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AgriQuora . 28th Oct, 2021

The Ultimate Guide on Yoghurt Making and Quality Control

The origin of yoghurt, a dairy product made through fermentation of milk using carefully selected bacteria, remains speculative. Some sources document the origin of yoghurt in Turkey from as early as 8000BC (download file). The word ‘yoghurt’ is derived from the Turkish term yogurmak, which translates to curdle, coagulate or thicken, hence “thick milk.” The product can be traced to any ancient culture where dairy animals were kept.


In retrospect, the fermentation of milk was mostly unintentional as milk would accidentally be exposed to plant or bacterium at the household level. Researchers have uncovered intentional combination of fermented milk with honey to produce Dahi.


Traditionally, yoghurt was prepared from boiling milk for extended periods and letting the water in it evaporate. This mimics modern processing techniques where water in the milk is removed using a vacuum pan.


The Foodand Drug Administration (FDA) recognizes yoghurt as a fermented dairy product derived from the fermenting of milk using certified bacterial cultures. The resulting product can have optional additions including milk powder, dried fruits, sugar, and food colour subject to preference. Notably, during production, the two traditional strains can be used separately or in combination with no stringent regulations in place. 


Criteria for Categorization of Yoghurts

Broadly, yoghurts are categorized as either standard culture yoghurt or probiotic yoghurt.


A. Standard yoghurts

Are made with a mixed strain culture containing Lactobacillus delbrueckii ssp Bulgaricus (L. Bulgaricus) and Streptococcus thermophilus (S. thermophilus).


Protocooperation

These two bacteria (L. Bulgaricus and S. thermophilus) co-exist in a symbiotic relationship (protocooperation). Protocooperation involves the exchange of several metabolites that provide mutual growth stimulating effects leading to increased acidification of the milk and terminal bacterial population.


S. thermophilus stimulates the growth of L. bulgaricus by producing pyruvic acid, formic acid, folic acid, ornithine, long-chain fatty acids, and CO2. The acid lowers the pH of the milk to an optimal level for L. bulgaricus. On the other hand, L. bulgaricus exhibits proteolysis yielding peptides, free amino acids, and putrescine which stimulate the growth of S. thermophilus (read more).


Benefits of protocooperation

Include modification of yoghurt texture (through production of exopolysaccharides (EPS)), rapid acidification and flavour enhancement (through production of acetaldehyde) leading to a stable final product.


B. Bio-yoghurts

Are made by culturing milk with probiotic strains of Bifidobacteria and L. acidophilus. To qualify as a Bio-yoghurt, the contents must meet minimum thresholds including the right bacteria strains used and in the right quantities. With this criterion met, bio-yoghurts can impart additional health benefits to the consumers..


Other than these two broader categories, yoghurt can be further classified to account for the different varieties available in the market today.


The categorization criteria may include:


1. Based on the physical nature of yoghurt

Yoghurt can be solid, semi-solid or fluid. Stirred yoghurt is less viscous with semi-solid characteristics. Drinking yoghurt is usually homogenized and protein suspensions added for stabilization.


2. Based on the flavour of yoghurt

Flavouring is usually done to enhance sensory appeal to consumers. Here, you can have plain/natural yoghurt, which is unsweetened and no other flavour or colour added. There can also be sweetened and flavoured yoghurts with a variety of additives for different flavours including fruits and cereals depending on consumer preference.


3. Based on the chemical composition of yoghurt

Yoghurts are categorized as either regular yoghurt, low-fat yoghurt and non-fat yoghurt based on the fat content of the product. Regular yoghurt is produced with the full fat milk containing at least 3.35% butterfat. Low-fat yoghurt and non-fat yoghurt are produced with low fat milk and skim milk respectively.


Types of Yoghurt


1. Plain set yoghurt

This is often considered the traditionally prepared yoghurt where no sugar or sweeteners are added to the fermented milk. The cultured yoghurt milk is packaged before fermentation begins. Fermentation usually takes places in the packets.


2. Flavoured yoghurt

Popular type of yoghurt due to huge variety that appeals different consumers’ sensory preferences. Sugar and flavours are usually added during processing.


3. Greek yoghurt

Traditionally prepared by straining water from plain yoghurt to make it thicker, richer and creamier. It is a popular option as it is considered healthier, especially since it has no added sugar and contains more protein.


4. Frozen yoghurt

Frozen yoghurt is more like ice-cream with a hint of yoghurt. Typically made of frozen ice milk and is served with toppings.


5. Drinking yoghurt

Made by mixing regular yoghurt with reduced milk solids, making it less viscous (flows easily). They come in a variety of flavours but Kefir and lassi are the most popular. 


Benefits of Yoghurt

Yoghurt has a rich nutritional profile and improves the overall quality of diet. Some of the benefits of consuming yoghurt include:


1. Yoghurt acts as an antidote when you are on antibiotics

The British Journal of General Practice (BJGP) has linked oral antibiotics with adverse effects including antibiotic-associated diarrhoea. Antibiotics are not selective and will indiscriminately destroy bacteria in the body, including beneficial bacteria. This upsets the intestinal balance leading to undesirable effects such as diarrhoea.


Consumption of yoghurt containing both the Lactobacilli spp and Bifidus spp bacteria provides relief for this diarrhoea as they replenish the intestinal microbial flora.


2. It boosts vitamin B production

Yoghurt, by virtue of being a milk product, contains significantly high amounts of vitamin B12. This Journal of Dairy Science article illustrates how vitamin B12 is more bioavailable in milk, with upwards of 51-79% absorption rate.


Vitamin B12 not only boosts neurological functions but is also critical in the production and growth of red blood cells. Consequently, consumption of yoghurt helps in boosting cognitive abilities and ensures proper growth, especially in infants.


3. It facilitates the production of immune system boosters

Probiotics in yoghurt can boost the immunity of the consumers. Due to its ability to replenish the normal microbial flora of the gut, yoghurt can help immune-compromised individuals fight both bacterial and viral infections. Yoghurt consumption can also help fight off fungus-related infections by preventing their overgrowth.


4. Yoghurt accelerates bowel movement

The lactic acid in yoghurt lowers the pH of the gut leading to elimination of some harmful bacteria. This can help in prevention of constipation and diarrhoeal illnesses. The increased gut acidity also helps speed up the process of digestion, easing any discomfort associated with slow digestion.


5. It protects against inflammatory infections

Yoghurt culture bacteria exhibit inflammation dampening characteristics, which may help prevent serious illnesses that if left unchecked may progress to cancers. This research article links higher yoghurt intakes to lower risk of conventional adenoma, a form of colorectal cancer.  


6. Protects against peptic ulcers

According to research, consuming yoghurt with active cultures can prevent ulcers by inhibiting the multiplication of H. pylori in the gut, effectively helping ulcers heal faster. It is an effective alternative for nonsteroidal anti-inflammatory drugs as it alleviates the stress associated with taking such drugs.


7. Yoghurt prevents atopic diseases

A 2017study published  in the Journal of Dermatological Science indicates how  habitual consumption of yoghurt can prevent atopic dermatitis and food sensitization in children under five. Incorporating natural yoghurt to maternal diet during pregnancy or lactation and to children prevents the development of eczematous skin.


8. It provides relief to women with yeast infections

Even though consumption of yoghurt on its own does not prevent yeast infections, taking it together with prescription drugs

seem to provide notable relief


Principles of Yoghurt-Making

Yoghurt is produced by fermenting milk by the action of two bacterial strains – Lactobacillus bulgaricus and Streptococcus thermophilus. The quality of the final product depends on the quality of raw milk used, good starter culture, hygienic handling and processing conditions.


Consider the following parameters when setting out to make yoghurt:


  • Type of milk (cow, goat, buffalo, sheep, etc) and quality thereof
  • Yoghurt additives - stabilizers, emulsifiers, preservatives, fruits.
  • Choice of starter culture - bulk or direct vat set (DVS)
  • Yoghurt milk standardization – to the desired butter fat content
  • Heat treatment – use appropriate pasteurization regime
  • Design of the yoghurt process plant - batch or continuous system
  • Availability of utilities – including steam for heating, chilled water for cooling, compressed air, etc.
  • Homogenisation – 200+ bar with two stages of homogenisation for complete fat dispersion to avoid creaming.
  • Incubation temperatures should favour starter growth.


Starter Cultures Used in Yoghurt-Making

A starter culture is an essential component in all commercially fermented products. The mixed strain starter (Lactobacillus bulgaricus and Streptococcus thermophilus) used in yoghurt makingis suitable for improved nutritional value, preservation and modified sensory qualities.

You can read more about starter cultures here.


Over time, more functionalities of starter cultures have been unveiled. For instance, choosing a slower acid-producing starter can impart longer term freshness to the final product. Additionally, adding a flavour adjunct culture to the starter culture significantly improves the ripening of yoghurt.


Equipment for Yoghurt-Making


The equipment necessary for making yoghurt include:

  • Milk reception units
  • Homogenizers
  • Pasteurizer
  • Incubation units
  • CIP system
  • Filling and sealing machines
  • Packaging machines


Industrial Process of Yoghurt-Making

The flow chart below summarizes the process of making (stirred) yoghurt.




 Step One: Receiving Good Quality Milk

Upon arriving at the processing plant, all prerequisite quality assurance tests must be done to ascertain the quality of milk intended for yoghurt processing. Milk that meets the prescribed standards is accepted for processing.


Formulation and Standardization

The quality assurance officer must determine the purpose of the batch under production. This helps with prescribing standards for that particular batch. For instance, they must determine where the batch is intended for producing standard yoghurt or low-fat yoghurt.


Once this is determined, they can then set the machines to separate sufficient quantities of cream to ensure the yoghurt milk is of the right butter fat content. There may also be need to raise total solids content by adding skim milk powder.


Step Two: Clarification and Standardization

Clarification removes all the soil and strenuous matter in the milk. Standardization helps separate cream to reduce the butterfat content to the desired level. Milk is pre-heated to 45℃ to facilitate efficient cream separation. The process takes place in a disk separator.


Step Three: Homogenisation

Homogenization involves subjecting milk to very high pressures (2000-2500 psi) leading to break down of the fat globules into tiny droplets to avoid creaming (cream separation) when the milk is left standing for long. Homogenization contributes to the consistency of the final product. Homogenization takes place at 65 inside a homogenizer.


Step Four: Pasteurisation

Pasteurization process eliminates all pathogenic microorganisms and their spores and logarithmically reduces the load of spoilage types. In addition, the prolonged holding time is intended to denature all serum proteins, which significantly influences the final body and texture of the yoghurt.


Milk is heated to 85°C for 30 minutes or 95°C for 10 minutes. This process can happen continuously or in batches.

If making sweetened yoghurt or yoghurt with cereal additives, sugar and such additives should be added at 65 before pasteurization begins. This will safeguard against any form of threat to product safety that may arise from these additives.


Step Five: Cooling

Cool down the yoghurt milk to 65 if you are intending to add colour. This is because these additives are heat labile and are likely to evaporate away if added before pasteurization. Adding them at 65 is also a precautionary measure against any threat to product safety.


Cool down the milk further to 43 and inoculate with appropriate starter culture.


Step Six: Fermentation

After cooling, inoculate the milk with is 2-3% v/v starter culture or as per the manufacturer’s instructions when using DVS culture. Stir gently to ensure the starter is evenly distributed in the milk then incubate at 43 to ferment undisturbed.


Monitor the acidity of the product until it reaches 0.7%. The starter cultures will metabolize the milk to produce various metabolites that modify the taste, texture and flavour of the final product.


Step Seven: Cooling and Packaging

Upon attaining the terminal acidity, fermentation is terminated through a fast cooling procedure. The curd is broken by stirring gently and fruits/fruit pulp / flavour can be stirred in at this point.


The cooled yoghurt is then packaged into appropriate containers under aseptic conditions and hermetically sealed (form-fill-seal technology).


Step Eight: Storage and Distribution

The packaged containers are packed into crates and stacked in a cold room awaiting distribution to market. Cold-chain distribution is usually done using reefers (refrigerated trucks).


 

How to Make Yoghurt at Home


Making yoghurt for you and your family at home is a fun-filled activity since the process is pretty straight forward. You do not require specialized equipment either because you can modify what you already have at home.


Recipe for making yoghurt at home


Ingredients

  1. One unit of milk
  2. Starter culture
  3. Sweetener either sugar or honey (optional)
  4. Flavours (Optional)
  5. Food colour (Optional)


Equipment

  1. A thermometer
  2. Timer
  3. Cooking pot
  4. Holding jars
  5. Mixing spoons and a glass tumbler
  6. Whisk / wooden spatula
  7. Oven / cooking stove
  8. Cooler / water bath
  9. Cooler box (optional)
  10. Yoghurt maker (optional)


Procedure

If you have a yoghurt maker, follow the attached manufacturer’s instructions. That is all we’ll say about a yoghurt maker.


Now let’s begin making our yoghurt, assuming you don’t have a yoghurt maker at home. Make sure to maintain high levels of hygiene throughout the process.


1. Prep your starter culture

Starter cultures are quite expensive, so, we need to circumvent any additional cost to our activity. This we do by using yoghurt itself as a starter culture. You can buy yoghurt from the store or use a leftover yoghurt from an earlier batch.


The yoghurt contains an active starter that will initiate the fermentation of your new batch. Add small amount of warm pasteurized milk into the yoghurt starter in a glass tumbler and mix properly to activate the culture awaiting inoculation.


2. Prepare the milk for making yoghurt

Add one unit of the milk into the cooking pot (make sure it does not exceed ¾ of the pot’s volume). Put the milk on medium heat and bring it to 65 then add 6% sugar (if making sweetened yoghurt) otherwise heat to 85 and hold it at that temperature for 30 minutes.


Make sure to follow this step carefully otherwise you will end up with watery yoghurt. Heating to 85 and holding for 30 minutes ensures that all pathogenic bacteria are eliminated and spoilage types reduced logarithmically. Also, the pasteurization conditions ensure that all the serum proteins are denatured hence the yoghurt will not whey off.


Denatured serum proteins bind water and prevents wheying off.


After 30 minutes, cool down the milk to 65 and add appropriate colour, otherwise cool down to 43 for inoculation with starter culture.


3. Innoculate the batch with starter culture

Add 2-3% v/v of the starter culture to the previously pasteurized batch at 43 and mix gently with a wooden spatula to ensure the starter is evenly distributed in the batch.


After mixing uniformly, transfer the inoculated milk into a holding jar and incubate in a water bath set at 43 and keep undisturbed for about 4 hours (until the curd forms). Incubating for longer will result in a thicker tangier product. The texture could be desirable but it’s likely to be too sour to consume.


4. Cooling

After fermentation, cool down the yoghurt using cold water bath to about 6 and transfer the jar into a cooler box or fridge for setting. You can add fruits and flavour before allowing the yoghurt to set.  


After setting, you can enjoy your fresh homemade yoghurt.


Common Mistakes During Yoghurt Production


a) Using too much starter

Many people make the mistake of using too much starter thinking it would speed up fermentation and yoghurt setting. This is counter-productive because the process ends up being too acidic and unsuitable for the starter culture bacteria. They end up dying instead.


b) Using the wrong kind of milk

Using shelf-stable milk is a common problem among homemade yoghurt DIYers. Pasteurized fluid milk does not follow the pasteurization regime for yoghurt milk. The holding time is not sufficient to denature all the serum proteins hence the yoghurt ends up wheying off (separating).


Also, make sure to use milk from the right animal. Camel milk for instance has a strong lactoperoxidase system hence does not coagulate as readily as cow’s milk.


c) Longer incubation periods

Many people will incubate yoghurt for longer with the intention of getting a thicker product. This works against them as the product’s palatability greatly reduces due to high acidity. Do not incubate for prolonged periods.


d) Using poor quality / contaminated milk

Exposing the milk to unsanitary conditions will lead to contamination. Since milk is a highly nutritious product, the bacteria multiply fast and compromises its quality. Make sure to use clean equipment and maintain high standards of hygiene.


e) Insufficient amount of water in the water bath

Make sure the water in the water bath cover the jars up to ¾ way up the jar and the level of yoghurt milk in the jar is below the level of water in the water bath. Change the water in the water bath as often as is required to maintain the incubation temperature.


f) Faulty equipment

Make sure the thermometer is working properly and is correctly calibrated, otherwise you may end up over/underheating the product. 


Defects of Yoghurt, Causes and How to Control


Quality assurance parameters of yoghurt are prescribed in a sensory evaluation score card. It checks the most desirable characteristics in yoghurt including appearance, texture and flavour.


a) Appearance

Great yoghurt should have a uniform, pleasing and attractive colour with no visible foreign material. The colour should be white with no browning (unless intentional colour has been added). Additionally, it should have a smooth, glossy appearance with no browning.


b) Flavour

The most important aspect for yoghurt has to be the flavour and taste. When done right, the aroma should be sweet with mild clean acid taste. Yoghurt is largely acidic which gives it its characteristic tangy flavour. According to this Journal of Dairy Science article, an acid level of between 0.75% - 0.85% is considered the best for good quality yoghurt.


c) Texture

Yoghurt is expected to have a smooth and glossy homogenous body. It should have a creamy layer without gas. Top-quality yoghurt should be creamy, viscous and non-pasty.


Any deviations from these qualities are usually considered a defect in the final product. These could include:


A bitter, sharp taste with a very acidic / unfavourable smell

High acidity is due to prolonged incubation leading to overproduction of lactic acid. Also check the quality of milk used as some animals feed on certain fodder / feeds that taint milk.


Lack of uniformity and unnatural colour

This could include wheying off, fat separation and gassiness. The problem is likely to be due to poor handling and insufficient heating during pasteurization. To control this, maintain high levels of hygiene and use the correct heating regime. Also make sure your thermometer is properly calibrated.


Metallic and rancid flavour

This is caused by oxidative rancidity of the milk. This can be attributed to using milk with rancid butter fat or insufficient pasteurization leading to rancidification of the product. Make sure to use high quality milk and sufficiently pasteurize the yoghurt milk.


Watery / runny yoghurt

Occurs as a result of whey separation. Whey separates because the serum proteins are not sufficiently denatured during pasteurization. Make sure to heat and hold the milk according to specifications to ensure that all the serum proteins are denatured.


Sticky / gummy yoghurt

Occurs when excessive stabilizers are used leading to gel formation. To avoid this defect, use only the recommended quantity of additives.


Gassiness

Exhibited as swollen packets or air holes in the product. The former is most likely due to contamination with coliforms, which is an indication of unsanitary handling. Gas holes in the product could indicate contamination with yeast.


Failure to coagulate / set

There could be a possibility of antibiotics present in the milk, which then kills off the starter culture. There could also be a possibility that the incubation temperatures are not suitable. Make sure to use milk without antibiotics and set the right incubation temperature to allow starter culture activity. 


Read more about fermentation defects here.


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