Biology 代写 不同类型牛奶发酵率

Biology 代写 生物论文代写

In Kenya, many of our people prefer raw cow milk over 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 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 health benefits.

The making of yoghurt involves both chemical and biological processes. 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.

Fermentation is the action of milk bacteria upon the milk sugar which converts it into lactic acid which gives the sour taste to the milk and causes the milk protein to curdle. After the lactic acid is produced, its presence in the milk makes it hard for bacteria to grow. After the souring/ fermentation, it is said that the milk would for sometime not undergo any further changes and this can be detected by noting the pH.

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.

“Milk normally has a pH of about 6.5-6.7 and at this pH value the casein is without protons and has a negative charge which makes the casein micelles relatively soluble as 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 an equal number of positive charges and negative charges. The positive part of each micelle is attracted by the negative part of the other causing the formation of ionic bonds among the micelles which work against the dipole-dipole bonds with water so that the protein precipitates in the form of deminerilized casein and in the solution remains 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

Structural formula of lactic acid:

Lactic acid is a weak acid because it only partially dissociates. When this dissociation occurs, an ion lactate and H+ ions are formed. The reaction is reversible.

CH3CH(OH)CO2H CH3CH(OH)CO2 + H+ (Ka=1.38×10-4)

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.

Since lactic acid is weakly acidic, the rate of fermentation reaction can be monitored by measuring pH. 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.

In this experiment I compared the rate of fermentation of 3 different types of milk with and without a bacterial starter culture. I expect to find that the milk samples with the starter culture will ferment much faster than the milk samples without the starter culture, this is because I would find that the enzymes of the bacteria would speed up the reaction. 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

VARIABLES

Independent Variable: -milk type

-presence or absence of culture

Dependent Variable: -pH change with time

Controlled Variable: -milk volume

-culture mass

-temperature of milk samples