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Effect of Heat on Bacterial Expansion in Milk


The purpose of this test was to comprehend the conditions which encouraged and inhibited bacterial growth by observing the rate of bacterial reproduction in milk samples stored at different temperature. The results gathered after the six day amount of observation unveiled that factors which influenced the speed of bacterial progress were temperature, pH level, water and the amount of food resources available. It was concluded that milk is a perfect environment for the duplication of bacteria because of its high water content, pH value close to 7, and elements. Warmer conditions provoked bacteria to replicate quicker as opposed to winter which constrained bacterial progress. Furthermore, the lactobacilli bacteria existent in milk produced lactic acid solution by fermenting the sugar in milk. As time transferred by, this reduced the pH level of the milk examples, which brought on the dairy to curdle and create a pungent sour smell.


The Kingdom Bacterias consisted of anaerobic unicellular microorganisms with exceptional skills of adapting to vast amounts of environment conditions. Bacterias were the one living microorganisms that existed on the planet for approximately 3. 5 billion years implying these prokaryotic organisms could actually endure through all the harsh environment changes in Earth's background. Furthermore, bacteria existed all over; in the deserts and the oceans, in glaciers and hot springs, in the bodies of other living organisms and even in the Earth's atmosphere ("Bacteria, " 2014).

Classification of the Bacterias Kingdom was based on the shape, framework/thickness of cell surfaces, resources of food and energy, and the research of RNA sequences. Bacteria could obtain energy from various resources. They may be photoautotrophic, photoheterotrophic, chemoautotrophic, or chemo- heterotrophic. Also, these organisms arrived in several different shapes; the most typical being cocci (circular), bacilli (rod-shaped), and spirilli (spiral designed) ("Classification of bacterias, " 2012). Each condition offered distinctive advantages. Coccus possessed protection against blow drying, bacillus bacteria got great surface area for nutrition absorption and spirilla bacterias were able to travel through fluids with ease. Moreover, bacteria commonly grew in distinguishing preparations. Some grew in pair (diplo), some in clusters (staphylo), while others grew in a chain (strepto) ("Bacterial figures and, " 2014).

A populace of bacteria could increase greatly in a reasonably little timeframe due to the fact they reproduce exponentially. In favourable conditions, bacteria would reproduce asexually through a kind of cell section process called binary fission. In this method of asexual reproduction, the single DNA molecule would replicate to create a backup of its original solitary chromosome. As the cell persisted to grow, it could elongate and start to create a septum between your two DNA substances. Finally, a distinct cell wall structure would be produced between your two DNA molecules splitting the initial call into two smaller genetically indistinguishable daughter cells (Bailey, 2014). In unfavourable conditions, a few bacterias were able to reproduce sexually by an activity called conjugation with the intent of increasing the survival probability. This process allowed new gene combinations to be presented which may provide the daughter cells a much better potential for adapting to the changing conditions. During conjugation, two bacterial cells with marginally different genetic make-ups were linked to one another through protein tube buildings called pili. The donor bacterium would transfer all or part of its chromosome to the receiving bacterium. The receiving cell would then undergo binary fission with the new gene content to create more cells with this new gene blend. There were also other styles of reproduction that bacteria proceed through such as change, transduction, and spore development (Galbraith et al. , 2001).


  • To understand the favourable and unfavourable conditions for bacterial reproduction
  • To monitor and compare the speed of bacterial development in dairy stored at warm and cool temperatures


  • 8 glass cups
  • pH paper
  • Plastic Wrap
  • Skim Milk
  • Whole Milk
  • Chocolate Milk
  • Buttermilk


  1. 4 glass mugs were labeled with different types of milk: entire, skim, butter, and chocolate.
  2. The other 4 goblet cups were labeled as; complete, skim, butter, and chocolates as well.
  3. A small test of different types of dairy (~50mL) was poured in the equivalent labeled glass cups.
  4. 8 observation desks were created; 2 dining tables for each type of milk; refrigerator vs. cupboard.
  5. The temp was taken for every sample of dairy and saved in the correct observation table.
  6. The pH level was used for each milk sample and registered in the correct observation table.
  7. The odour of the samples of milk was described and registered in the correct observation table.
  8. The coloring of the milk samples were recognized and known in the matching observation stand.
  9. A information of the looks of the milk examples were made and observed in the appropriate observation table.
  10. 4 glass mugs each with another type of dairy sample were put in the refrigerator.
  11. The other 4 goblet cups filled with different kinds of milk examples were positioned in a cupboard/cabinet.
  12. The steps 5-11 were repeated for 6 days
  13. At the end of the experiment; on the 6th day, all samples were removed properly and all glassware was carefully cleaned.


The results following the six day observation period proved that the four types of milk which are maintained in the fridge underwent little to no changes in appearance, odour, pH level or shade, whereas the milk that were kept in the cupboards shown drastic changes in all aspects. The chocolate milk in both fridge and the cupboard seemed to have undergone the greatest amount of change after the six days compared to the other three types of dairy kept in the corresponding conditions. On the list of four types of dairy still left in the fridge, the chocolate dairy and the butter dairy were the sole samples that experienced a slight shade and appearance change. Among the samples remaining in the cupboard, the skim milk and chocolate milk showed the greatest amount of difference in appearance from day 0; the start, to day 6; the end of the test. All samples of milk lowered in pH levels and it appeared like by the fourth day, all samples retained in the cupboards got produced a cheese-like stable compound that was ornamented by a slim water residue.

Milk has ideal conditions for bacterial development having high drinking water content, plentiful nutrition, and a pH level that's very near to neutral (6. 4-6. 8). Also, the chemicals in milk such as sugars (lactose, blood sugar), milk fats, protein, and other ingredients provided the bacterias with a great amount of food resource for energy. The bacteria present in dairy could be organized into two main categories; pathogenic, and spoilage bacterias. Pathogenic bacteria induced food poisoning such asBacillus cereus whereas spoilage bacterias were only with the capacity of producing pungent odours, unappetizing flavours, and changes in texture and appearance of milk. The most common bacteria within milk were the lactobacilli. These bacteria converted the sugar in dairy (lactose) into lactic acidity ("Biochemical changes in, " 2014).

There were several factors that influence the rate of bacterial development. The most significant factor proven in this test was temp. Several food companies reported that at heat between 5 to 60C several foodborne bacterias could actually grow; this was known as the "danger zone. " This simple fact explained the real reason for the extremely slow rate of bacterial duplication in the fridge milk samples considering that refrigerators were usually retained under 4C to inhibit quick bacterial development in the foods. Furthermore, research exhibited that food bacterias reproduce the quickest at temperature ranges varying between 21 and 47C. Hence, the bacterias in the milk samples held in the cupboards were multiplying at a much faster rate than the milk samples in the refrigerator. Because of this, after approximately two days, the subject can notice distinctive changes in the odour and appearance of the dairy samples from the cupboards ("Danger area, " 2014).

The nutrients in the four different types of dairy were also a big factor that influenced the speed of bacterial growth. Chocolate milk possessed about 10g of sugars per 100mL ("Delicious chocolate dairy, " 2014), dairy experienced about 5. 2 grams ("Nutrition facts, " 2014), skim milk got 4. 9g ("Skim dairy, " 2014), and buttermilk with a 5 g sugars content per 100mL ("Buttermilk, " 2014). Predicated on the sugar content of the milks, it was shown that the speed of bacterial duplication in the delicious chocolate dairy was faster than the other styles of milk due to its higher sugar content that was about two times the sweets content of the other types of dairy. The lactobacillus bacteria existent in milk fermented the carbohydrates such as sugar and lactose in dairy into lactic acid. Therefore, the greater amount of available food supply, the faster the bacterias multiply in the dairy ("What's lactobacillus?, " 2014).

The pH worth were another factor that affected the bacterial reproduction in the dairy samples. Most bacterias preferred to are in a pH value of 7; natural. The pH levels of all the dairy samples excluding the buttermilk, were very close the neutral, therefore they motivated bacterial expansion. However, due to the low temp that the examples in the fridge were kept in, the bacterial development was constrained, hence less lactic acidity was produced keeping the pH levels of the milks close to 7, while the samples placed in the cupboards all slipped in pH levels significantly; became more acidic following the six days. This is because both the warm temperature and the near to neutral pH level provided the bacterias in dairy, lactobacilli, with a favourable environment for reproduction allowing more bacterias to ferment lactose which produced more lactic acidity and increased the acidity of the milk ("Microbiology, " 2014).

However, most bacterias cannot endure in very acidic surroundings; a low pH level. As the increasing bacteria population in the cupboard milk samples continued to produce lactic acid, these were creating an unfavourable condition for themselves. Therefore, after some time the cupboard examples would slowly increase in pH level; are more basic, as all the bacteria die anticipated the extremely acidic environment and thus the bacterial growth rate would reduce drastically. At this time, the dairy would become more vulnerable spoilage caused by mould and candida which have the ability to survive in pH prices below 4. 5, extremely acid solution conditions ("What pH is, " 2012).

The milk samples retained in the cupboards for six times produced a pungent sour odour while the examples in the refrigerator got no or a faint sour smell. This is due to difference in the amounts of lactic acid present in the two sets of samples. A physical property of acids was sourness, therefore considering that there have been more lactobacilli bacterias in the cupboard samples breaking down sugars and producing lactic acidity as a by-product, there was a greater level of acidity in the examples building the strong sour smell. Whereas, the dairy samples in the refrigerator got less amount of lactic acid solution present and produced a faint or no sour odour (Chua, 2008).

All the dairy samples maintained in the cupboard developed curd after the six day observations; this was as a result of ingredients in milk. Milk covered various compounds, the key ones being excessive fat, protein, and glucose. Milk was an emulsified colloid where in fact the protein molecules are suspended and dispersed within a water-based solution. The colour of dairy was because of the ability of the protein molecules to refract light. These protein molecules repelled one another, but when the pH level lessens, the molecules suddenly became drawn to one another forming chunks. This was what occurred at the molecular level when dairy developed curd. Hence, the clumps of a cheese-like element that was developed at the end of the experiment in the cupboard examples were health proteins (casein) molecules. The greenish yellowish liquid residue encompassing the floating clump was a remedy of translucent whey. The curdling procedure for milk occurred quicker at warmer heat compared to winter, therefore only the examples in the cupboard developed curd (Moncel, 2014).


It could be concluded that several factors influenced the bacterial expansion rate in milk. A few factors were temperature, pH levels, drinking water content, and available food resources. Normally the one was temperatures. Warm temperatures provided bacteria a favourable environment for duplication which caused these to multiply quickly as opposed to cooler temperature. The pH values also affected the bacterial growth rate. Various bacteria could grow within an environment with the pH level near to 7; hence in a near neutral environment, bacteria reproduce rapidly. However in dairy, as the lactobacilli inhabitants grew rapidly, the amount of lactic acid increased greatly also, considering that these bacteria broke down the lactose in dairy and released a by-product of lactic acid. This reduced the pH level greatly which triggered the death of bacteria. Therefore, after a few days the pH prices of buttermilk test in the cupboards increased as more bacterias died and halted producing lactic acidity. The chocolate milk with amount of sugars content, which provided the bacteria more food supply, spoiled the quickest compared to the other types of milk. The speed of the curdling of dairy is induced by both the warm temp and acidic conditions in the cupboard dairy samples. Finally, the distinct sour odour of the cupboard samples after a few days were made by the lactic acid solution present in the dairy.


Errors which took place during this test included inaccurate measurements of the pH ideals considering that the colors of the red litmus newspaper strip were difficult to differentiate. Also, different individuals might interpret the shades differently and then for the buttermilk and whole milk, the beliefs of the precise pH levels weren't documented. These errors damaged the analysis greatly because the information wasn't specific and correct. Furthermore, the results could've been different depending on how each individual recognized the changes that had occurred to the dairy samples. Another factor that could've affected the outcome was the different expiry dates for the types of dairy. These problems would've changed the observations that were made significantly.


The factors that control the pace of bacterial growth learned in this test could be employed to everyday life. For instance, the development of sour cream, yogurt, and mozzarella cheese were all results of the fermentation of dairy where in fact the lactobacilli broke down lactose in milk into lactic acidity. The drop of the pH prices in the milk triggered by the lactic acid resulted in various different modifications of the dairy, appearance and feel smart producing different fermented milk products ("Milk, " 2014).

Furthermore, foods should be held refrigerated to be able to inhibit bacterial development. Foods which were kept in warmer conditions such as with the oven for at least a day or two should not be consumed; they could be dangerous to one's health considering that warm temperature ranges encourage bacterial duplication. When the factors were considered, one would have the ability to control the expansion of bacterias. Kitchen utensils and equipment should be stored dry and clean in order to restrict the amount of bacteria produced since moisture level, and the quantity of available nutrition are both factors that influence the rate of bacterial reproduction ("Dairy bacteriology, " 2013).

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