Rates Of Fermentation Of Cow Milk Biology Essay

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I will be researching on the different rates of fermentation of different types of cow milk with and without a Thermophilic culture to make yoghurt. The types of cow milk used were UHT milk (Ultra Heat Treatment), Pasteurized milk and Unpasteurized or Raw cow milk. I found this an interesting question as I have always wondered how yoghurt is manufactured and since I had an idea that different types of animal milk make different kinds of yoghurt but I was fascinated to find out how cow milk and its varieties would ferment under the conditions of having a Thermophilic culture and conditions without.

The three kinds of milk under the two conditions were done in triplicate so in total 18 tests were done all at the same time. At the beginning of the experiment, the pH for the pasteurized and Unpasteurized milk was both higher than that of the UHT milk. The textures of the fermented milks were all different. The milk with the Thermophilic culture varied with the UHT milk having the smoothest texture while the Unpasteurized milk had a very watery and the pasteurized milk was soft but thick in texture. The milk without the Thermophilic culture also varied UHT milk formed soft tiny curdles; the Unpasteurized milk was less watery compared to the Unpasteurized milk which was very water. In the case of the formation of curdle over a 24 hour period, this also varied. The milk with the added culture-UHT no curdle was visible, Pasteurized had soft tiny curdle formed while the Unpasteurized had big chunks of curdle formed that was suspended in water fluid. The milk without a culture-UHT had tiny curdle formed, pasteurized had no sign of visible curdle while the Unpasteurized had curdle formed at the bottom in water.

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Milk can be defined as the lacteal secretion obtained from the mammary glands of mammals by the process called milking. In Kenya, one can find many different sources of milk. That is from cows, goats, sheep and even camels. The most common source of milk is that from cows as they are a lot more than the other animals in the country. Also the cows are easy to milk and they are abundant in milk quantity that can satisfy the needs and wants of the population as a whole.

Milk production in Kenya is dominated by the small-scale producers located mainly in the Rift-Valley, Central and Eastern provinces. As of the year 2008 the estimated cattle population was estimated to be about 3.9 million.

From the cow milk samples I used in this experiment, there were 3 different types of cow milk used. Raw cow milk or Unpasteurized cow milk, pasteurized cow milk and UHT pasteurized cow milk.

Raw cow milk can be defined as the natural unaltered milk from cows. This milk is not heated above the animal's highest body temperature which would usually be 38 degrees Celsius. Raw cow milk is therefore not pasteurized, homogenized or frozen, nor has it been altered with additives, chemicals, light or homogenization.

Pasteurization can be defined as the process which slows microbial growth in food by heating the food, which would normally be a liquid, to a specific temperature for a definite length of time, and the cooling it immediately. The process was named after its creator, a French chemist and microbiologist, named Louis Pasteur. The first ever pasteurization test was conducted by Louis Pasteur and Claide Bernard on April 20th 1864. This process was originally created as a way of preventing wine and beer from souring.

This process is not intended to destroy or kill all pathogenic micro-organisms in the milk; however, pasteurization aims to decrease the number of viable pathogens so they are unlikely to cause illness among its consumers. The pasteurization of milk inactivates or kills the organisms that grow rapidly in the milk but does not, however, destroy the organisms that grow slowly or produce spores. Also another thing to note is that the pasteurization of milk typically requires temperatures below boiling since at very high temperatures casein micelles will irreversibly curdle.

Two out of the three different types of cow milk I used during this experiment have come from some type of pasteurization. These types of pasteurization are: high temperature, short-time pasteurization (HTST) and Ultra high temperature pasteurization (UHT).

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Ultra-high temperature pasteurization involves exposing the milk to temperatures exceeding 135 degrees Celsius (275 degrees farenheight) for one or two seconds.

High temperature, short-time pasteurization involves the milk being exposed to heat or temperatures as high as 160 to 165 degrees farenheight (71 to 74 degrees Celsius) for 10 to 20 seconds.

Between these two different pasteurization techniques one could say that the most effective technique is the Ultra high temperature technique is the most effective in killing all the pathogens in the milk. Milk that has undergone UHT pasteurization has a longer shelf life and can be kept at room temperature as long as its container is not opened.

Though UHT processing could be said to be the most effective means to eradicate ALL spores in milk, in Kenya, it is not as popular as the milk that has undergone the normal pasteurization (HTST). This could be because the UHT processed milk has a slightly different taste to that of the normal pasteurized milk. A more 'burnt' taste that does not appeal to the milk consumers in my country

In Kenya, many of our people prefer raw cow milk over any kind of pasteurized milk. This is because it is 20 to 50 % cheaper than pasteurized milk, its supply involves fewer costs, people prefer its tastes and its high butter milk content and most importantly consumers feel, justifiably, that simply boiling the milk removes most health hazards. This type of mentality doesn't occur with Kenyans, in the 'higher-income bracket', who would much prefer pasteurized milk to raw milk due to its many health benefits.

One may think that the process of yoghurt making is as simple as just adding live bacteria in the milk and yoghurt is made. However there are chemical and also biological processes that occur during this process.

According to the code of Federal Regulations of the FDA (FDA,1996,c), yoghurt is defined as the food produced by culturing one or more optional dairy ingredients with a characterizing bacteria culture that contains the lactic acid bacteria, Lactobacillus bulgaricus and Streptococcus thermphilus.

Souring/fermentation is a certain action of a milk bacteria upon the milk sugar which converts it into lactic acid and this acid gives the sour taste and causes the milk protein to curdle. After the lactic acid is produces, its presence in the milk makes it hard for bacteria to grow in the milk. After the souring/ fermentation, it is said that the milk would for sometime not undergo any further changes.

Yoghurt was accidentally invented thousands of years ago but has gained a lot of popularity around the world. This is because of the introduction of fruit and other flavourings put in the yoghurt and it being convenient as a ready made fast food and the image of yoghurt as a low fat healthy food.

In normal conditions, milk has a pH of about 6.5-6.7 and at this pH value the casein is without proteins it has a negative charge and therefore the casein micelles are relatively soluble because they repel each other. In an acidic environment, the milk coagulates because the casein has its isoelectric point at about a pH of 4.6. At this pH value it has a quantity of positive charges equal to the quantity of negative charges and the positive part of each micelle is attracted by the negative part of the other causing the formation of ionic bonds among the micelles working against the dipole-dipole bonds with water so that the protein precipitates in the form of deminerilized casein and in the solutions remain soluble calcium salts. So the increased acidity due to the conversion of lactose to lactic acid causes milk proteins (casein) to tangle into solid masses.

Equation for lactic acid formation from lactose:

C12H22O11 + H2O 4C3H6O3

Lactose is a sugar molecule called a disaccharide. It is composed of two molecules: one glucose molecule and one galactose molecule joined together to make one molecule of lactose.

The pH probe helps us to monitor the pH by measuring pH as the activity of hydrogen ions surrounding the thin-walled glass bulb at its tip. The probe produces small voltages (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter.

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In this experiment I expect to find that the milk samples with the Thermophilic starter culture will ferment much faster than the milk samples without the Thermophilic starter culture, this is because I would find that the culture acts as a catalyst and would speed up the process. Also I would expect to find that the Unpasteurized milk will ferment much quicker than the rest of the milk while the UHT milk will ferment much slower because the UHT milk has gone through such a high temperature processing that virtually all the bacteria in the milk would have been killed.

MATERIALS AND METHODS

MATERIALS USED

100mL of Long Life UHT cow milk (whole milk)

100mL of Pasteurized cow milk (whole milk)

100mL of Unpasteurized cow milk (raw milk)

18 test tubes

Three 10mL measuring cylinders (+/- 0.05mL)

Water bath

Weighing scale (+/- 0.005g)

Vernier pH probe

6 containers

1 thermometers

Clock

Naming tags

Test tube racks

Thermophilic starter culture containing Lactobacillus bulgaricus and Streptococcus thermphilus

SOURCE OF MATERIALS USED

All the milk, I personally bought with the exception of the raw cow milk that was given to me by a farmer in the rural Kiambu area of Central Kenya. The raw milk had not gone through any further processing after the cow was milked and was packed in a sterilized white plastic bottle.

The Thermophilic starter culture was acquired courtesy of and with permission from the manager of the Palmhouse Dairies in Kiambu. The manager also took time to carefully explain to me the precautions, storage and handling of the Thermophilic starter culture.

The rest of the equipment was provided to me by the Braeburn College laboratory with the help of one lab technician.

PROCEDURE UNDERTAKEN

All equipment needed was first sterilized before the experiment was conducted in hot boiling water

pH of the milk samples was taken before the experiment began. The pH probe after each pH recording was rinsed with distilled water and wiped with tissue

18 test tubes were placed in racks in the water bath of constant temperature. 9 test tubes were placed in each rack so as to differentiate the ones that had the Thermophilic starter culture from the ones that didn't have. They were then correctly labeled( using milk type and trial)

Three 10mL measuring cylinders were used to measure the volume of each milk tyoe. I used 10mL of milk for the experiment for the different types of milks and for each trial. Different measuring cylinders were used so as to avoid any mixing between the different milk types as these could affect the results I get.

The Thermophilic starter culture was weighed on a weighing scale. A white sheet of paper was palced on the weighing scale to avoid the culture form sticking on the surface of the weighing scale. This also made it easier in transporting the culture and putting it in the 9 milk samples that required it.

The culture was mixed thoroughly with the milk using a wooden splint till all the culture was dissolved.

pH was then, for the first hour, taken after every 20minutes then after that, after every hour for a total of 7hours.

NOTE:

pH of the milk was determined by inserting the Vernier pH probe directly into the milk before the Thermophilic culture was added into the milk and also during the fermentation process of all the 18 samples. pH of the milk without the Thermophilic starter culture was taken beforehand so as to avoid contamination with the ones that contained the Thermophilic starter culture. Between measuring the pH of each and every sample, the pH electrode was rinsed with distilled water and wiped with tissue each and every time.

For the first one hour during the experiment, pH was taken for the first 20minutes, 3 times. This was done so as to see or detect how fast the fermentation process was taking place for each and every sample. After the first hour, pH measurements were taken after every hour for the next 6 hours and then finally pH reading was taken after 24 hours were finalized. During pH readings, samples were still in the water bath so temperature conditions were kept constant. The culture used, the milk volume and temperature of the water bath should be noted that they were kept constant during the experiment.

Mass of Culture used

Approximately 0.11g

Temperature of water bath

42 degrees Celsius

Volume of milk used

10mL

RESULTS

pH of the different types of cow milk was taken from the samples that I got before I conducted the experiment and that is why I only have one pH reading for each:

Type of cow milk used

pH

Unpasteurized

7.15

Pasteurized

7.00

UHT

6.93

pH of the different types of cow milk with and without a culture in a water bath of constant temperature:

Raw Data:

Samples with Thermophilic starter culture

pH

TIME

UNPASTEURIZED

PASTEURIZED

UHT

TRIALS

1

2

3

1

2

3

1

2

3

20MINS

5.15

5.17

5.22

5.34

5.31

5.25

5.26

5.11

5.23

40MINS

5.06

5.04

4.95

4.95

5.02

4.83

4.98

4.72

4.94

60MINS

4.96

4.98

4.77

4.78

4.92

4.77

4.80

4.67

4.83

2 HRS

4.77

4.89

4.79

4.78

4.86

4.78

4.81

4.69

4.78

3 HRS

4.76

4.77

4.67

4.74

4.80

4.73

4.69

4.65

4.71

4 HRS

4.73

4.69

4.60

4.61

4.68

4.63

4.59

4.59

4.54

5 HRS

4.63

4.51

4.48

4.50

4.54

4.46

4.47

4.45

4.49

6 HRS

4.74

4.61

4.68

4.53

4.54

4.46

4.42

4.42

4.49

7 HRS

4.66

4.57

4.45

4.51

4.53

4.46

4.38

4.37

4.46

24 HRS

4.20

4.06

4.11

4.18

4.26

4.24

4.26

4.26

4.28

Samples without Thermophilic starter culture

pH

TIME

UNPASTEURIZED

PASTEURIZED

UHT

TRIALS

1

2

3

1

2

3

1

2

3

20MINS

6.83

6.80

6.78

6.86

6.84

6.85

6.92

6.90

6.88

40MINS

6.80

6.79

6.78

6.86

6.82

6.85

6.94

6.93

6.89

60MINS

6.82

6.79

6.80

6.86

6.85

6.85

6.94

6.97

6.96

2 HRS

6.86

6.88

6.88

6.98

6.98

6.95

6.96

7.07

7.06

3 HRS

6.85

6.89

6.88

6.99

7.00

7.00

7.09

7.06

7.10

4 HRS

6.84

6.54

6.67

6.97

7.01

6.85

6.87

6.88

7.04

5 HRS

6.43

6.34

6.41

6.86

6.84

6.77

6.83

6.76

6.77

6 HRS

6.10

5.85

5.97

6.56

6.50

6.40

6.40

5.97

6.32

7 HRS

6.00

5.40

5.94

6.03

5.76

5.95

5.72

5.41

5.56

24 HRS

4.47

4.31

4.46

4.30

4.27

4.26

4.16

4.19

4.23

Averaged data:

Samples with Thermophilic starter culture

pH

TIME

UNPASTEURIZED

PASTEURIZED

UHT

20MINS

5.18

5.3

5.2

40MINS

5.02

4.93

4.88

60MINS

4.9

4.82

4.77

2 HRS

4.82

4.81

4.76

3 HRS

4.73

4.76

4.68

4 HRS

4.68

4.64

4.57

5 HRS

4.54

4.5

4.47

6 HRS

4.68

4.51

4.44

7 HRS

4.56

4.5

4.40

24 HRS

4.12

4.23

4.27

Samples without Thermophilic starter culture

pH

TIME

UNPASTEURIZED

PASTEURIZED

UHT

20MINS

6.80

6.85

6.9

40MINS

6.80

6.84

6.92

60MINS

6.80

6.85

6.96

2 HRS

6.87

6.97

7.03

3 HRS

6.87

7.00

7.08

4 HRS

6.68

6.94

6.93

5 HRS

6.39

6.82

6.79

6 HRS

5.97

6.49

6.23

7 HRS

5.78

5.91

5.56

24 HRS

4.41

4.28

4.19

Now I will process these results by converting the pH value to the Hydrogen concentration of the 3 different milk samples. Calculating the Hydrogen concentration will be done by using the formula:

[H+] = 10^ (-pH)

Samples with Thermophilic starter culture

Hydrogen concentration

TIME

UNPASTEURIZED

PASTEURIZED

UHT

20MINS

6.61 x 10-6

5.01 x 10-6

6.31 x 10-6

40MINS

9.55 x 10-6

1.17 x 10-5

1.32 x 10-5

60MINS

1.26 x 10-5

1.51 x 10-5

1.70 x 10-5

2 HRS

1.51 x 10-5

1.55 x 10-5

1.74 x 10-5

3 HRS

1.86 x 10-5

1.74 x 10-5

2.09 x 10-5

4 HRS

2.09 x 10-5

2.29 x 10-5

2.69 x 10-5

5 HRS

2.88 x 10-5

3.16 x 10-5

3.39 x 10-5

6 HRS

2.09 x 10-5

3.09 x 10-5

3.63 x 10-5

7 HRS

2.75 x 10-5

3.16 x 10-5

3.98 x 10-5

24 HRS

7.59 x 10-5

5.89 x 10-5

5.37 x 10-5

D. Samples without Thermophilic starter culture

Hydrogen Concentration

TIME

UNPASTEURIZED

PASTEURIZED

UHT

20MINS

1.58 x 10-7

1.41 x 10-7

1.26 x 10-7

40MINS

1.58 x 10-7

1.45 x 10-7

1.20 x 10-7

60MINS

1.58 x 10-7

1.41 x 10-7

1.10 x 10-7

2 HRS

1.35 x 10-7

1.07 x 10-7

9.33 x 10-8

3 HRS

1.35 x 10-7

1.00 x 10-7

8.32 x 10-8

4 HRS

2.09 x 10-7

1.15 x 10-7

1.17 x 10-7

5 HRS

4.07 x 10-7

1.51 x 10-7

1.62 x 10-7

6 HRS

1.07 x 10-6

3.24 x 10-7

5.89 x 10-7

7 HRS

1.66 x 10-6

1.23 x 10-6

2.75 x 10-6

24 HRS

3.89 x 10-5

5.25 x 10-5

6.46 x 10-5

Why does H+ concentration increase with decrease in pH?

*QUOTE FROM THE OCEAN ACIDIFICATION NETWORK:

The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases).  This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold.*

C. Appearance of the cow milk after fermentation

1. With Thermophilic starter culture:

COW MILK TYPE

APPEARANCE

UNPASTEURIZED

Very watery curdle; big curdle suspended in water

PASTEURIZED

Soft but thick tiny curdle mixed with water

UHT

No curdle seen; very smooth liquid

2. Without Thermophilic starter culture:

COW MILK TYPE

APPEARANCE

UNPASTEURIZED

Watery curdle; curdle at the bottom

PASTEURIZED

No sign of curdling

UHT

Soft tiny curdle

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 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. This is practice is true, when I compare the results I got from the experiments using samples with the starter culture and the samples without the starter culture. 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 samples of milk to ferment in normal conditions( without the addition of the Thermophilic starter culture.

In the fermentation process to make yoghurt a Thermophilic starter culture is used that normally has a combination of Lactobacillus bulgaricus and Streptococcus thermphilus. The growth of these two bacteria's 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 is much faster and this growth 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 for use by the Streptococcus thermphilus. The lactic acid produced during fermentation in the milk causes proteins to coagulate leading to curds and a thickening of the mil 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 results gave me an idea of it using the data got.

In the milk samples without the Thermophilic starter culture occurred more slowly because they were left to sour/ferment by themselves. 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 starters, the Unpasteurized milk hasn't gone through any heat treatment at all and this would therefore increase the rate at which it sours/ferments as it contains more bacteria more in it that will be used to turn more lactose into lactic acid. The Pasteurized milk has gone through heat treatment but this heat treatment kills the bacteria that cause disease and not all the bacteria in the milk will be removed meaning 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 goes through virtually kills all the bacteria in the milk so we 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) because the air also has some bacteria. Because there isn't supposed to be any bacteria in the milk itself, the bacteria from the air would cause a much slower souring process than the Pasteurized milk meaning that we would expect the UHT milk to be the slowest fermentation/souring process in the experiment.

Using the results obtained in the experiment, I found that there were fluctuations 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 types could be due to a possible increase and decrease in microbial activity. Noting that H+ ions are made during the lactose to lactic acid formation but fluctuations in pH could mean that the H+ ions are being used up in some other processes.

CONCLUSION:

When I began the experiment I assumed that if I started measuring the pH changes of the milk WITHOUT the Thermophilic starter culture then measure the pH of the milk WITH the Thermophilic starter culture and use the time interval between the next reading as a scope that would be able to eliminates the chance of any bacteria/ Thermophilic starter culture to be caught in the probe as I would make sure that I had distilled water each and every time after I took pH values of the milk and note the expected pH of the distilled water (when I placed the pH probe in the distilled water).

However using the results obtained from the experiment and what I had assumed would occur before I began the experiment, they aren't consistent. The inconsistency comes from the rate at which the individual milk types ferment. I had previously assumed that the rate of each individual milk type would occur fastest in the Unpasteurized milk then the Pasteurized milk and then followed by the UHT milk due to their different (or lack thereof) heat treatments they had undergone through. My experimental results contradicted the theory behind it leaving my experiment with a number of flaws that could have caused this contradiction.

From research, when UHT milk is left in open air it will spoil and this is actually a condition the UHT milk processors state on their packaging and as long as the UHT milk is sealed it will not spoil. However this doesn't explain why it fermented as fast as it did. The only possible explanation for this would have been that there was contamination between the samples caused by the use of the pH probe and ineffective sterilization.

I will now look at the possible weakness and improvements that could be used to improve this experiment.

WEAKNESSES AND LIMITATIONS

IMPROVEMENTS

Distilled water was used to sterilize the pH during the experiment and this could have proved inefficient this is because the bacteria/culture that was on the pH probe wouldn't have been killed in between pH measurements.

Use of alcohol/ ethanol as a form of sterilization. One would need to use 70% ethanol mixed in 30% water because the water helps to penetrate the bacteria cell wall around the pH probe. If one uses 100% ethanol, the bacterial cell wall could resist the effect of the ethanol.

Use of 1 pH probe during the experiment due to the fact that the laboratory only had 1 that I could use at the time. This made taking pH readings and measurements difficult as I had to keep sterilizing the probe all the time. This also increased the chances of contamination in the milk.

The use of at least 6 pH probes so as to find the pH changes for the 3 milk samples with the culture and the 3 milk samples without the culture. This would avoid contamination of the culture to the milk samples without the culture and also cross contamination between the different types of milk. These pH probes would need to be calibrated using the same standard solution so as to increase result accuracy.

Lactose content wasn't stated on the different types of milk and this proved to be a problem because lactose content does affect the rate at which milk ferments. A higher lactose content in one of the milks results in the increase in the rate of fermentation and vice versa and this could have had a clear effect on my results.

Use milk types that have an indication of the lactose content in them. Also if the lactose content isn't specified I could measure the lactose content by the use of a polarimetry*.

*(

The different milk types were taken from 3 different dairy plants. This could affect my results in the experiment as these milk types would have gone through different methods of heat treatments some more extreme than others and also using different types of cows to produce these milks

I could have obtained the different milk types from the same dairy plant making sure that the same cow was used to obtain the milk before any kind of heat treatment occurred. Also I could have done the heat treatment myself and be as accurate as possible to heat to the accepted temperatures. The only limitation with that is that the UHT milk goes through such a high heat treatment that it would be difficult to heat the milk that high temperature.

Immediately the culture was added it was difficult to take the pH measurements and it meant that some of the milk samples had a 'head start' in the fermentation process.

This would have been improved with the use of the 6 pH probes I mentioned earlier.

There were some successes during the experiment such as how I managed to keep the temperature of all the milk samples the same using a water bath.

However some questions remain unanswered even after the amount of research I did. I couldn't find a good enough explanation for the fluctuation of pH during the experiment. During my research the process of putrefaction came up as a cause for the fluctuations but I didn't understand how the decay process could be involved in the fermentation processes as I would assume that it would only occur after the fermentation process is complete. This is one of the phenomena's that I didn't understand and I'm hoping that with this Extended Essay I could inspire someone to explore the possible answers for it.