Process of evaporation

Concentration of liquid foods is an important operation of many food processes and there are several technologies available, such as evaporation under vacuum and membrane concentration.

Evaporation has historically been the primary technology for liquid concentration in the food industry.

Evaporation removes most of the water resulting in concentrated product which may be used as such or processed further, e.g. by drying.

Evaporation is employed in the food industry to reduce weight and volume of fluids, with subsequent reduction of packaging, transportation, and storage costs. It also used to reduce the energy consumption at the drying operation.

Another important application is the production of fruit (mainly grape, apple, and orange) and vegetable (mainly tomato products) juice concentration.

In the dairy industry, evaporation is used for concentration duties, such as milk, skimmed milk and whey. It is also used as a preliminary step to drying. The thermal efficiency of evaporators for removing water is much higher (e.g 90%), compared to the efficiency of dryers (e.g. 60%).

Evaporation of water from the solution by heating is feasible, but usually the products to be evaporated are heat sensitive and heating can change some their physicochemical characteristics at this elevated temperature. Evaporation might result in crustal precipitation.

Evaporation might also result in foaming of the concentrate. Evaporation at low temperatures, under vacuum, reduces thermal degradation of food properties (textural and nutritional) and aroma recovery schemes allow collection of essential flavors and aroma compounds.
Process of evaporation

Tubular heat exchangers

The simplest heat exchanger is the double pipe type which consists of two centric pipes with the cold stream flowing usually in the inner tube and the heating medium in the annulus. It operates in either co-current or counter-current flow.

The concentric tube heat exchanger is the most common type of the tubular heat exchanger. The hot liquid, or steam, flows through the outer jacket and the cold liquid flows through the inner tube.

The shell and tube heat exchangers are less expensive than other types of food heat exchangers especially when high loads are transferred. They can be operating at higher temperature and pressures (e.g. steam at 6 bar and 160 ° C) than plate heat exchangers. Shell and tube exchangers are tubular consisting of bundles of parallel tubes inside a larger cylindrical jacket (shell).

In a type of tubular exchanger known as a Joule effect heater, the tube wall is electrically heated. The length of most commercial tubular system has been standardized at six meters; therefore, tubular heat exchangers are long and thin in terms of their geometry.

Most tabular heat exchanger now use corrugations on the shell and tubes to enhance heat transfer with the heating and cooling media, typically water. Tubular heat exchangers are suitable for heating or cooling highly viscous products and where relatively high pressure must be applied.

They are therefore utilized for the bulk in-flow sterilization of products containing solid particles or for the heat treatment of cooling of tomato paste prior to aseptic packaging.

For fruit juices with fibers of up to 15 mm length and for relatively water-like foods, a multitube tubular heat exchanger is preferably used. Also fluid of moderate to high viscosity with only small particulates will flow through a multitube heat exchanger without problem.
Tubular heat exchangers

Steam injection

Direct exchangers are based on mixing of heating steam with the liquid food product resulting in very fast heat transfer rate. Steam injection and steam infusion are each used to intimately combine the product with potable steam.

In steam injection, the steam is injected into the fluid food through small hole. A typical example is the production of UHT milk by direct steam injection.

Milk is pre-heated to 75 ° C and then brought into contact with saturated steam, resulting in almost instantaneous heating of the UHT temperature. The milk plus the injected water come in the expansion vacuum vessel, where the added water of the steam injection is removed through condensation in the following condenser.

Control of the pressure in this system is needed for two reasons. First, this is to ensure that the correct amount of water is driven off, and second, that any undesirable volatile odors that may have developed during the injection process are removed.

The advantages of direct steam injection are their quick and accurate temperature control (within less than 1 ° C), the restricted pressure drop under normal flow rates (less than 0.14 bar) and their relatively low noise during operation (less than 80 dB).
Steam injection