FERMENTED MILK PRODUCTS
Milk products prepared by lactic acid fermentation (e.g. yoghurt) or a combination of this and yeast fermentation (e.g. Kefir) are called fermented or cultured milks. The term fermented will be used in this chapter.
Fermented milk is the collective name for products such as yoghurt, ymer, kefir, cultured buttermilk, filmjölk (Scandinavian sour milk), cultured cream and koumiss (a product based on mares’ milk). The generic name of fermented milk is derived from the fact that the milk for the product is inoculated with a starter culture which converts part of the lactose to lactic acid. Dependent on the type of lactic acid bacteria used carbon dioxide, acetic acid, diacetyl, acetaldehyde and several other substances are formed in the conversion process, and these give the products their characteristic fresh taste and aroma. The microorganisms used in the production of kefir and koumiss also produce ethyl alcohol.
Fermented milk originates from the Near East and subsequently became popular in Eastern and Central Europe. The first example of fermented milk was presumably produced accidentally by nomads. This milk turned sour and coagulated under the influence of certain microorganisms. As luck would have it, the bacteria were of the harmless, acidifying type and were not toxin-producing organisms.
A legend says that yoghurt and kefir were born on the slopes of Mount Elbrus in the Caucasus range by a miracle of nature. Microorganisms of various kinds happened to land in a pitcher of milk at the same time and at the right temperature, and found that they could live in symbiosis.
On the southern slope of Mount Elbrus, microorganisms preferring relatively high temperatures, 40 – 45 °C, came together in a milk pitcher that probably belonged to a Turkish nomad, and the result was what the Turks called “Yogurut”. Some sources say that this name was introduced in the 8th Century and that it was changed in the 11th Century to its present form, yoghurt.
It is further claimed, however much truth there may be in the story, that yoghurt acts as a “preservative” against human ageing; that if you happen to meet a Cossack galloping along bareback in some Caucasian valley, he is likely to be 130 to 140 years old!
Kefir, the legend goes on to relate, was created on the northern slope by a mixture of microorganisms that are not so fond of heat. They thrive best at 25 – 28 °C. The name kefir may be derived from Turkish. The first syllable of the name, kef, is Turkish and means pleasurable, which was probably the shepherd’s first comment on the flavour.
Kefir contains several different types of microorganisms, among which yeast is most famous as it is capable of forming alcohol. The maximum alcohol content of kefir is about 0.8%.
General requirements for fermented milk production
The conversion of lactose into lactic acid has a preservative effect on milk. The low pH of cultured milk inhibits the growth of putrefactive bacteria and other detrimental organisms, thereby prolonging the shelf life of the product. On the other hand, acidified milk is a very favourable environment for yeasts and moulds, which cause off-flavours, blown packages etc. if allowed to infect the products.
The digestive systems of some people lack the lactase enzyme. As a result, lactose is not broken down in the digestive process into simpler types of sugars. These people can consume only very small volumes of ordinary milk. They can, however, consume fermented milk, in which the lactose is already partly broken down by the bacterial enzymes.
In the production of fermented milk, the best possible growth conditions must be created for the starter culture. These are achieved by heat treatment of the milk to destroy any competing microorganisms. In addition, the milk must be held at the optimum temperature for the relevant starter culture. When the best possible flavour and aroma have been achieved, the cultured milk must be cooled quickly, to stop the fermentation process. If the fermentation time is too long or too short, the flavour will be impaired and the consistency wrong.
In addition to flavour and aroma, correct appearance and consistency are important features. These are determined by the choice of pre-processing parameters. Adequate heat treatment and homogenization of the milk, sometimes combined with methods to increase the MSNF content, as for milk intended for yoghurt, are essential “foundation stones” for the construction of the coagulum during the incubation period.
Some of the most important fermented milk products are described below. The production techniques for other fermented products have many similarities; the pre-treatment of the milk, for example, is almost the same. The process descriptions for other products therefore concentrate primarily on the production stages which differ from those in yoghurt production.
Yoghurt is the best known of all fermented milk products, and the most popular worldwide.
The consistency, flavour and aroma vary from one district to another. In some areas, yoghurt is produced in the form of a highly viscous liquid, while in other countries it is in the form of a softer gel. Yoghurt is also produced in frozen form as a dessert, or as a drink. The flavour and aroma of yoghurt differ from those of other acidified products, and the volatile aromatic substances include small quantities of acetic acid and acetaldehyde.
Yoghurt is typically classified as follows:
- Set type: incubated and cooled in the package, Figure 11.3
- Stirred type: incubated in tanks and cooled before packing, Figure 11.4
- Drinking type: similar to stirred type, but the coagulum is broken down to a liquid before being packed, Figure 11.5
- Frozen type: incubated in tanks and frozen like ice cream, Figure 11.6
- Concentrated: incubated in tanks, concentrated and cooled before being packed. This type is sometimes called Greek yoghurt or strained yoghurt, sometimes labneh or labaneh, Figure 11.7
Fruit and flavoured yoghurts
Yoghurt with various fruits, flavouring and aroma additives is very popular, although the trend back towards natural yoghurt is clearly discernible in some markets. Common additives are fruit and berries in syrup, processed or as a purée. The proportion of fruit is usually about 15 %, of which about 50 % is sugar.
The fruit is mixed with the yoghurt before or in conjunction with packing; it can also be placed in the bottom of the pack, before the latter is filled with yoghurt. Alternatively, the fruit can be separately packed in a twin cup integrated with the basic cup.
Sometimes yoghurt is also flavoured with vanilla, honey, coffee essences, etc. Colouring and sugar in the form of sucrose, glucose or aspartame (a sugar-free diet sweetener) are often added together, with the flavouring.
When necessary stabilizers may also be added to modify the consistency.
The additives increase the DM (Dry Matter) content of the finished yoghurt;
a typical composition for fruit yoghurt is:
- Fat 0.5 – 3.0 %
- Lactose 3.0 – 4.5 %
- Milk solids non-fat (MSNF) 11.0 – 13.0 %
- Stabilizer (if used) 0.3 – 0.5 %
- Fruit 12.0 – 18.0 %
- Have a low bacteria count
- Not contain enzymes and chemical substances which may slow down the develop- ment of the yoghurt culture
- Not contain antibiotics and bacteriophages
Factors affecting the quality of yoghurt
Numerous factors must be carefully controlled during the manufacturing process in order to produce a high-quality yoghurt with the required flavour, aroma, viscosity, consistency, appearance, freedom from whey separation and long shelf life:
- Choice of milk
- Milk standardization
- Milk additives
- Heat treatment
- Choice of culture
- Plant design
Pre-treatment of the milk thus includes a number of measures which are all very important to the quality of the end product. The mechanical treatment to which yoghurt is subjected during production also affects its quality.
Choice of milk
In order to be able to produce a high quality yoghurt, the milk intended for yoghurt production must be of the highest bacteriological quality. It must have a low content of bacteria and substances which may impede the development of the yoghurt culture. The milk must not contain antibiotics, bacteriophages, residues of CIP solution or sterilizing agents. The dairy should therefore obtain the milk for yoghurt production from selected, approved producers. The milk must be very carefully analysed at the dairy.
The fat and dry solids contents of the milk are normally standardized according to the FAO/WHO code and principles described below.
Yoghurt may have a fat content of 0 to 10 %. A fat content of 0.5 – 3.5 % is, however, the most typical. Yoghurt can be classified in the following groups according to the FAO/WHO code and principles:
- Yoghurt Min. milk fat 3 %
- Partially skimmed yoghurt Max. milk fat <3 %
Min. milk fat >0.5 %
- Skimmed yoghurt Max.milk fat 0.5 %
Dry matter (DM) content
According to the FAO/WHO code and principles the minimum MSNF is 8.2 %. An increase in the total DM content, particularly the proportion of casein and whey proteins, will result in a firmer yoghurt coagulum, and the tendency to whey separation will then be reduced.
The most common ways to standardize the DM content are:
- Evaporation (10 – 20 % of the milk volume is normally evaporated)
- Addition of skim milk- or protein powder, usually 1 – 3 %
- Addition of milk concentrate
- Addition of UF or RO retentate from skim milk
Sugar or sweeteners and stabilizers may be used as additives in yoghurt production.
Sugar or sweetener
The disaccharide sucrose, or a monosaccharide such as glucose, can be added alone, or in conjunction with fruit addition. To satisfy dieters, among whom diabetics are an important category, sweeteners should be used. A sweetener has no nutritive value, but tastes very sweet, even in very small doses.
The fruit in question usually contains about 50 % sugar or a corresponding amount of sweetener, so the required sweetness can normally be supplied by adding 12 to 18 % fruit.
It should be noted that adding too much sugar (more than ~8 %) to the milk before the inoculation/incubation period has an adverse effect on fermentation conditions, because it changes the osmotic pressure of the milk.
Hydrophilic colloids can bind water. They increase the viscosity and help to prevent whey separation in yoghurt. The type of stabilizer and the rate at which it should be added must be determined experimentally by each manufacturer. The product may acquire a rubbery, hard consistency if the wrong stabilizer, or an excess of stabilizer, is used.
Correctly produced, natural yoghurt requires no addition of stabilizers, as a firm, fine gel with a high viscosity will occur naturally. Stabilizers can be used in fruit yoghurts and must be used in pasteurized and whipped yoghurt. Stabilizers (0.1 – 0.5 %) such as gelatine, pectin, starch and agar-agar are the most commonly used substances.
The air content of the milk used to make fermented milk products should be as low as possible. However, some admixture of air is unavoidable if the MSNF content is increased by addition of milk powder. If this is done, the milk should be deaerated as part of the subsequent processing.
When the MSNF content is increased by evaporation, deaeration is a part of that process.
The advantages gained through deaeration are:
- Improved stability and viscosity of the yoghurt
- Shortened fermentation time
- Improved working conditions for the homogenizer
- Less risk of fouling during heat treatment
- Removal of volatile off-flavours (deodorization)
The main motives for homogenizing milk intended for cultured milk production are to prevent creaming during the incubation period and to assure uniform distribution of the milk fat.
Homogenization also improves the stability and consistency of fermented milks, even those with low fat contents.
Homogenization with subsequent heating at high temperature, usually 90 – 95 °C for about five minutes, has a very good influence on the viscosity of the final yoghurt.
Table 11.1 illustrates the dual influence on the viscosity of a fermented milk (Swedish filmjölk; 3 % fat and about 8.7 % MSNF) when it is pre-treated at various homogenization pressures and heating temperatures. The homogenization temperature is 60 °C in all cases.
The viscosity is measured with a simple viscosimeter (SMR viscosimeter) at 20 °C, and the result is given in seconds for 100 ml of product to pass a nozzle of a certain diameter. Figure 11.8 shows a viscosimeter provided with exchangeable nozzles, each of a diameter of 2 – 6 mm.
The viscosity of full-stream homogenized milk runs parallel to the homogenization pressure, regardless of whether it has been subjected to ordinary heat treatment or not. The table also shows that high-temperature heat treatment makes the product more viscous.
As a general recommendation, the milk should be homogenized at 20 – 25 MPa and 65 – 70 °C to obtain optimum physical properties in the product. Homogenization is frequently utilized even in production of low-fat cultured milks.
Some producers homogenize their yoghurt milk up to 40 MPa (400 bar) and at temperatures up to 95 °C. For certain recipes these higher parameters have a positive influence on both viscosity and stability.
The question of single- or double-stage homogenization is sometimes discussed. Generally speaking, this is a matter of the design of the homogenization system and of the homogenizer head in particular.
The milk is heat treated before being inoculated with the starter in order to:
- Improve the properties of the milk as a substrate for the bacteria culture
- Ensure that the coagulum of the finished yoghurt will be firm
- Reduce the risk of whey separation in the end product
Optimum results are achieved by heat treatment at 90 – 95 °C and a holding time of about five minutes. That temperature/time combination denatures about 70 – 80 % of the whey proteins (99 % of the β-lactoglobulin). In particular, the β-lactoglobulin, which is the principal whey protein, interacts with the κ-casein, thereby helping to give the yoghurt a stable body.
UHT treatment and sterilization of milk intended for culturing do not, however, have the same favourable influence on viscosity, for reasons not yet fully understood.
Choice of culture
Culture laboratories today produce a wide range of customized yoghurt cultures. Dairies can choose branded yoghurt cultures or mix cultures themselves to get their own requirements for the final yoghurt. Some cultures will give the final yoghurt different mouth thickness and gel firmness. Other cultures that influence fermentation time and post acidification. Cultures are also adapted to the type of yoghurt that should be produced (e.g. stirred type, set type, drink type and concentrated).
In earlier times it was common that dairies bought a mother culture from a culture laboratory. The dairy then propagated the culture itself in water baths and bulk starter tanks to get enough for the yoghurt production. This system is rarely used today. It is so much easier and safer to use the highly concentrated cultures produced by the culture company. These cultures are distributed deep frozen or freeze dried.
The coagulum formed during fermentation is sensitive to mechanical treatment. This makes the selection and dimensioning of pipes, valves, pumps, coolers, etc., as well as the plant lay-out very important.
The pre-treatment of the milk is the same, regardless of whether set or stirred yoghurt is to be produced. It includes standardization of the fat and DM contents, homogenization and heat treatment.
Figure 11.9 shows an example of the design of a process line for yoghurt production. The milk storage tanks, from which the milk is pumped to the process line, are not shown in the figure. It is assumed that the milk has been standardized to the required fat content and pre-pasteurized before entering the line. In the example, standardization of the DM content takes place by adding milk powder or protein powder. The milk, increased in DM by adding milk powder, should preferably be deaerated to reduce the risk of whey separation in the final yoghurt.
Any additives, such as stabilizers, vitamins, etc., can be metered into the milk before the heat treatment. From the balance tank (1), the milk is pumped to the heat exchanger (2), where it is pre-heated regeneratively to about 65 °C and fed to the deaerator.
From the pre-heater the milk is deaerated in a vacuum vessel. The milk enters about 65 ºC. Due to the vacuum the outlet temperature from the deaerator will be 2-3 ºC lower than the inlet temperature.
The deaerated milk continues to the homogenizer (4) and is homogenized at a pressure of approx. 20 – 25 MPa (200 – 250 bar).
The homogenized milk flows back through the regenerative section to the pasteurization section of the heat exchanger (2) and is reheated to 90 – 95 °C. The milk then flows to a holding section dimensioned for a holding time of five minutes.
Other time/temperature programs can be used. The tubular holding section shown in Figure 11.10 offers a holding efficiency of 90 – 95 %, which is appreciably higher than when one holding tank is integrated in a continuously operated plant.
Cooling the milk
After pasteurization, the milk is cooled, first in the regenerative section and then with water, to the desired inoculation temperature (typically 40 – 45 °C). Alternatively, if set yoghurt is to be produced, and the pre-treatment capacity does not match the packing capacity, the milk is cooled to a temperature below 10 °C (preferably 5 °C).
Design of the yoghurt plant
When the yoghurt milk has been pre-treated and cooled to inoculation temperature, the procedure for further treatment depends on whether set, stirred, drink, frozen or concentrated yoghurt is to be produced. The block diagrams in Figures 11.11 – 11.13 show the various production stages for each process.
The quality of the yoghurt in terms of texture and flavour depends on the design of the plant, the treatment of the milk and the treatment of the product. Modern plants are designed to satisfy demands for high production, continuous treatment and high quality. The level of automation varies, and complete CIP systems are normally integrated into the plants.
The level of automation is usually high in large-scale production. Excessive mechanical treatment of the product must be avoided, as it may cause product defects such as thin consistency and whey separation. The total amount of treatment to which the product is subjected must be taken into consideration when the plant is designed. The choice of suitable equipment and the matching and optimization of the plant are consequently a question of achieving a suitable balance between cost and quality.
In modern plants, stirred and set types of yoghurt are often produced concurrently. In the production of set yoghurt, the product flow is continuously controlled from the point where the milk is accepted in the pre-treatment section to the packaging of the product. In the production of stirred yoghurt, the pre-treatment of the milk is continuous up to the point at which it is pumped into the incubation tanks, to which the culture is added. The continuity is interrupted by the time-consuming incubation, which must be free from any physical disturbance.
A typical plant for continuous production of a relatively large volume of stirred yoghurt is shown in Figure 11.14.
The pre-treated milk, cooled to incubation temperature, is pumped to the incubation tanks (7) in succession. Simultaneously, freeze dried or deep frozen culture is dosed into the milk stream. After a tank has been filled, agitation commences and continues for a short time to assure uniform distribution of the starter culture.
The incubation tanks are insulated, to ensure that the temperature remains constant during the incubation period. The tanks can be fitted with pH meters to check the development of acidity.
In typical production of stirred yoghurt the incubation period is 4-5 hours at 42 – 43 °C, when highly concentrated culture (about 0.02 % Inoculum) is used. The relative short incubation time indicates that the multiplication (generation) period is fast. For typical yoghurt bacteria, the generation period is some 20 – 30 minutes. To attain optimum quality conditions, cooling to 15 – 22 °C (from 42 – 43 °C) should be accomplished within 30 minutes after the ideal pH-value has been reached, to stop further development of bacteria.
Cooling the coagulum
In the final stage of incubation, when the required pH (normally about 4.2 – 4.5) has been reached, the yoghurt must be cooled to 15 – 22 °C. This temporarily stops any further increase in acidity. At the same time, the coagulum must be subjected to gentle mechanical treatment, so that the final product will have the correct consistency. In some cases a strainer or a structurizing valve is built into the line, prior to the cooler, in order to optimize the yoghurt structure and appearance.
Cooling takes place in a plate heat exchanger (8), which is designed to give a gentle mechanical treatment of the product. The capacities of pump and cooler are often dimensioned to empty a tank in about 30 minutes in order to maintain a uniform product quality. However, some cultures are specially adapted to stop when reaching pH e.g. 4.3 by themselves. These cultures are fermenting very slow in this pH area and thus a longer cooling time to 15-22 ºC can be accepted.
The cooled yoghurt is pumped to buffer tanks (9) before being routed to the filling machine(s) (12).
After cooling to 15 – 22 °C, the yoghurt is ready for packing. Fruit and various flavourings can be added (10) to the yoghurt when it is transferred from the buffer tanks to the filling machines. This is done continuously with a variable-speed metering pump, which feeds the ingredients into the yoghurt in the fruit-blending unit shown in Figure 11.15. The blending unit is static and hygienically designed to guarantee that the fruit is thoroughly mixed into the yoghurt. The fruit metering pump and the yoghurt feed pump operate synchronously.
The fruit additives can be:
- Sweet; normally 50 – 55 % ordinary sugar content
- Natural; unsweetened
The fruit should be as homogeneous as possible. A thickener in the form of pectin can be added. The proportion of pectin is hardly ever higher than 0.5%, which corresponds to 0.05 – 0.005 % of pectin in the end product.
Proper heat treatment is an extremely important stage in the pre-treatment of fruit additives. Scraped-surface heat exchangers, tubular heat exchangers or tanks with scraper units, can be used for adequate pasteurization of whole berries or fruit with solid particles. The temperature program should be such that all vegetative microorganisms are inactivated without impairing the taste and texture of the fruit. Continuous production, with rapid heating and cooling, is therefore important with regard to product quality and economic aspects.
Following the heat treatment, it is important that the fruit is packed in sterilized containers under aseptic conditions. Deterioration of cultured milk products is too often caused by reinfection from inadequately treated fruit.
Various types of filling machines are used to pack yoghurt. The sizes of the packages vary from one market to another. In general, the total packing capacity should match the capacity of the pasteurization plant, so as to obtain optimal running conditions for the plant as a whole.
As mentioned, the plant design is one important factor affecting the quality of the yoghurt and, of course, all other cultured products.
Figure 11.16 shows curves for the development of viscosity in stirred yoghurt from the moment it leaves the incubation tank, via packing and up to about 24 hours in cold storage.
Curve A represents the ideal situation, when all operations that influence the structure and viscosity are optimized.
It is inevitable that the product will become less viscous while being treated, since yoghurt belongs to the class of products with thixotropic flow behaviour. However, if all parameters and equipment are fully optimized, the viscosity will be almost fully regenerated, and the likelihood of syneresis occurring will be minimized.
Curve B shows the result when the product has been maltreated on its way from the incubation tank up to packaging and cold storage. If the yoghurt coagulum has been treated too hard, the viscosity will be too low, resulting in a liquid product with high risk for whey separation.
In order to reduce installation costs, it is possible to use the same plant for production of both stirred and set yoghurt. The pre-treatment of the milk intended for either product is identical up to cooling down to incubation temperature. Figure 11.17 shows how this kind of production can be arranged. The starter is metered into the stream of milk as it is pumped from an intermediate storage tank to the filling machine.
An alternative production concept
The most frequently used system for production of set yoghurt is illustrated in Figure 11.18. This system offers flexibility in production planning, because it is not necessary to match pre-treatment capacity to packing capacity.
The milk, pre-treated in the same way as for stirred yoghurt, is cooled to a temperature of less than 10 °C, preferably to 5 °C, and pumped into one, two or more tanks (1). Following culture inoculation and thorough stirring, the milk is ready to be heated in-line (2) to exact incubation temperature, before being packed (4) in containers.
Flavouring can be continuously metered into the milk stream prior to the filling machine. If fruit or additives with particles are added these have to be dosed into the packages or cups first before they are filled with inoculated milk. It is, however, important to remember that additives with low pH have a negative influence on fermentation.
Incubation and cooling
Following packaging the packages, after crating and palletizing, are trucked into either of two systems for incubation and subsequent cooling, viz.:
- Combined incubation/cooling chamber, when the pallets are stationary through both incubation and cooling, before being trucked to the final cold store.
- An incubation room able to accommodate a large number of filled pallets. After adequate incubation, the pallets are trucked to a conveyor passing through the cooling sections enclosed in a tunnel. This system offers continuous cooling and is illustrated in Figure 11.19.
The filled packages/containers are placed in crates of open design, and at a certain distance from each other, so that the circulating warm/cold air for the incubation and cooling room or chamber can reach every individual container. The crates are normally stacked on pallets, which are then trucked into the incubation room. This ensures uniform quality, provided that the temperature is accurately controlled.
When the empirically determined optimum pH (typically 4.5) is reached, it is time to start cooling. The normal target temperature is 18 – 20 °C; it is important to stop further growth quickly, which means that a temperature of about
35 °C should be reached within 30 minutes, and 18 – 20 °C after another
30 – 40 minutes.
Final cooling, normally down to 5 °C, takes place in the cold store, where the products are held to await distribution.
Cooling efficiency depends on the size of the individual package, the design and material of the packages, the depth of the crate stack, the spacing between individual packages in each crate, and the design of the crates.
At a depth of one metre, for example, the free cross section of the stack for air-flow must be not less than 25 % of the total area. A smaller, free cross-section will require higher airflows, which also means higher energy consumption.
The pallets (crates) are stationary during incubation. They are placed in the incubation room/chamber in such a way as to facilitate first in/first out handling. In a typical incubation period of 3 – 3.5 hours, it is very important that the product is not exposed to any mechanical disturbance during the last 2 – 2.5 hours, when it is most sensitive to the risk of whey separation.
The cooling capacity should be adequate to achieve the above-mentioned temperature program. As a guide, the total cooling time is about 65 – 70 minutes for small packages (0.175 – 0.2 kg sizes) and about 80 – 90 minutes for large packages (0.5 kg size).
Eventually, regardless of the type of incubation/cooling chamber, the set yoghurt is cooled to about 5 °C in the cold store.
A low-viscosity drinkable yoghurt, normally with a low fat content, is popular in many countries. The composition can be the same as for stirred yoghurt but can also be reduced in DM by e.g. dilution with water.
The yoghurt intended for production of drinking yoghurt is produced in the ordinary way with fermentation in tanks. In order to get a stable drinking yoghurt without sedimentation, a stabilizer (commonly pectin but also modified starch or CMC are used) should be added to the product before cooling. The yoghurt with added pectin is homogenized prior to cooling to get optimal stabilizing effect.
Shelf life of a fermented milk product is dependent on a number of visible and organoleptical factors like whey separation, changes in viscosity, structure, colour, acidity and aroma. It is of course also dependent on bacteriological defects.
Because of the tendency towards larger and more centralized production units, the markets are becoming geographically larger and transport distances longer. In some cases, the sales district may be so large that only one delivery per week is economically justifiable. This, in turn, necessitates methods which extend the shelf life of the product beyond normal. In some countries, it is difficult to maintain the integrity of the cooling chain. Therefore, there is a demand for a sterilized yoghurt that can be stored at room temperature.
The shelf life of cultured milk products can be extended in two ways:
- Production and packing under aseptic conditions
- Heat treatment of the finished product, either immediately before packing or in the package.
It should be noted that if the microorganisms in the yoghurt are killed by heating, the product is then, according to the definition in many countries, not allowed to be called yoghurt. They can however be called "yoghurt based".
Production under aseptic conditions
In aseptic production, measures are taken to prevent the yoghurt from being infected by yeast and moulds. These microorganisms would destroy the product, as they can survive and multiply in an acid environment and can cause off-flavours and whey separation. The prime measure is thorough cleaning and sterilization of all surfaces in contact with the product. The special feature of aseptic production is, however, that it takes place under aseptic conditions; using aseptic tanks which are permanently pressurized with sterile air, remote-controlled aseptic valves, aseptic metering devices for fruit and aseptic filling machines. Infection by airborne microorganisms can then be prevented. This extends the shelf life of the product significantly.
Clean Room production conditions
Hygenic conditions must be maintained in all food industries, not only in the equipment coming in direct contact with the product, but also in the premises where production takes place.
A system based on filtration of the air through absolute filters, as shown in Figure 11.21, can be installed to clean the air in processing rooms, tanks, etc. to a high standard of purity. In this system one main filter and a fan are serving four tanks. An alternative is that each tank is equipped with its own filter. An absolute filter is capable of trapping particles larger than 0.3 microns and will capture most microorganisms, as the average diameters of cocci, bacilli and fungi (yeasts and moulds) are 0.9; 0.25 – 10 and 3 – 15 microns respectively.
Each system or tank to be supplied with air is equipped with an extra pipe for the air and a safety system to prevent the tank from imploding as a result of the vacuum created by the drop in temperature after cleaning.
Air velocity is approx. 0.5 m/s and the tank is positively pressurized to approx. 5 – 10 m water gauge, corresponding to about 0.05 – 0.1 bar.
The filter is normally placed in the process room, with the result that all contaminant particles in the ambient air will eventually be filtered out, thereby creating Clean Room conditions.
Similar systems are used in bacteriological laboratories, hospital operating theatres and pharmaceutical factories.
The ”clean room” conditions will improve the production safety and minimize the risk for re-infection. However, the most critical areas for re-infection are at fruit addition and packaging. It is therefore of high importance that contamination can be excluded during these operations and that a high hygienic filling machine is used.
Production and packing under aseptic or high hygiene conditions are important prerequisites to improve shelf life and production safety of a yoghurt for cold distribution.
Heat treatment of yoghurt
Heat treatment of yoghurt is another method to prolong its shelf life. Dependent on temperature used the product can be stored chilled or ambient. The heat treatment temperature is dependent on a number of factors as: milk quality, milk pre-treatment, pH of yoghurt, fruit quality, particle size, stabilizer type and microbiological requirements of the final product.
All types of yoghurt (stirred, set, drinking and concentrated) can be prolonged in shelf life by heating.
Heat treatment of yoghurt prolongs its shelf life by:
- Inactivating the starter bacteria and their enzymes
- Inactivating contaminants such as yeasts and moulds
Long-life stirred yoghurt
In production of stirred yoghurt, the coagulum from the incubation tanks can be heat-treated at 60 – 70 °C for a few seconds. This heat-treatment will minimize post-acidification, giving the yoghurt a bacteriological shelf life in cold store of 1 – 2 months if packed under high hygienic conditions.
If the aim is to produce a yoghurt for ambient storage the heating temperature should be in the range of 75 – 110 °C for some seconds and dependent on factors as milk quality, milk treatment, pH of yoghurt etc.
Different processing solutions can be used.
- Yoghurt and fruit mixed. Heat-treated and cooled together.
- Yoghurt and fruit heat-treated and cooled separately prior to mixing.
- Yoghurt heat-treated and cooled. Fruit heat-treated and mixed warm to the cold yoghurt.
The product should, in all cases, be packed in an aseptic filling machine to prevent reinfection, as in Figure 11.22.
Viscosity reduction and whey separation are associated with heating of fermented milk. These problems can however be avoided by using stabilizers. The stabilizers will re-build the reological properties of the product.
Long-life set yoghurt
Set yoghurt can be heat-treated at e.g. 60 – 70 °C for 30 minutes in the packages, in special pasteurizing chambers. The time is of course dependent on the size and shape of the package. Also for set type a stabilizer should be used.
Long-life drinking yoghurt
Drinking yoghurt may have the same composition as ordinary milk. It is however popular in many countries to dilute the product with water. In certain regions drinking yoghurt can be a mixture of 30 % yoghurt and 70 % water.
Pectin is a common stabilizer used to avoid sedimentation and whey separation as well as to improve the viscosity and the mouth feel of the product after heating. Other stabilizers which can be used are modified starch e.g. CMC. If pectin is used it is preferably added as a water solution to the yoghurt prior to the final heat treatment. In order to get the optimal stabilizing effect of the pectin, mechanical treatment e.g. homogenization, should take place. In some countries the use of stabilizers is forbidden by law or is only permitted to a limited extent. Other additives to the drinking yoghurt are sugar and fruit concentrate or aroma.
Heating to a temperature of about 75 °C and above kills all the virulent microorganisms in the yoghurt.
A process line for heat treatment of yoghurt can also be used for production of pudding and desserts.
Frozen yoghurt can be manufactured in two ways. Either, the yoghurt is mixed with an ice cream mix or an ice cream mix is fermented, before further processing.
In the latter alternative a conventional line for production of stirred type yoghurt can be used. About 4 – 6 % starter is dosed into the pipeline as the mix is pumped to the incubation tanks. The incubation time of the yoghurt mix is appreciably longer than for normal yoghurt production. This is because the yoghurt mix contains much more carbohydrates than normal yoghurt. An incubation time of 7 – 8 hours is required at a saccharose content of 10 – 12 % to attain the characteristic acidity of yoghurt, which occurs at pH 4.5. For both alternatives further processing will be identical with the conventional production of ice cream. (See Chapter 19 Ice cream.)
Frozen yoghurt can be divided into soft-served and hard-frozen types. The mix intended for soft-served yoghurt differs somewhat from that of the hard-frozen type. Typical recipes are:
Concentrated yoghurt is produced in many countries. It Is also known under names as Strained yoghurt, Greek yoghurt, Labneh, Skyr, etc., dependent on country or region. In concentrated yoghurt the DM of the product is increased after fermentation. Whey is drained off from the coagulum. There are two main production methods used today:
Fat 4 6
Sugar 11 – 14 12 – 15
MSNF 10 – 11 12
Stabilizer, emulsifier 0.85 0.85
Water 71 66
In both cases standardized milk is heat treated 90-95 ºC for 5 minutes before it may be homogenized (if required) and then cooled to fermentation temperature before it is fed to the incubation tanks. After ready fermentation the yoghurt is evenly agitated before it is thermized 55-60 ºC for 2-3 minutes and cooled to separation temperature. The product is fed to either a nozzle separator or to an ultrafiltration unit plant. Here the whey/permeate is separated and the concentrated yoghurt is pumped out for cooling and packing. In the latter methods there are mainly three different types of membrane systems which can be used. These are: ceramic, spiral wound or plate & frame. Which one to use is dependent on several factors such as product dry matter, running time needed, investment costs, etc.
Except for these two production methods It is also possible to standardize the milk to final composition before fermentation. Care has to be taken when using this method, as if protein content is increased too much there is a risk of getting off flavours and sandiness in the finished product. No matter which method is used to produce the concentrated yoghurt, the final product is often very viscous. Compared to a stirred yoghurt line extra care has to be taken in the design and dimensioning of the buffer tanks and surrounding equipment to make sure that the thick product can be emptied from the tanks.
Kefir is one of the oldest cultured milk products. It originates from the Caucasus region and is today produced in many countries. The raw material is milk from goats, sheep or cows. .
Kefir should be viscous and homogenous, and have a shiny surface. The taste should be fresh and acid, with a slight flavour of yeast. The pH of the product is usually 4.3 – 4.4.
A special culture, known as Kefir grain, is used for the production of Kefir. The grains consist of proteins, polysaccharides and a mixture of several types of microorganisms, such as yeasts and aroma and lactic-acid forming bacteria. The yeasts represent about 5 – 10 % of the total microflora.
The Kefir grains are yellowish in colour and about the size of a cauliflower florette, i.e. about 15 to 20 mm in diameter. The shape of the grains is irregular, as seen in Figure 11.25. They are insoluble in water and in most solvents. When steeped in milk, the grains swell and become white. During the fermentation process, the lactic-acid bacteria produce lactic acid, whereas the lactose-fermenting yeast cells produce alcohol and carbon dioxide. Some breakdown of protein also takes place in the yeast metabolism, from which Kefir derives its special yeast aroma. The contents of lactic acid, alcohol and carbon dioxide are controlled by the incubation temperature during production.
A The yoghurt bacteria Lactobacillus bulgaricus (rod shaped) and Streptococcus thermophilus (spherical) live together.
B Yeast and lactic acid bacteria at the surface of a kefir grain. The “ball” in the centre is a yeast fungus and the rods are different kinds of bacteria.
C The centre of a kefir grain. Y east and bacteria are united by a network consisting mainly of proteins and polysaccharides.
Depending on local conditions and requirements, the equipment and pro-cess variables may differ significantly from one manufacturer to another.
Kefir type products can also be produced in the same way as stirred type yoghurt by using special concentrated freeze dried starter culture.
As with other cultured milk products, the quality of the raw material is of major importance. It must not contain any antibiotics or other bactericidal agents. The raw material for kefir manufacture can be milk from goats, sheep or cows.
Production of starter culture
Kefir culture is normally produced from milk of various fat contents, but skim milk and reconstituted skim milk, too, have lately been utilized for better control of the microbial composition of the kefir grains.
As in propagation of starter cultures for other cultured milk products, the milk substrate must be thoroughly heat-treated to inactivate bacteriophages.
Production takes place in two stages. The basic reason for this is that kefir grains are bulky and awkward to handle, whereas relatively small volumes of mother culture are easier to control. Figure 11.28 shows the various process stages.
In the first stage, the pre-treated substrate is inoculated with active kefir grains. Incubation takes place at about 23 °C, and the proportion of grains is about 5 % (1 part grains to 20 parts substrate) or 3.5 % (1 part grains to 30 parts milk). The incubation time is about 20 hours; as the grains tend to sink to the bottom, intermittent stirring for about 10 – 15 minutes every 2 – 5 hours is recommended. When the desired pH value (say 4.5) has been reached, the culture is stirred before the grains are strained off from the mother culture, now also called filtrate. The strainer has holes with a diameter of 3 – 4 mm.
The grains are washed in the strainer with boiled and cooled water (sometimes skim milk). They can then be reused to incubate a new batch of mother culture. The microbial population grows by about 10% per week during incubation, so the grains must be weighed and the surplus removed, before the batch is reused.
In the second stage, the filtrate can be cooled to about 10 °C if it has to be stored for a few hours before being used. Alternatively, if large quantities of kefir are going to be produced, the filtrate can be immediately inoculated into the pre-treated milk intended as the substrate for the bulk starter. The dosage is 3 – 5 % of the volume of the substrate. After incubation at 23 °C for about 20 hours, the bulk starter is ready for inoculation into the kefir milk.
Production of kefir
The process stages are much the same as for most fermented milk products. The following combination is typical for traditional production of kefir:
- Fat standardization (not always practised)
- Pasteurization and cooling to incubation temperature
- Inoculation with starter culture (here also called 'filtrate')
- Incubation in two stages (this, together with the specific culture, is characteristic of kefir)
The fat content of kefir is reported to vary between 0.5 % and 6 %. The raw milk is often used with its original fat content. However, fat contents of 2.5 to 3.5 % are frequently specified.
Following fat standardization, if any, the milk is homogenized at about 65 – 70 °C and 17.5 – 20 MPa (175 – 200 bar).
The heat treatment program is the same as for yoghurt and most cultured milks: 90 – 95 °C for five minutes.
Following heat treatment, the milk is cooled to inoculation temperature, usually about 23 °C, after which 2 – 3% starter is added.
The incubation period is normally divided into two stages, acidulation and ripening.
The acidulation stage
The acidulation stage lasts until a pH value of 4.5 is reached or, expressed as acidity, until 85 – 100 °Th (35 – 40 °SH) has developed. This takes about 12 hours. The coagulum is then stirred and pre-cooled while still in the tank. At a temperature of 14 – 16 °C, cooling is stopped and agitation discontinued.
The ripening stage
The typical slightly yeasty flavour starts to develop during the following 12 – 14 hours. Final cooling commences when the acidity has reached 110 – 120 °Th (pH about 4.4).
The product is cooled rapidly to 5 – 8 °C in a heat exchanger. This stops any further reduction in pH. It is of vital importance that the product is treated gently when cooled and during subsequent packing. Mechanical agitation in pumps, pipes and filling machines must therefore be minimized. Air entrainment must also be avoided, as air increases the risk of syneresis in the product.
Alternative kefir production
As previously mentioned, the traditional method of preparing bulk starter for kefir manufacture is laborious. This, in combination with the complexity of the microflora, sometimes leads to unacceptable variations in product quality.
To overcome these problems, freeze-dried concentrated kefir culture that is handled in the same way as similar forms of other cultures, have been developed at culture laboratories.
After thorough examination of kefir grains obtained from various sources, strains of bacteria and yeasts were isolated and tested for various growth characteristics, lactic acid production, aroma formation, etc. The composition of the freeze-dried culture was then chosen to obtain a balance of microorganisms in the bulk starter and product comparable to that of traditional kefir manufactured with grains in a mother culture.
Concentrated freeze-dried kefir cultures for direct use in the milk inten-ded for the end product are now commercially available. The block chart in Figure 11.29 illustrates the processing stages.
Compared to traditional bulk starter production, the technique based on freeze-dried culture reduces the number of process stages, and with it the risk of reinfecting the culture.
It should however be noted that kefir produced by this type of freeze dried culture is not allowed to be called "kefir" in some countries. It is instead called something like "kefir type".
Cultured cream has been used for years in some countries. It forms the basis of many dishes in the same manner as yoghurt. Cultured cream can have a fat content of 10 – 12 % or 20 – 30 %. The starter culture contains Lc. lactis subsp. lactis and Lc. lactis subsp. cremoris (O cultures) whereas Lc. lactis subsp. lactis biovar. diacetylactis and Leuc. mesenteorides subsp. cremoris (LD and L cultures) bacteria are used for the aroma.
Cultured cream has a uniform structure and is relatively viscous.
The taste should be mild and slightly acidic. Cultured cream, like other cultured products, has a limited shelf life. Strict hygiene is important to ensure product quality.
Yeast and moulds can develop in packages which are not airtight. These microorganisms occur mainly on the surface of the cultured cream. In the event of extended storage, the lactic acid bacteria enzymes, which break down b-lactoglobulin, become active and the cultured cream goes bitter. The cultured cream also loses its flavour because carbon dioxide and other aromatic substances diffuse through the packaging.
Long-life cultured cream can also be produced by heat treatment of the product prior to the packing. Stabilizers are added as for other heat-treated fermented dairy products.
The process line for production of cultured cream includes equipment for standardization of the fat content, homogenization and heat treatment of the cream, and also inoculation and packing.
The cream is homogenized. For cream with 10 – 12 % fat the homogenization pressure is normally 15 – 20 MPa (150 – 200 bar) at 60 – 70 °C. Up to a certain point, an increase in homogenization temperature improves the consistency.
For cream with 20 – 30 % fat, the homogenization pressure should be lower, 10 – 12 MPa (100 – 120 bar), as there is not enough protein (casein) to form membranes on the enlarged total fat surface.
The homogenized cream is normally heat treated for five minutes at 90 °C. Other time/temperature combinations can be used if the homogenization technique is carefully matched to the heat treatment.
Inoculation and packing
The pre-treated cream is cooled to an inoculation temperature of 18 – 21 °C. 0.01 % concentrated culture or 1 – 2 % of bulk starter culture is added.
Inoculation can take place in a tank or in the packages. The fermentation time is 18 – 20 hours. When fermentation is completed, the cultured cream is cooled quickly, to prevent any further pH reduction. For low fat (10-12 %) products the cooling can take place in a plate heat exchanger as the viscosity is rather moderate. The higher fat content the more viscous the fermented cream is. This will make cooling in a plate heat exchanger difficult (due to the high pressure drop). The fermented cream can then be fed to the packaging directly and the product is then cooled in the package (Fig. 11.30).
The cream is sometimes inoculated, packaged and fermented in the packages to avoid mechanical treatment. This is especially the case when a high fat cream is produced.
After inoculation of the cream and subsequent packing, the product is stored at 20 °C until the acidity of the fat-free phase is about 85 °Th, which takes about 16 – 18 hours. The packages are then carefully transferred to the chilled store, where they are kept for at least 24 hours at a temperature of about 6 °C before distribution.
Cultured cream is often used in cooking.
Long-life cultured cream
The shelf life of the cultured cream can be prolonged by heat treatment. Stabilizers are added either in the cream before fermentation or in the fermented cream before final heat treatment. The viscosity of the ready product is dependent on the choice of stabilizer as well on the design of the plant.
Buttermilk is a by-product of butter production from sweet or fermented cream.
The fat content is about 0.5 %, and it contains a lot of membrane material including lecithin. The shelf life is short, as the taste of the buttermilk changes fairly quickly because of oxidation of the membrane material content. Whey separation is common in buttermilk from the manufacture of butter, based on fermented cream, and product defects are therefore difficult to prevent.
Fermented buttermilk is manufactured on many markets in order to overcome problems such as off-flavours and short shelf life. The raw material can be sweet buttermilk from the manufacture of butter based on sweet cream, skim milk or low-fat milk.
In all cases the raw material is heat treated at 90 – 95 °C for about 5 minutes before being cooled to inoculation temperature. Ordinary lactic-acid bacteria are most commonly used. In some cases, when the raw material is skim milk or low fat milk, grains of butter are also added to the product to make it look more like buttermilk. Buttermilk may also be flavoured with e.g. fruit concentrate.
Trends in fermented milk products
The latest years there has been increased focus on Functional Foods. Within this category certain types of lactic acid bacteria plays a large role.
For a number of years it has been known, at least in the northern part of Sweden, that a certain type of cultured milk called Långfil has been used to heal wounds and treat vaginal fungus infections. However, studies of lactic acid bacteria and their importance to health can be traced back to the beginning of the twentieth century. Professor Elie Metchnikoff of the Pasteur Institute in Paris, France, knew that many people in his Russian home district consumed a great deal of yoghurt and lived for a long time. (Professor Metchnikoff was awarded a Nobel Prize in Medicine in 1908, but that was for the discovery of phagocytosis, in which white blood corpuscles, leucocytes, eat bacteria that have invaded the body.)
Metchnikoff argued that lactobacilli ingested by consumption of yoghurt pass through the stomach and destroy putrefactive bacteria in the colon. By doing so they inhibit the production of “poisonous” waste products that cause chronic morbid alterations in the system, especially arteriosclerosis.
This theory of Metchnikoff’s was plausible, but it has also been criticised on the grounds that lactobacilli cannot survive the low pH, approximately 2, that prevails in the stomach. However that may be, the following fragments of information reflect the situation in the final decade of the twentieth century.
Interest in the deliberate use of lactic acid bacteria as a health-giving constituent of certain foods and forage products has snowballed in the past few years. The greatest enthusiasts claim that living lactic acid bacteria will be the 21st century’s answer to the 20th century’s penicillin and sulfa drugs.
The expression “functional food” is applied to foods with near-medicinal properties that promote health. “Food for special health use” is another term for the same thing.
Lactic acid bacteria have been used since time immemorial to ferment foods. The special strains of bacteria normally used in production of yoghurt, as well as other types such as Lactobacillus acidophilus, L. reuteri, Bifido bacteria and certain species of Lactococcus lactis, are among those that have been found of interest for production of functional foods.
What properties must a lactic acid bacterium have to be able to function in the intestine? The following four characteristics that are of primary importance:
- Ability to colonize and survive
- Adhesive capacity
- Ability to aggregate
- Antagonistic effects
L. acidophilus and Bifido bacteria are important members of the human intestinal flora. The former normally predominates in the small intestine and the latter in the large intestine.
Production of these important bacteria is reduced in some people as a result of medication, stress or old age. In many people, reduced production of intestinal bacteria can cause symptoms such as swelling, indigestion and pronounced illness.
Consumption of live L. acidophilus and Bifido bacteria in milk products is an ideal way to restore the balance of the intestinal flora.
Apart from the possible prevention and relief of diarrhoea, literature indicates that L. acidophilus and Bifido bacteria may help to:
- Reduce the cholesterol level in the blood
- Relieve lactose malabsorption (lactose intolerance)
- Strengthen the immune system
- Reduce the risk of stomach cancer.
(Nutrish cultures, Chr. Hansen’s Laboratories, Hørsholm, Denmark)
These microorganisms can be utilized alone or in combination with other cultures, e.g. thermophilic, yoghurt or mesophilic cultures.
Thus, lactic acid bacteria may have a great potential for promoting the health of both human beings and animals. The claimed effects, however, are by no means fully documented. It is therefore important that sufficient resources are invested in this field in the near future, both to find new interesting health effects of lactic acid bacteria and to compile scientific documentation. Other trend Is protein enriched fermented milk products. These are concentrated fermented products where the protein content has been increased often around 3 times that of normal milk. The product may contain 0-10 % fat. This concentrated yoghurt has many names, depending on where it is produced. It is called Labneh, strained type yoghurt, Greek yoghurt, Skyr, etc.
Many people in the world are lactose-intolerant. Thus they get stomach problems when consuming dairy products like milk containing lactose. In fermented milk products, however, some of the lactose has been converted to glucose and galactose by lactic acid bacteria. This allows some lactose-intolerant people to consume yoghurt, for example, without problems. There are many lactose-free milk products available today In shops for those who are extremely lactose-intolerant. This trend not only covers consumption milk but also fermented milk products like yoghurt.