Higher order taxa:
Introduction of Yogurt Niche
Description of Niche
Yogurt is a dairy product that is made by blending fermented milk with different ingredients that gives flavor and color. Yogurt was discovered accidentally thousands of years ago. It has been said that the origin of yogurt is from Mesopotamia because evidence shows that these people reared goats and sheep at around 5000 B.C. The milk from the animals was stored in gourds, which formed a curd naturally in warm climate. The early form of yogurt was the curd. Yogurt is made with a variety of ingredients including milk, sugars, stabilizers fruits and flavors and a bacterial culture called Lactobacillus bulgaricus.In the process of fermentation, the organism interacts with the milk and converts it to a curd. In addition, it changes the flavor of milk to a yogurt flavor and acetaldehyde contributes to this process.Yogurt is a good source of vitamin B, protein and calcium, it is more digestible than fresh milk and it keeps the intestinal system populated with good bacteria and in healthy condition.
Yogurt is a commercial product that is available in grocery stores, supermarkets, and restaurants.
In making of the modern day yogurt, bacterias such as L.acidophilus or a lactose-fermenting yeast is introduced into concentrated sterilized milk and milk solids. The product is kept for 4 to 5 hours at 110 – 112 °F (43 – 44 °C) until it forms a cord. Yogurt is mostly stored in the fridge and if frozen it destroys most of the beneficial bacteria.
Microbes Specific to Yogurt
Yogurt Microbial Community
The most common microbial ingredients inside yogurt includes the species Streptococcus Salivarius subsp. thermophilis (http://microbewiki.kenyon.edu/index.php/Streptococcus_salivarius) and Lactobacillus delbrueckii subsp. bulgaris (http://microbewiki.kenyon.edu/index.php/Lactobacillus_delbrueckii). Often these two are both co-cultured with other lactic acid bacteria for health and taste effects. Some of these probiotics include but are not limited to L. acidophilus (http://microbewiki.kenyon.edu/index.php/Lactobacillus_acidophilus), L. casei(http://microbewiki.kenyon.edu/index.php/Lactobacillus_casei) and Bifidobactium(http://microbewiki.kenyon.edu/index.php/Bifidobacterium_adolescentis) species.
Prescence of Non-microbe
Initially none, but when yogurt is exposed to high temperatures and when theres lack of refrigeration, yeast will grow.
Yogurt is a simple ecosystem, a niche of which relies on interactions between two lactic acid bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (Lb. bulgaricus). It is preserved by its acidity which inhibits the growth of putrefactive or pathogenic bacteria. Different combinations of lactic acid starter and probiotic cultures allow the production of differnt yogurts with target medicinal characteristics, and potential nutritional and health benefits (Juillard et al., 1987). Under the various microbial interactions, protocooperation or antagonism among these cultures may generate changes in the composition of microflora during the manufacture and preservation of yogurt(Bellengier et al., 1997). To study this, both sides of bacterial association has to be considered, this was done by proteolysis and quantifying formic acid in both pure and mixed cultures of S. thermophilus and Lb. bulgaricus. In the pure cultures, S. thermophilus CNRZ385 exhibited better growth than the two Lb. bularicus strains(1038 and CNRZ397). In the mixed cultures, the different proteolytic capacities of the two strains of Lb. bulgaricus(1038 & 397) attributed to both positive(1038) and negative(397) associations. In contrast, the growth of Lb. bulgaricus was not significantly hindered or inhibited, possibly because of an insufficient production of the acid-production inhibitor formic acid by the S. thermophilus strain(Courtin, 2003), while the addition of formic acid reduces the formation of lactic acid due to the reduced activity of microorganisms as a result of an increase in acidity. In a similar study, Lb. delbrueckii subsp. bulgaricus was to be able to inhibit S. thermophilus strains. And Lb. acidophilus was the sole species that was inhibited by the others (Lb. casei and Bifidobacterium), among probiotic cultures. This demonstrated that stimulation, delay, complete inhibition of growth, and no effects are four types of interactions that occur between lactic acid and probiotic bacterias(Vinderola, 2002). Contrary to these broad range of microbial behaviors, S. thermophilus and Lb. bulgarius also promotes the production of volatile molecules involved in flavour development.
Other Niches Affecting Microbes in Yogurt
Influence by Adjacent Communities
The yogurt niche and its environment overlap its similarity between some of the other dairy niches, specifically being the milk niche which is the root of all dairy niches. The making of yogurt is first through the transition of the milk niche. Firstly, yogurt is made from fermented milk. Milk is rich in sugars, more specifically the sugar being lactose. An environment rich in sugars is an environment that microbes love to thrive in; thus, milk is a great feast for microbes. The following are a list of microbes that thrive in milk. Of all the microbes that live in milk, the Bacillus family and the Streptococcus family is the one that overlaps into the yogurt niche. However, there are only two particular microbes that feast in yogurt which are Lactobacillus bulgaricus and Streptococcus thermophilus. So, as we can see there is a similarity between the microbes that thrive in milk to the ones that thrive in yogurt.
Related Microbes in Adjacent Communities
1. Streptococcus lactis
Purpose: Souring Method: Lactose-lactic acid precipitation
2. Streptococcus bulgaricus
Purpose: Souring Method: Lactose-lactic acid precipitation
3. Lactobacillus casei
Purpose: Cheese ripening Method: Controls altermentation.
4. E coil
Purpose: Souring & gassiness Method: Lactic acid & gases and affects cheese ripening.
5. Bacillus substallis
Purpose: Protecolysis Method: flavors change.
6. Alkaligenes viscus
Purpose: Ropiness Method: Ropi milk
7. S Streptococcus liquifiecence
Purpose: Bitter Flavour Method:Bitter flavour to cream &butter.
8. Bacillus substallis
Purpose: Sweet curdling Method: Curd formation
9. Streptococcus paracitrovorus
Purpose: Attacks citric acid Method: Flavors curd.
Conditions under which the environment changes
In the process of using the lactose sugar from the milk, Lactobacillus produces acid which makes the yogurt sour and a less suitable place for other microbes. This is why there is a dramatic decrease in the amount of microbes that live in the yogurt niche when compared to the milk niche. Thus the essential conversion between milk to yogurt is the acidic levels. The increase in acidic levels is the sourness that is tasted in yogurt which lacks in milk, another characteristic difference. Lactic acid also known as lactate is not good for bacteria. So, as a response to this change in environment, they excrete lactate into their environment. This again is what causes the pH to fall to become more acidic. Another affect of excreting lactate is that the protein molecules in the milk become denatured. What this means is that the protiens unfold from their normal structures and become disordered. After becoming distroted, the protein molecules begin to stick to each other forming a semi-solid matrix. Thus, this is what gives the yogurt a semi-solid state, another characteristic different from the liquid-milk.
Microbe Metabolism and Its effect on environment and human body
Lactobacillus is found to be living in highly acidic environments of pH 4-5 or lower, thereby altering the pH and suppressing pathogens by producing lactic acid. Under the optimal temperature of 37°C, it derives the energy, such as ATP, by converting the glucose to lactic acid through homolactic fermentation. Nevertheless, it is unable to breakdown complex sugars, like ribose, under the optimal temperature. In addition, Lactobacillus secretes nonbacteriocin antibacterial substances. In humans, Lactobacillus is found in the gut and vagina. In the vagina, it plays an important role by keeping the pH low to deter infection.
Under the optimal temperature of 42°C, Streptococcus thermophilus, can generate ATP through fermentation. In contrast to Lactobacillus, it is also able to produce ATP through aerobic respiration in the presence of oxygen. Through fermentation, it converts lactose to lactic acid at the optimal pH of 4.6. In humans, Streptococcus thermophilus is found in the upper part of the intestine and can help people with lactase-deficiency to digest lactose due to the low level of lactase they produce.
Probiotics that are specific to genus Lactobascillus are found in foods and also food supplements. It is the most common bacteria that is classified as a probiotic as it is considered to be a "friendly" bacteria. While inhabiting in the intestine and the vagina, the purpose of this friendly microbe is to prevent other "bad" microbes which cause diseases. This is done by proliferating at the intesntine. Thus, the Lactrobascillus microbe acts as a defense system. This is accomplished through a variety of mechanisms. For example, the breakdown of food by Lactobascillus acidophilus leads to production of lactic acid, hydrogen peroxide, and other byproducts that make the environment hostile for undesired organisms. Other probiotics responses include improving lactose absorption digestion in people who are lactose intolerant, enhancing the immune response, and alleviating symptoms of the bowel syndrome.
Milk fermented with yogurt cultures and Lactobacillus casei compared with yogurt and gelled milk: influence on intestinal microflora in healthy infants (4)
Consuming fermented dairy products have profound health effects, such as providing the minerals and vitamins to humans. They regulate the equilibrium and metabolism of microflora in healthy infants. The experiment was based on comparing the effects of consumption of regular yogurt, milk fermented with yogurt cultures and Lactobacillus casei (YC), and nonfermented gelled milk on the fecal microflora of healthy infants who were divided in three groups, and each group received one of three products. The results show indexes, such as anaerobes, bifidobacteria, bacteroides were not modified during supplementation period. However, in the yogurt group, the number of enterococci in fecal samples had increased. In the YC group, the number of fecal lactobacillus had increased in the colon where lactobacillus may provide physiological benefits. This may prove that Lactobacillus can be used for preventing infectious diseases and stimulating the immune system.
Lactrobacillus species properties and Lactose intolerance (5)
This paper talks about the experiments done to test the isolated Lactobacilli from the stomachs of piglets that were related to the properties of fermented milk. Lactobacillus was extracted from piglets because it is likely that there is a high concentration of Lactobacilli. This was specifically directed to the lactose- intolerant humans. There were several properties that were tested for. It was tested for the pH of the fermented milk over a span of 30 days, the reduction of the concentration of the lactose in milk, the viability, and the b- galactosidase activity. Optimal pH levels and ability to grow in a environment of bile salts were determined. Finally, commercial yogurt was also test to products varied in characteristics. The results were found that the Lactobacillus sp. Isolated from pigs had lower b-galactosidase activity than did Lactobacillus delbrueckii. Also, the optimal temperature was found to be 4 degrees Celsius for the b- galactosidase activity of the fermented milk to decrease with a lactose concentration of 4% and an active pH of 5.5. Lastly, when comparing the Lactobacilllus bacteria found in the piglet’s stomach to that found in yogurt, it has been proposed that for lactose-intolerant subjects, improved fermented milk products could result by using strains of this bacterium that possess high b galactosidase activity at acidic pH. And thus, these microbial cells that are more resistant to gastric digestion.
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(8.)Bennie C. Viljoen, Analie Lourens-Hattingha, Bridget Ikalafenga and Gabor Peterb. "Temperature abuse initiating yeast growth in yoghurt". Food Research International, Volume 36, Issue 2, 2003, pages 193-197.
Edited by [Chung Abbott, Ibukun Osindele, Anusha Sridharan, Jerry Wang], students of Rachel Larsen