson Advanced Science: Respiration and Coordination (revised edition). Cheltenham: Nelson Thornes Ltd.