Yoghourt Production

Homogenization

Milk is a normal oil in water (o/w) emulsion with milk fat globules (MFG) appearing because the oil droplets and the milk fats globules membrane as the emulsifier. But, due to the reaction of agglutinins and interfacial tension, the fats globules tend to collide, either by way of sharing the membrane or due to the Laplace principle, according to which the stress is more inner small globules than inner large globules and, subsequently, there is a bent for big fats globules to develop at the cost of the smallest. This phenomenon, similarly to Brownian motion, forces the milk fat to upward push to the floor of the milk and as a consequence creates the unwanted impact of separation. In an effort to prevent this effect, standardized milk undergoes homogenization. The basic principle of milk homogenization is to problem MFG to intense situations with a view to disrupting the membrane surrounding them after which preserve the new globules in a dispersion at the same time as a brand new membrane is shaped on the fats serum interface. The severe situations that purpose milk homogenization may be accomplished with the aid of the application of pressure, high velocity go with the flow of the milk, or high-frequency vibrations (>10 kHz). The shear strain and temperature gradient developed underneath those conditions result in a cavitation phenomenon that contributes to the homogenization procedure. Homogenization is done by way of the software of strain. Particularly, the pressure commonly implemented inside the dairy industry is 10–20 Mpa . The main homogenization consequences are a decrease of the diameter of the MFG from 2–10 μm to 0.1–1 μm and altering the composition of the MFG membrane. Consistent with Cano-Ruiz and Richter, the membrane absorbs protein molecules, more often than not caseins, from the milk serum to grow to be sufficient to emulsify the brand new-shaped globules, since the fats surface place will increase because of homogenization. Aguilera and Kessler confirmed that a discount of MFG length and the changes in the MFG membrane as a result of homogenization contribute to milk emulsion stability. Furthermore, homogenization affects the characteristics of acidified milk gels, like yogurt. According to Cho, et al, the smaller MFG facilitate the incorporation of fat into the protein network, whilst their elevated surface vicinity favors the interactions between fat and milk proteins, casein and denatured whey, during acidification and next gel formation.

Heat treatment

Heat remedy of milk is completed to make sure the safety of the product, whether or not it's miles milk itself or some other dairy product, and to make the most several outcomes that expanded temperature has on sure milk components facilitating further processes for dairy merchandise manufacture . Heat treatment of milk reduces the number of pathogenic microorganisms to safe limits for the client’s fitness. Numerous warmth treatments can be carried out, which are classified primarily based on the duration and the temperature . The maximum common are referred to as thermalization low and excessive pasteurization, sterilization and UHT (extremely warmness treatment). Low pasteurization refers to heat treatment of milk at sixty-three–sixty-five °C for 20 min or at 72–75 °C for 15–20 s (HTST, high-Temperature quick Time). All through this procedure, maximum pathogens, vegetative microorganism, yeast, and molds are killed. Additionally, with low-temperature pasteurization, several enzymes end up inactive, at the same time as the taste of milk is rarely altered. Furthermore, very little serum proteins are denatured, and bloodless agglutination and bacteriostatic homes continue to be certainly intact . A greater extreme warmth remedy is excessive temperature pasteurization that calls for a temperature of 85 °C for 20–30 min or 90–95 °C for five min. At some point of high-temperature pasteurization most vegetative microorganisms are killed, besides from spores; maximum enzymes are deactivated (except milk proteinase, plasmin mainly, a few bacterial proteinases and lipases); most whey proteins are denatured, and a wonderful “cooked” taste is developed because of the formation, often, of ketones; no in addition irreversible changes occur. Sterilization results in the extermination of all microbial content of milk, which includes bacterial spores, and it is carried out at a hundred and ten °C for 30 min or at a hundred thirty °C for forty s. Similarly, sterilization reasons inactivation of maximum milk enzymes (besides numerous bacterial lipases), darkening of the milk color due to the Maillard response, evaporation of most flavor volatiles, hence weakening the flavor of the milk, and vast harm to all milk proteins, even caseins. Eventually, UHT is completed at a 145 °C for 1–2 s and achieves equal bacterial eradication as from sterilization, minimal taste deterioration and causes denaturation of several whey proteins (β-lactoglobulin, serum albumin, and some immunoglobulins). UHT treatment and excessive pasteurization produce much volatiles in milk, which includes: 2-pentanone, 2-heptanone, 2-nonanone, 2-undecanone, 2,6-dimethylpyrazine, 2-ethylpyrazine, 2-ethyl-three-methylpyrazine, methional, pentanoic acid, benzothiazole vanillin, hexanal, benzothiazole, Deca lactone, H2S, methanethiol, dimethylsulphide, and carbonyl sulphide. Those sulfur-containing molecules are liable for the “cooked” off taste developed in the course of UHT and excessive temperature pasteurization. It has to be cited that the maximum usually used heat treatment within the yogurt manufacturing technique is the high-temperature pasteurization at 85 °C for 20 min .

Pathogens that may grow in milk, due to bad hygiene practices or hardware failure during the levels of processing, encompass Mycobacterium tuberculosis, Coxiella burnet ii, Staphylococcus aureus, Salmonella species, Listeria monocytogenes, and Campylobacter jejune. Those microorganisms are killed by using even slight warmth treatment making sure that processed milk is safe for intake. The declare that milk is secure after a moderate heat treatment would possibly sound frivolous, but most high-heat resistant pathogens both do no longer arise in milk (e.g., Bacillus anthracis) or are outnumbered by using other native microorganisms (e.g., Clostridium perfringens), or reason spoilage earlier than their amount is enough to motive fitness problems (e.g., Bacillus cereus).

Similarly to the reduction or whole extermination of microbiological load, warmth remedy causes launch of CO2 and O2, a growth in the amount of insoluble colloidal calcium phosphate, a decrease in calcium cations, and forces lactose isomerization, degradation and Maillard reaction, as a result affecting the pH of the milk and taste. eventually, for yogurt, the maximum essential modifications at some point of warmth remedy of milk difficulty the milk proteins; the reactions of milk proteins, at some point of heat treatment, have an extreme effect on the yogurt curd formation and might be defined more very well .

The casein molecules in milk are in the shape of micelles or aggregates of submicelles that are formed from αs1-, αs2- and β-caseins stabilized by means of κ-casein molecules held collectively with the aid of calcium and calcium phosphate. This shape is strong and requires an excessive quantity of energy to be disrupted. alternatively, whey proteins in solution have a globular form. Whey proteins, because of their structure, are fairly strong and do not have interaction with fatty molecules, calcium ions or caseins of their native state. but, within the case of whey proteins (β-lactoglobulin, serum albumin) over 80 °C, their peptide chains spread, for this reason denaturating irreversibly. This deformation of the peptide chains exposes their thiol companies and enables them to have interaction with other molecules forming S–S bonds. depending on the pH of the surroundings and the proximity of molecules to be had, whey proteins can shape bonds with different whey proteins and caseins (κ- and αs1- mainly) and additionally they may be integrated into the MFG membrane. The denatured whey proteins, especially at pH values decrease than 6.five, have the tendency to accomplice with casein micelles . All the above phenomena are of paramount significance and are exploited all through yogurt manufacture. Yogurt curd formation is based on the isoelectric precipitation of casein. but, whey proteins can be involved; if the thiol groups of the whey proteins are exposed, an interaction between casein and whey protein molecules occur, and the formation of casein-whey bonds are facilitated. therefore, whey proteins are integrated into the curd matrix, strengthening the latter and ensuing in a more firm yogurt. therefore, the warmth induced denaturation of whey protein flavors the yogurt formation with excessive firmness and viscosity values .

References

Cassida, L.E Jr.Industrial microbiology.New age into publishers, 1996.

I, Stever.Biochemistry.new york Wall freeman company, 1995.

JE, Smith.Biotechnology.Sinauer Association, 2000.

Nelson, D L and M M Cox.Leininger Principle of Biochemistry.Fifth. Freeman publication, 2004.