El suero tradicional permanece en la preparación de la mantequilla de la crema agria. Durante el batido de leche, la grasa de la leche se agrupa porque las membranas de grasa se rompen. La grasa láctea agrupada fluye entonces como mantequilla encima del suero. Los fosfolípidos de estas membranas grasas son muy sensibles a la oxidación, lo que estropea el suero rápidamente. Hoy en día, casi todo el suero se hace agregando bacterias de ácido láctico a la leche desnatada pasteurizada.
In general, skimmed milk is used as a base for buttermilk production. This milk must be of a high microbiological quality and not contain any antibiotics or disinfectants. For the best flavor it is recommended to use milk with a fat content of at least 1 to 1.8%.
The flavor of the buttermilk is, to a large extent, determined by the diacetyl production of the lactic acid bacteria. To stimulate the diacetyl production by the lactic acid bacteria, often 0.10% w/w of sodium citrate is added. To prevent syneresis, stabilizers can be added, like modified starch or carrageenan.
Before the milk, used in the production of buttermilk, is pumped to the storage tanks, the following things have to be checked:
- Directly aerobic colony count;
- Presence of antibiotics;
- Sensory quality;
When the milk is stored in storage tanks, its acidity is checked regularly.
The milk is standardized on non-fat solids (at least 9%). Before the milk is pasteurized, 0.1 to 0.2% salt is added, to obtain the desired flavor.
Before the standardized milk is pasteurized, it is homogenized under high pressure (125 bar), at a temperature of about 49°C, creating small fat globules, which the prevent the fat from floating.
In order to reduce the bacterial count and increase the viscosity, through denaturation of whey proteins, the homogenized milk is pasteurized at 90°C for 2 to 5 minutes.
After pasteurization step, the milk is cooled to a temperature of between 22.2 and 23.3°C and pumped to the fermentation tank. Here, the milk is inoculated with about 1 to 3% of a mesophilic starter. This so-called mesophilic starter consists of the lactic acid bacteria Lactococcus lactis ssp. lactis, Lactococcus lactis ssp. cremoris and Leuconostoc mesenteroides ssp. cremoris. The first-mentioned lactic acid bacteria are responsible for producing lactic acid from lactose, while the latter produces the aroma. It is very important these lactic acid bacteria are well-balanced. A maximum of 20% of the bacteria can be of the aroma producing kind.
After adding the mesophilic starter, the mixture is stirred for about 15 to 30 minutes, depending on the type of tank and its agitators, at a high speed. After which, the milk is allowed to ferment, and after 12 to 15 hours the pH is measured until a pH of 4.6 is reached. At this pH level, casein sinks to the bottom of the tank, making the mixture curdle. The milk is then left to rest for about 1 to 2 hours, this will lead to the desired taste and aroma.
After the buttermilk is fermented sufficiently, the buttermilk has to be cooled rapidly. In order to prevent post-contamination and to terminate the production of acid. This rapid cooling starts with the circulation of ice water between the outer layers of the fermentation tank, for about 10 to 15 minutes. Next, the whole is stirred at a high speed until the product has become smooth. Then, the stirring speed is reduced and the buttermilk is stirred until it is cooled down to 17°C.
For the filling process to start, stirring should be stopped, preventing air from entering the buttermilk. Any air could may lead to syneresis.
The cooled buttermilk is pumped to the filling machine and filled into bottles or cardboard packs.
When the buttermilk has been bottled, it must be stored at 4°C. If kept at this temperature, the buttermilk will have a shelf life up 2 to 3 weeks.
Food Safety & Hygienic Design
Milk is the ideal environment for all microorganisms to grow. Between a temperature of from 7 to 63°C these microorganisms can develop relatively quickly into undesirable quantities.
The whole buttermilk production process should therefore be hygienically designed: cleanable to a microbial level and able to be set aside to dry after cleaning. If the latter is not possible, this part of the plant will lose its clean status, within 4 hours after cleaning, and will have to be cleaned and especially disinfected again.
The fermentation step is a critical part of the process. The temperature is ideal for microbial growth, it also enables the good and wanted bacteria to do their job. During the fermentation, however, it is not desirable that other, unwanted, microorganisms also develop further. This part of the process should preferably be aseptic. I.e. cleanable to microbial level, including sterilizable with steam and bacteria-proof - as according to the EHEDG guidelines 5 and 7.
After the acidification has occurred – and the buttermilk is at a pH of less than 4.6 - the product is no longer susceptible to the growth of pathogenic microorganisms. If the buttermilk is kept cool, then the following steps in the process; the including filling/ bottling machine do not need to be aseptic. Hygienic design is hereby sufficient.
A note about the pasteurizer. Frequently, plate heat exchangers with a regeneration section are used. The regeneration section saves energy, because the unpasteurized milk is heated with the pasteurized milk, which in turn, is cooled. It is known that these plates even when new, but even more likely after some time, will start to leak. This is not desirable for the fermentation process. The usage of a hygienically designed tubular heat exchanger of the type tube-in-tube, is preferred.
It cannot always be confirmed that the multi-tube-in-tube heat exchanger is fully clean, because of its increased viscosity, which occurs during the cooling of the buttermilk after fermentation. When one of the parallel tubes clogs, the CIP (Cleaning-in-place) cannot always be used to get the tube free again, causing a contamination to remain in the system. This will not be discovered right away. Therefore, here the preference is also given to a tube-in-tube heat exchanger.