Yoghurt is a centuries old dairy product packed with a whole lot of nutritional benefits that is relished by the young and old alike.

As opposed to regular milk, yoghurt is an excellent vehicle for probiotics that aid in lactose digestion hence is well tolerated by people with lactose intolerance. Yoghurt contains 10 essential nutrients including calcium, vitamin A, vitamin B12 and riboflavin. 2013 Australian Dietary Guidelines state that yoghurt can help protect us against heart disease, stroke, high blood pressure, some cancers, Type 2 diabetes, and help contribute to stronger bones.

With its dynamic nature and variable viscosity, yoghurt requires specialist processing knowledge to get a high-quality and safe product through the plant and into the package. The production processes, the type of milk, practices adopted during feeding, milking, and beyond can affect the quality and safety characteristics of yoghurt. Let’s troubleshoot some of these issues encountered by manufacturers.

Raw materials receipt and storage

Total microorganisms count is a good indicator of the general quality, hygiene and safety of milk, since increased microbial growth influences the efficiency of heat treatment. Raw milk may be contaminated by antibiotics, the residues of various livestock therapeutic treatments. Besides their cumulative effects on consumers, antibiotics can also inhibit yoghurt fermentation. In addition, high somatic cells count is a good indicator of mastitis that degrades milk hygiene and should not exceed certain limits.

Raw milk may also contain heavy metals such as lead, mycotoxins such as aflatoxin M1 and dioxins that may impact people health if consumed. Lead is implicated for mental disorder and cardiovascular diseases, while aflatoxin M1 and dioxins are considered by the European Commission as  carcinogenic.

Cream and milk powder which are used as raw materials may also be contaminated by airborne pathogens during storage and transportation from suppliers. Since these pathogens are virtually identical to those found in raw milk, cream need always be maintained chilled at 0- 40 C. However, bacteria like Listeria monocytogenes grow even at low temperatures. While these hazards may also generally appear in milk powder, heat treatment during yoghurt production ensures that they are effectively eliminated.

Quality of starter culture crucial

 The presence of a non-vital culture can inhibit milk coagulation. Culture vitality, measured by the coagulation time and the amount of produced lactic acid, is therefore an indicator of whether or not the fermentation process will take place as desired.

Vitality can be affected by a number of reasons chief among them being bacteriophage-attack. Bacteriophages are viruses that infect and destroy the lactic acid bacteria needed in the production of yoghurt. Phages can slow down fermentation, which can negatively impact the flavor, yield and texture of dairy products. As phages are strain-specific, culture rotation is an essential practice in the dairy industry to combat infection by phages. DSM, a Dutch multinational corporation, has developed an industry-leading, proactive phage management approach. The company supports dairy manufacturers on location by assessing the design of the process for phage risks and conducting phage surveys. The data collected can subsequently be used to give individual dairy producers advice on hygiene and a suitable culture rotation scheme to avoid phage issues. Manufacturers are also turning to Direct Vat Inoculation (DVI) as it provides relative immunity to phage attacks. DVI involves inoculating the yoghurt mix directly with a very large number of freeze-dried starter organisms.

Contamination from added fruits

Moulds can be introduced through the addition of fruits and other foods to the yoghurt. Commercially most fruit for blending with yoghurt is supplied in pasteurized sealed bags and should be free of pathogenic or spoilage bacteria and molds. Typically, packs should come with a certificate of analysis to confirm the absence of coliform bacteria and molds. Auspiciously, most fruits are acidic with a pH of 4.5 or less so their addition should not affect the safety of the product. Melons and guava however may have a pH above 4.5 and theoretically could increase the pH of the yoghurt/fruit blend or result in low acid “patches” in the yoghurt. This could support the growth of spoilage bacteria, should any find their way into the blended yoghurt. To this end, melon and guava fruits may need to be acidified by the manufacturer before pasteurizing and packing to ensure their safety.

In addition, fruits must be free from heavy metals and pesticides that can be extremely harmful above critical limits. Additives should be added in conformance with national and international legislation due to their chemical texture, which can be hazardous above certain limits.

Adding other foods to yoghurt

Confectionery and nuts are very dry foods and do not support the growth of micro-organisms. Nuts are a potential source of moulds and could, particularly if the storage temperature is not maintained, reduce the shelf life of the yoghurt. The presence of each nut variety added to a blend must be clearly declared to assist people who may have a nut allergy. Care must be taken with cleaning and with handling equipment such as ladles to ensure that nut residues do not find their way into blends that are not meant to contain nuts. Flavoring such as chocolate, strawberry or vanilla should not impact on the safety or shelf life of the product.

Yoghurt’s acidity keeping micro-organisms at bay

Due to the high acidity of yoghurt attributed to the lactic acid bacteria, most micro-organisms that cause foodborne illnesses do not get an ambient environment to thrive hence die off. In case there is a problem with the acidity, the low temperature at which yoghurt is stored comes in handy to slow down bacterial growth. Moreover, storing at the recommended temperature (50C or less) ensures that the yoghurt remains fresh over the course of its storage life. It also guards against the growth of acid-tolerant micro-organisms that, while they are unlikely to cause illness, can cause the yoghurt to develop off-flavors or go moldy and spoil.

Good Manufacturing Practices

Notwithstanding the acidity in yoghurt contributing to its safety, good manufacturing practices (GMP) must be adhered to, to control the risk of micro-organisms getting into a blend and reducing its shelf life. There is also the risk that if bacteria like Salmonella find their way into the blend, they may cause serious illness. This is because the infectious dose of Salmonella is very low and, even though they can’t grow in yoghurt, if any are present there is a chance that they could cause illness.

Existence of yeasts or moulds in industrial yoghurts is an indicator of poor hygienic practices in manufacturing.

Sugar and acidity are two important parameters in yoghurt and must be tested, as the target contents of both vary in different types of yoghurt.


Final processing and packaging

During incubation and packaging airborne pathogens such as coliforms, moulds and yeasts constitute possible hazards, which cannot be timely controlled at this stage. Packaging materials could be a source of microbial contamination, which is effectively eliminated under aseptic packaging conditions. The presence of foreign bodies such as packaging material fragments like metals, and adhesives is a critical subject that can impose a physical hazard. Metal detectors and optical control are necessary for the detection of these hazards and their elimination.

Hygienic quality for yield and specifications

Advances in analytical technology for checking the hygiene quality of milk such as the flow cytometry technology has made it possible to test milk within minutes so that a result is available before it is unloaded from the delivery truck and mixed with the rest of the supply. The analyzer incubates a sample of milk and then uses a laser to count individual bacteria and somatic cells that determine the hygiene quality and subsequent suitability for processing.

Effective finished product analysis

To limit liabilities and potentially damaging product recalls, analysis at the critical final stage of production is vital. The majority of this testing can be performed using rapid routine analysis with Near-infrared (NIR), but certified chemical analysis methods are also often required for validation of compliance with certain end-product criteria.

Degree of pasteurization is measured by an enzyme known as phosphatase and performing this test is required before fermentation may proceed. Sugar and acidity are two important quality parameters in yogurt and must be tested as the target contents of both vary in different types of yoghurt. Both are essential guidelines of taste in the finished product.

Consistency is king

All processors want their product to have consistent quality. It is vital to treat all the yoghurt in a line in the same way to guarantee the same final-product characteristics within and between batches. Many types of yoghurt require gentle treatment. The process of pumping, cooling and agitating a fermented product can damage its structure. Inadequate process control is another potential source of product inconsistency. The solution is effective automation and accurate temperature and pressure control throughout the process. Inconsistent processing parameters can cause separation, variable viscosity, graininess, and flavor variations in the final product.

As such, agitation should be kept to a minimum in the vat, during cooling and before filling. This can be achieved through intermittent agitation with timers being placed on the yoghurt vat agitators to minimize shear in the vat during cooling. At times separation of whey from the rest of the yogurt occurs, and although not a safety issue, it has serious quality implications and may impact consumer acceptance.

Ever been taken aback after opening a yoghurt pack only to be greeted with a thin layer of a watery liquid sitting atop your yoghurt? Well, the substance that forms is called whey and is a product when the milk added to the yoghurt has been strained and curdled. To prevent this as a manufacturer, you can either increase your stabilizer level or use different stabilizers. Stabilizers not only help in preventing syneresis, it also improves the body and texture by increasing firmness in yoghurt.

Codex specifications for safe yogurt production

The Codex Alimentarius Commission recommends the use of milk and/or products obtained from milk in making yoghurt and potable water for the use in reconstitution or recombination as raw materials. Cultures of Lactobacillus bulgaricus and Streptococcus thermophilus must also be included.

Ingredients like whole milk powder, skimmed milk powder, unfermented buttermilk, concentrated whey, whey powder, whey proteins, whey protein concentrate, water-soluble milk proteins, edible casein, caseinates, manufactured from pasteurized products can also be added only in amounts functionally necessary as governed by Good Manufacturing Practice. These substances may be added either before or after adding the non-dairy ingredients. It is vital to note that the Commission does not permit whey removal after fermentation.

The Commission has also stipulated in the General Standard for Contaminants and Toxins in Foods and Feeds (CXS193-1995), the Maximum Levels (MLs) for contaminants to be permitted in the yoghurt. The milk used in the manufacture of yoghurt is also expected to comply with the maximum residue limits for veterinary drug residues and pesticides established for milk.

Codex recommends that yoghurt be prepared and handled in accordance with the appropriate sections of the General Principles of Food Hygiene (CXC 1-1969), the Code of Hygienic Practice for Milk and Milk Products (CXC 57-2004) and other relevant Codex texts such as Codes of Hygienic Practice and Codes of Practice. The products should comply with any microbiological criteria established, in accordance with the Principles and Guidelines for the Establishment and Application of Microbiological Criteria Related to Foods (CXG 21-1997).

For flavored yoghurts, Codex stipulates that the manufacturer should include the name of the principal flavoring substance or flavor added on the label. Water added as an ingredient to fermented milk should be declared in the list of ingredients and the percentage of fermented milk used (m/m) shall clearly appear on the label. To avoid misleading consumers, the milk fat content which should not be more than 15%, should also be declared in a manner acceptable in the country of sale to the final consumer. This can be either as a percentage of mass or volume, or in grams per serving as qualified in the label, provided that the number of servings is stated.

If necessary, storage instructions, should also be given either on the container or in accompanying documents, except that the name of the product, lot identification, and the name and address of the manufacturer or packer, shall appear on the container. However, lot identification and the name and address of the manufacturer or packager may be replaced by an identification mark, provided that such a mark is clearly identifiable with the accompanying documents.

This feature appeared in the May/June 2022 issue of Food Safety Africa. You can read the magazine HERE