The freeze-drying process was developed as a commercial technique that enabled heat sensitive products to be made stable and viable without refrigeration.
Freeze drying is an industrial process which began as early as 1890 with a limited success and reached turning point during World War II when medications were needed to treat the wounded in the field, and because of the lack of refrigerated transport, many medical supplies spoiled before reaching their recipients.
In the 1950s–1960s, freeze drying began to be recognised as a multi-purpose tool for both pharmaceuticals and food processing, when freeze-dried foods became a major component of astronaut and military rations which needed to preserve the food for long with no refrigeration.
The process of freeze-drying, also known as lyophilization or cryodesiccation, is a low-temperature dehydration process that involves freezing the product, lowering pressure, and refinement.
The primary purpose of freeze-drying within the food industry is to extend the shelf-life of the food while maintaining the quality and due to the cost, the process is applied to high value food products. Examples of high-value freeze-dried products foods are those used by military or astronauts as well other specific products like yogurt and kefir which can be recultured many times after the first initial batch, effectively reducing the investment.
So, as you know the basics, let's describe how the freeze-dried yogurt and kefir starters are made
Fresh dairy milk from selected farmers is used. The fresh milk goes through preliminary microbiological inspections and standardization of the milk fat content.
The juice extracted yogurt cultures are made as a high in protein juice is used instead of dairy milk.
The first step in starter manufacture is the heat treatment of the milk in a fully automatic bioreactor responsible for processing and monitoring of the milk. It is heated first to 55 – 65°C and 14-20 MPa to homogenize the milk and then is heated to 93 – 95 °C and held for 15 – 30 min. This improves the milk structure and sanitizes the milk.
After heat treatment, the medium is cooled to inoculation temperature in the bioreactor. Typical inoculation temperature ranges are 20 – 30 °C for mesophilic types of cultures like kefir starter and 42 – 45 °C for thermophilic types like yogurt starter cultures.
For inoculation, a determined quantity of bacterial culture which is already processed in the laboratory is transferred automatically from the laboratory sample box to the heat-treated medium in the bioreactor, after the temperature has been adjusted to the correct level. Fully automatic monitoring of the PH level is constantly performed.
As soon as inoculation has taken place and the starter has been mixed into the medium, the bacteria begin to multiply – incubation begins. The incubation time is determined by the types of bacteria in the culture, the inoculation dosage, etc., and can vary from 3 to 20 hours.
Cooling is started at an empirically determined acidity which again is monitored automatically to stop bacterial growth and thus to preserve the activity of the culture at a high level.
This is a low-temperature dehydration process (-50 to - 80 °C) that involves freezing the product, lowering pressure, then removing the ice by refinement.
The lyophilized product goes to the process of pulverization.
The final microbiological sample of the product is then examined and inspected for quality control purposes in a sterile environment to avoid any contamination from the environment bacteria and microbiological check at the moment of the finished process before packaging.