The standardization can of course be carried out in dry form, i.e. by a simple dry mixing of the various components. But one of the most important things to be taken into consideration when producing baby food is the bacteriological quality of the end product.
As it is much easier to pasteurize the product in liquid form than it is to keep dry products uninfected during processing (some of them may even contain an unaccept-able level of bacteria), it is preferred to mix all the ingredients in liquid form prior to pasteurizing and drying.
This mixing is carried out to a solids content of 40-45% using pre-evaporated milk with 20-30% solids as basis. Demineralized and/or ultrafiltrated whey powder com-ponents are mixed. Further, carbohydrates and fat are added, if necessary. The mixing is carried out in big tanks equipped with volume or weight indication. The mixture is pasteurized and evaporated to about 50-55% solids depending on composition, after which the concentrate is homogenized and finally dried. It is important to avoid contamination after the pasteurization. Open buffer tanks must therefore be avoided.
The baby food concentrate can be dried in a normal spray dryer of the type usually used in the dairy industry.
However, due to the composition the products do not behave like ordinary milk powders in the dryer. This is caused by the higher content of carbohydrates and fat. The powder becomes thermoplastic and sticky, and wall deposits and cyclone problems may well occur, resulting in product loss and risk of contamination.
Average running conditions are usually:
Solids in feed: 50-55%
Feed temperature: 70-80ºC
Drying temperature: 180-190ºC
In case a product with very high content of carbohydrates is wanted, it may be necessary to use a lower drying temperature than described above.
By applying fines return, an agglomerated, instant, and free-flowing product is obtained. The excess moisture is evaporated in the Vibro-Fluidizer, in which also the cooling of the powder is carried out. Lecithin is sometimes sprayed on to the powder thus improving the instant properties.
However, the most commonly used drying plants for baby food are either the Tall-Form Dryer or the Multi-Stage Dryer, if a product with big agglomerates is wanted. These two dryers offer many advantages for this type of product:
- They operate with nozzles resulting in a more free-flowing powder.
- The plug-flow air stream minimizes wall deposits, so that longer operation time between cleaning is possible.
- The special air outlet design in the Tall-Form dryer reduces the cyclone fraction leading to less cyclone problems. This plant can produce agglomerated and non-agglomerated products.
- The special air outlet design in the Multi-Stage dryer offers an optimal agglomeration of the powder
In Fig. 143 is shown how the evaporator and dryer are connected to avoid contamination. The evaporator is working as "slave" to the spray dryer and is equipped with an enclosed buffer tank with level control below the calandria. Control loops ensure a constant solids content in the concentrate fed to the spray dryer.
In some cases the final product may contain an unacceptable number of bacteria originating from the raw material used in the formulas. It is now possible to UHT treat the premix prior to the evaporation. The high temperature (120-140ºC) is reached by direct steam injection into the premix. The temperature is immediately flashed back to 80ºC in a flash-regenerative preheater just prior to the evaporation. See Fig. 144.
Fig. 143 Flow-sheet of baby food plant with wet mixing and high-concentrator for feed preparation
The powder should be packed in cans of 1-2 lbs. To prolong the shelf-life a gas packing with N2 is recommended. Oxygen content should be less than 2%.
Fig. 144 UHT plant for baby food