The bioprocess of yogurt


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This report discusses the bioprocess of manufacturing yoghurt on an industrial scale. On a industrial scale the procedure for yoghurt production follows: preparation and standardization of milk, Pasteurization, homogenization, Cooling to incubation temperature for inoculation with starter culture, fermentation, post-fermentation treatment i.e. adding additive such as flavouring, fruit addition, refrigeration and packaging. In examining each step and identifying limitation, improvements in the general procedure have been suggested. Some improvements include; ensuring correct standardization of milk i.e. yoghurt standard contents are met, changes in technique used to alter fat content, effective heat treatments to kill off unwanted organisms, verification of fermentation process to give and starter culture used in inoculation provides high yields, handling is limited to avoid contamination of and damage to the final product.


Yoghurt is one of the most popular dairy products consumed worldwide and is made principally on the bacterial fermentation of milk. The word is from the Turkish Yogen, meaning thick. Originating in the slopes of Mount Elbrus the discovery of yoghurt was by complete chance; when a Turkish nomad's pitcher of milk was contaminated by some organism that thrived in warm milk. Since then yoghurt has been produced and consumed worldwide for at least 5 400 years and continues to be one of the most nutritious foods proving unique nutritional value to the consumer.

Yoghurt is produced when a lactic acid bacterial starter culture is used to ferment the sugar Lactose found in milk to Lactic Acid which then acts on the protein to give yogurt its characteristic texture and form. Dairy yoghurt is produced using a mixed culture of Lactobacillus bulgaricus and Streptococcus thermophilus bacteria.

Although the consistency, flavour and aroma may vary from one region to another, the basic ingredients and manufacturing process are essentially consistent.

The general manufacturing procedure includes;

  1. Adjusting milk composition and addition of other ingredient to meet the required Code of Federal Regulations (CFR) in terms of fat and non-fat content.
  2. Pasteurization of milk to kill unwanted bacteria
  3. Homogenization of milk to ensure uniform composition
  4. Cooling of milk to optimum starter culture incubation temperature
  5. Fermentation of milk by addition of starter culture
  6. Addition of flavours and fruit
  7. Storage and packaging

The implantation of this general manufacturing procedure is seen in the production of Yoplait yogurt (as seen in the flowchart below).

Preparation of Milk

In the production of yoghurt a variety of milk can be employed; whole fat, semi-skimmed milk or low fat milk, the type of milk used depends upon the type of yoghurt one intends to produce. However, one of the main importance when considering the type used is the absence of any material (disinfectant) or organisms that will hinder or prevent the growth of the starter organism, for example, bacteriophages which are a group of virus that can attack the starter organism causing extremely long periods of incubation (Moralee, 2002).

When the milk arrives at the plant, its composition is altered before any further processing can be completed. This standardization process usually involves increasing the total solids content and decreasing the fat content. The fat content is reduced by a process termed centrifugation. Centrifugation is a separation process commonly used in the industrial treatment of milk. In this process the centrifugal force is utilized to separate insoluble from the liquids thereby resulting in the fat globules separating from the rest of the milk suspension. The milk should be kept at temperature of 40oC before entering the centrifuge (Costa & Goomes, 2008). The lower the fat content the higher the lactose levels (Carpers, 2005) hence lowering the fat content will be beneficial when the milk later undergoes fermentation. Several methods can be used to raise the solid contents in milk; evaporation, addition of skim milk powder, addition of milk concentrates etc. In industrial yoghurt manufacturing, 'the solids content of yogurt is often adjusted above the 8.25% minimum' (Food and drug administration, 1999). Commonly the standard solids content is increase to 16% with 1-5% being fat and 11-14% being solid non-fat (SNF). Increasing the dry matter contents (solid non milk fat) not only improves the nutritional value of the yoghurt, but also provides a thicker, firmer and fuller bodied to the finshed yoghurt (Watson, 2004).


Evaporation of milk prior to the fermentation process for concentrated milk (higher non-fat solid content) is utilized in order to give a better body and texture to the yoghurt. Due to the ever spiralling energy costs, the increase in cost of concentrating milk is 'becoming prohibitive ( Moralee, 2002)'. Consequently, larger manufacturing plants have begun replace the evaporation method with a new technique known as fortification. Fortification is the addition of 'dried milk solids to the base of milk prior to inoculation (Moralee, 2002)'. Several commercial yoghurt manufacturing plants are 'fortifying the base milk with a cocktail of milk and non milk solids' (Smith, 2002). The addition of stabilizers may also be employed to improve the body of the yogurts as they increase the firmness and viscosity, preventing separation of the whey; liquid remaining after milk has been curdled, from the yoghurt, a problem know as syneresis (Watson dairy consulting, 2007). This is achieved through their ability to form gel structures in water and as a result leaving behind less free water for syneresis. Stabilizers are more important in yoghurt produced commercially as yoghurt mix 'has to be pumped, stirred, fruited and filled and will often break down to a runny liquid without the addition of stabilisers' (Watson, 2004). The amount of beneficial stabilizers to be added to the yoghurt mixture is required to be determined simply through 'batch sampling experimentation' by each manufacturer as 'Too much stabiliser and the yoghurt can take on a rubbery texture, far too much stabiliser and the yoghurt can become a hard solid mass' (Moralee, 2002). Some stabilizers used in yoghurt manufacturing include gelatins, gums (locust bean, guar), pectins, and starch.


Generally the fresh milk obtained from healthy animals is either beneficial or harmless, however, changes when improperly handled, or deterioration of the animals' health can create conditions in which the bacteria are able to multiply. The large presence of unknown organisms in the raw milk would 'make the fermentation too unreliable and unpredictable for commercial operations' (Csutak, 2005). Pasteurization is the process by which heat treatment to each particle of milk at relatively high temperature for a specified period of time is capable of killing off harmful bacteria for example pathogens, and improve the storage quality of milk by inactivating certain undesirable enzymes and spoilage bacteria (Watson 2007). Typical temperatures for milk pasteurization are 85-95oC for 20-30minutes (Smith 2002). This technique of heat treatment is also successful in achieving a more stable gel as it denatures the whey (serum) proteins. Furthermore, high heat treatment can also ensure little competition from spoilage organisms providing a much better environment for grown of starter organisms later in the process. It is important that the pasteurization of the milk occurs before addition of starter cultures as this ensures they act as prebiotics; remain active in yoghurt after fermentation (moralee, 2002).


Whilst pasteurization of the milk is effective in destroying microorganisms, 'improper handling after pasteurization can recontaminate the milk' (Brock, 2005). Several safe handling practices can be employed such as sterilised equipment should be used, preferable stainless steel utensils as these are more easy to clean and durable, pure water streams should be utilize when cleansing to prevent any contamination, rapid cooling to approximately 5oC or less and storage in closed vessels before and after pasteurization to ensure unaltered quality and flavour of milk (Brock, 2005).

Pasteurization can be a continuous or batch process, however, commercially it is better in continuous stirring vessels as this continuous agitation ensures every particle of milk including the foam receives the minimum heat treatment. The temperature within each vessel should be monitored with an accurate metal or glass thermometer (Smith P, 1981).

Apart from utilising pasteurization, Ultra-high temperature (UHT) is another type of heat treatment that is currently being employed. In this type of processing organisms are destroyed destroy more effectively by heating the mixture for a shorter period of time, approximately 1-2 seconds, at an extremely high temperature exceeding 135oC (Dairy Science and Technology, 2007). UHT treatment essentially sterilizes the milk and allows it to be stored at room temperature for up to 8 weeks with out change in flavour (Brock, 2005).


In the commercial production of yoghurt milk is always virtually homogenize prior to fermentation in order to give a uniform product. Homogenization of milk aids in the prevention of cream that is, the fat from rising to the surface during the fermentation process. Using a homogenizer or viscolizer fat globules are broken down into much smaller and more evenly dispersed particles and subsequently producing a much smoother and creamier end product (Watson, 2007). When milk enters a homogenizer it is force through small orifices at extremely high pressures breaking down globules as a result of the high shearing forces and dispersing the fat evenly throughout the milk which prohibits the fat from rising to the top of the container (Steane, 2008). Milk mixtures are typically homogenized under pressures of 2000-2500 psi (Moralee, 2002). Additionally this process also prevents wheying off during the incubation and storage stage thus enhancing the stability and consistency of the yoghurt. Once the homogenized mixture has been cooled to the optimum growth temperature, the starter culture is added. It is important for the mixture to be cooled to a typical temperature of approximately 45oC for inoculation as temperatures higher than this may kill off culture (Watson, 2007).

Following the cooling of milk to a temperature of 40-45oC, the milk is inoculated with a fermentation culture. The main starter cultures for yoghurt production are Lactobacillus bulgaricus and Streptococcus thermophilus. The function of the starter cultures is to ferment the lactose, sugars found in milk, to produce lactic acid.

C12H22O11 + H2O ? 4 C3H6O3

Lactose Lactic Acid

The increase in lactic acid production corresponds to a decrease in pH levels and causes the milk to clot, or form the soft gel 'curd' that is characteristic of yogurt. The fermentation of lactose also produces flavour compounds, acetaldehyde, that are characteristic of yogurt. The lactic acid levels can be measured by simply performing a titration with sodium hydroxide. The current minimum standards for yoghurt manufacturing require a value of at least 0.9% acidity and a pH of about 4.4 (Food and drug administration, 1999). The fermented milk is typically incubated in large sealed vats in a temperature controlled environment, temperature is monitored carefully to maintain at a temperature optimum for starter culture growth. Inoculation typically occurs at a rate of 0.5-5% (Watson, 2007). When the desired acidity, that is pH level of about 4.4 is reached, rapid cooling to approximately 7 oC is effected in order to stop the fermentation process. It is vital to note that 'too long or too short a fermentation process will produce a product that is inferior in either its flavour or texture. Too long fermentation will give other organisms the change to become established, with the associated risks of "off" flavours and smells' (Morale, 2002).

In the commercial production of yoghurt, emphasis is not only on the fermentation ability but also on characteristics imparted by the lactic acid cultures. For satisfactory performance the starter culture must be capable of rapid acid production, flavouring of yoghurt, production o full body and texture yoghurt. The determination of whether the starter culture activity is satisfactory or not is 'determined by direct microscopic counts of culture slides stained with methylene blue' (Smith, 2002). In doing so there is also the benefit of observing the physiological state of the cultured cells. The lactic starter culture used for yoghurt production is thermophillic has an optimum temperature of 42oC. The two cultures (Lactobacillus and streptococcus thermophillus) can be grown independently; however, there is a higher rate of acid production when used in together (mixed strain), that is that make use of each other's metabolites in order to effect the acid production efficiency. Streptococcus thermophilus grows at a faster rate producing carbon dioxide and lactic acid. The formate along with the carbon dioxide stimulates the growth of Lactobacillus (Smith, 2002). During fermentation the yoghurt mixture coagulates when the pH level drops. 'The streptococci are responsible for the initial pH drop of the yogurt mix to approximately 5.0. The lactobacilli are responsible for a further decrease to pH 4.5' (Steane, 2008). Inoculation of lactic starter cultures with yoghurt milk produces lactic acid, acetaldehyde and acetic acid.


As mentioned above when selecting the type of milk used in manufacture of yoghurt it is important for the milk to possess a low bacteria count, be free from sanitizing chemicals, rancid milk and have no contamination by bacteriophages. 'Serious economic losses in the yoghurt industry have been attributed to phage attack (Smith, 2004)'. Accompanying the decrease in texture and flavour, bacteriophage attacks decreases the rate of acid production. Large scale manufacturers typically have 'laboratory facilities to check incoming milk to eliminate the possibilities of other starter inhibiting substances' (Moralee, 2002), however, the presence of bacteriophages is always a risk. 'Large manufacturers "Phage" are usually found in the drains and floor gullies of a dairy producing any cultured product, poor hygiene and a lack of general housekeeping increase the risk (Dairy Science Technology, 2007).' Thus in order to reduce the possibility of bacteriophage present 'strict sanitation procedures would ensure prevention of phage attack' (Smith, 2004). This include; hand-washing between each handling, sterile tanks, floors and bench surface. s

Also, since the starter culture is one of the most critical ingredients in yoghurt manufacture synchronisation of rate of acid production and plant production schedules is extremely vital. For example, if frozen cultures are used then a longer incubation period of approximately 5 hours at 43oC is required for acid production (Smith 2002). Similarly as noted above, maintenance of temperature is extremely important for starter culture. This is because if the temperature of incubation is too low, culture grows at such a slow rate that it is unable to adequately acidify milk and to achieve a good texture vice versa if the temperature is too high the culture is killed.

Traditionally, when the lactic acid starter culture is grown in a suitable medium, small quantities will be extracted to inoculate each new batch of yoghurt. This is commonly referred to as using "bulk starter". However, due to the high risk of bacteriophage infections and the delay in time when each new batch is prepared, employment of "bulk starter " is becoming increasingly uncommon amongst commercial producers (Smith, 2002). A new technique referred to as DVI (Direct Vat Inoculation) is fast becoming the most preferred choice which 'involves inoculating the yoghurt mix directly with a very large number of freeze dried starter organisms' (Watson,2007). Even though there may be a longer incubation time, relative immunity to "phage" attack makes this a more 'safe' option for manufacturing plants to ensure production of yoghurt is efficient every time.

The design of the container in which the fermentation process is carried out in can also improve the process. Commercially there has been a shift in design of 'cone bottom processers to facilitate the draining of relatively viscous fluids after incubation' (Carpers, 2005). Critical step in the fermentation process is the maintenance of the optimum temperature for growth of starter cultures; hence well insulted incubation vats should be utilized.

Addition of fruit and Packaging

At this stage fruits and addition desired flavours may be added at or prior to filling preformed pots. Common additives include, fruit flavours, colouring agents, sweetening agents. Depending on the type of yoghurt, the above fermentation process may occur before this addition of fruit or after. For example, in 'set style yoghurt' fruit is added to the bottom of the cup and inoculated yoghurt is poured over the top, the fermentation takes place within the cup. For stirred yoghurt, the fruit is blended with the cooled ferment yoghurt and yoghurt texture can be made smoother by pumping it through a cone before packaging (Moralee, 2002). The yoghurt is refrigerated until it is required for package and transportation.


In undertaking an engineering analysis of the bioprocess of yoghurt and exploring each step carefully it can be seen that although yoghurt follows a general manufacturing procedure. The bioprocess of yoghurt production involves preparation of milk, followed by pasteurisation and homogenisation of the milk, cooling down period, then inoculation with a lactic acid bacterial starter culture to ferment the milk. Measurement of acid production measures the success in fermenting milk. Additions of fruit and other additives are added just prior to storage and packaging. The exploitation of the fact that milk contains the sugar lactose, and when undergoing fermentation with a starter culture produces lactic acid, allows observation of how yoghurt is produced. The production of yoghurt on an industrial scale requires great improvement in the handling of the mixture to ensure no contamination, choice of raw materials will greatly affect the final product, employing a successful method to obtain a starter culture that is free from unwanted materials and operation in fermentation vats that give efficient outcomes.


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