Quality Milk is a versatile fermentation medium containing all the growth factors necessary for lactic acid bacteria. However, if the quality parameters of the final product are to be controlled:
- Quality of the raw milk must be high
- Plate count must be low
- Content of free fatty acids must be low
- Must be free of chemical residues - Low number of somatic cells
To produce high-quality yogurt, it is often necessary to increase the solid non-fat (SNF) content. This can be carried out by the addition of milk powder or by concentration. Careful control of re-combination conditions is necessary to ensure full hydration.
By letting the milk undergo proper treatment, it is possible to influence its characteristics and improve the viscosity and the structural stability - the curd properties - of the final product.
For many products, the curd properties are improved by increasing the protein content of the milk. The most common methods of increasing the total solids content are evaporation and membrane filtration. The choice of method will usually be determined by factors such as local regulations, energy prices and other running costs, investment costs, capacities, and product range.
The degree of evaporation depends on the composition of the raw milk and the desired composition of the final product. For production of yogurt, the raw milk SNF is normally concentrated to 9.5-12%. Evaporation can take place either in a separate evaporator (external evaporation) or in-line as part of the yogurt processing line (internal evaporation). External evaporators are more expensive than the in-line solution, but the running costs may be lower. Therefore, external evaporators are used only when the amount of water to be evaporated is large, or when the evaporators are to be used for other products as well.
Internal evaporation has some capacity limits and will normally only be used for a maximum of 1,500 l/h of evaporated water. The advantages of using an internal evaporator include: no extra buffer tanks for the concentrated milk, no extra CIP cleaning of equipment, minimal space requirement, less product waste, and fewer operating steps.
Membrane filtration for concentration of milk usually involves either reverse osmosis or ultrafiltration. Reverse osmosis concentrates all milk solids and can therefore be compared with evaporation. So far, reverse osmosis has not been used extensively in the production of cultured milk products, but the number of production plants and the competitiveness and reliability of reverse osmosis plants is increasing. In ultrafiltration, selective concentration takes place, i.e. the fat and proteins are concentrated whereas the other solids remain almost unchanged. The protein content is normally concentrated to 3.8-4.7%. Today, ultrafiltration is used in many cultured milk plants around the world. The main advantage is that the protein content can be substantially increased without significant changes to the lactose content.
Reverse osmosis and ultrafiltration plants can be either external plants or internal plants incorporated into the milk processing line. However, these plants must always be cleaned separately, even when they are installed as in-line plants.
The viscosity of the final product is highly dependent on the fat content and the size of the fat globules. To obtain yogurt with a uniform and well-defined fat content, the fat content of the milk must be adjusted to the required level, and all production parameters must be kept constant. Fat standardisation will usually take place before the yogurt milk is processed but can also be performed in-line.
Heat treatment improves the viscosity of cultured milk products. Optimum improvement of stability and viscosity is typically achieved by subjecting the milk to a heat treatment of 90°C for a minimum of 10 minutes. However, a pasteurisation temperature of 90°C is not always satisfactory in terms of bacteriological composition. To destroy important bacteria spores and enzymes, additional pasteurisation at 120-130°C with a holding time of 4-20 seconds is necessary. Heat treatment causes changes in the milk proteins so that the whey proteins are denaturated and form complexes with the caseins. The whey proteins function as natural stabilisers. Furthermore, heat treament de-activates most of the substances which inhibit bacterial growth in the milk, it reduces the content of dissolved oxygen, and it improves the milk as a growth medium of lactic acid bacteria.
Since the starter culture bacteria are anaerobic bacteria, de-aeration of milk will improve the growth conditions. De-aeration will also improve the working conditions for the homogenisers. If an internal evaporator is used, it can also function as a de-areator. Homogenisation When milk is homogenised, some of the milk proteins create new fat globule membranes. This creates an apparent concentration of the protein content so that the fat globules act as protein particles. This also means that the more efficient the homogenisation, the greater the improvement in the stability and viscosity of the curd structure. Efficient homogenisation is a combination of temperature and pressure. The fat content of the milk is the main basis upon which the homogenisation pressure is chosen; typically it will be between 250 and 400 bar, with the highest pressure being applied to the products with the lowest fat content. Other parameters in the choice of homogenisation pressure are the amount of protein in the milk and the protein-fat ratio. Finally, the type and number of homogeniser heads play an important role in the choice of the correct homogenisation pressure.
The pH value of milk can be lowered either by the addition of acid or by bacteriological fermentation of the lactose - the basic component from which curd is formed when the pH is lowered is the milk protein and especially the casein. Milk has a pH value of 6.6. At a pH of approx. 5.2, curd formation begins. With a further reduction of the pH value, the maximum curd firmness is obtained around pH 4.6.
For this type of product, the pasteurised yogurt milk is cooled to 5-6Â°C and led to a buffer tank. There are three basic ways of adding the starter culture: in the filling line before the buffer tank, directly into the buffer tank, or between the buffer tank and the filling machine. Whatever method is used, it is important that the starter culture is mixed thoroughly with the milk before it enters the filling machine. Likewise, the dosage must be kept constant to ensure uniform fermentation in the individual packages. On its way from the buffer tank to the filling machine, the yogurt milk is heated to the fermentation temperature - or slightly higher.
Incubation and cooling
When the yogurt milk has been filled into the final packages, these must be incubated at the fermentation temperature in an incubation chamber/room. It is very important that the packages are not handled in any way during fermentation as any vibration will disturb the curd formation. The temperature in the incubation chamber/room must be kept constant and the circulation of air must be properly controlled.
The choice between a chamber system and a room system depends primarily on the type and volumes of products to be produced. In the chamber system, each individual pallet may be subjected to its own individual heat treatment, and there is no need to move the products before they have been cooled. Incubation rooms, however, have certain limitations regarding the individual treatment of pallets. These must be transferred from the incubation room to the blast cooling area, and such transfers must be carried out without any vibration or shaking taking place since this will destroy the stability of the products. Cooling of the products also involves special care, the products must be cooled as quickly as possible to stop fermentation, but wheying-off in the packages must also be avoided and this requires fairly slow, controlled cooling.