Liquid food, heat processing

Evaporation is the oldest process for the concentration of liquid foods. Temperatures are higher compared to those of the more modern membrane filtration or freeze concentration processes. Tocopherols, carotenes, ascorbic acid, flavonoids and other phenolic antioxidants are partially destroyed by heating. Therefore, it is necessary to minimise the time needed for evaporation, and heating to the evaporation temperature should be carried out very rapidly. The temperature may be decreased if the pressure is reduced. The process is then more expensive, but losses of antioxidants become substantially lower. 

The changes that occur after the heat processing of food systems are often monitored by different parameters, such as total carotenoid content (and therefore isomerization and oxidation are underestimated), individual carotenoids (overall changes may be missed), and CIELAB color parameters (no information on carotenoid degradation mechanism). The data given in Table 12.3 reflects the influence of matrix composition, food state (liquid or solid), and measured parameter on the carotenoid degradation kinetics. 

Freeze concentration involves the concentration of an aqueous solution by partial freezing and subsequent separation of the resulting ice crystals. It is considered to be one of the most advantageous concentration processes because of the many positive characteristics related with its application. Concentration processes such as evaporation or distillation usually result in removal of volatiles responsible for arom in addition the heat addition in these processes causes a breakdown in the chemical structure that affects flavor characteristics and nutritive properties. In contrast freeze concentration is capable of concentrating various comestible liquids without appreciable change in flavor, aroma, color or nutritive value (1.2.3) The concentrate contains almost all the original amounts of solutes present in the liquid food. 

Aseptic processing is used to achieve commercial sterility in a continuous flow of liquid or semi-liquid food by heating the food to a suitable temperature before placing it in previously sterilised packaging. The sterilisation of liquids containing particulates presents a number of... 

Semibatch processes are also important in solids processing such as in foods and microelectronics, where it is more practical to load the reactor with a batch of solids (bread dough or silicon wafers) and subject the solids to heat and gas or liquid reactants. The processed solids are then withdrawn after a suitable time, and the reactor is reloaded. 

Frequently, simple carbohydrates are considered as a source for sweet pleasant tastes, and complex carbohydrates, such as starch, as thickeners. In actuality, both simple and complex carbohydrates can be heat processed either in the dry or liquid state, alone or in combination with other reactants, to produce a vast array of aromas and flavors. Thus, carbohydrates serve as flavor precursors/components in the commercial formulation of processed foods. 

Osmotic distillation also removes the solvent from a solution through a microporous membrane that is not wetted by the liquid phase. Unlike membrane distillation, which uses a thermal gradient to manipulate the activity of the solvent on the two sides of the membrane, an activity gradient in osmotic distillation is created by using a brine or other concentrated solution in which the activity of the solvent is depressed. Solvent transport occurs at a rate proportional to the local activity gradient. Since the process operates essentially isothermally, heat-sensitive solutions may be concentrated quickly without an adverse effect. Commercially, osmotic distillation has been used to de-water fruit juices and liquid foods. In principle, pharmaceuticals and other delicate solutes may also be processed in this way. 

Kumar, A. and Bhattacharya, M. 1991. Numerical analysis of aseptic processing of a non-Newtonian liquid food in a tubular heat exchanger. Chem. Eng. Comm. 103 27-51. 

Today, multistage vacuum evaporation is the predominant method used for liquid food concentrahon. Its major drawback is that it causes heat-induced deterioration of sensory (color, taste, and aroma) and nutritional (vitamins, etc.) value of the hnished product. The food industry has developed alternate methods, such as freeze concentration and thermally accelerated short time evaporation (TASTE) [20] for recovery and blending of such labile constiments for producing concentrates. Though they are currently practiced commercially, the final products in some cases do not satisfy the consumer requirement of their fresh or natural qualities. Some of these processes are energy intensive and therefore unattractive. 

Solidifying processes are important in cryogenics, food and process industries and also in metallurgy. A main point of interest is the speed at which the boundary between the solid and the liquid phase moves. Prom this the time required for solidifying layers of a given thickness can be calculated. The modelling of these processes belongs to the held of transient heat conduction, as the enthalpy of... 

A few trends are evident from the heat-processing data in Table III. Processing increased bioavailability of added iron when the process involved heating a predominantly aqueous food (i.e., wet-heat processing), as well as when ascorbic acid was added before heating. A greater bioavailability resulted after the processing of canned liquid milk-based infant formula (13),... 

Although some liquid foods, such as beer and fruit juices, may be pasteurized after filling containers (with warm water or steam applied to raise the temperature appropriately), most are pasteurized in a vat process or a continuous-flow process and then packaged. The vat (or batch) process involves heating in a well-agitated tank for the required time... 

Thus from this recall of heat transfer, with the similarity between the two processes of heat transfer and mass transfer controlled by diffusion, the necessity of admitting without ambiguity that the course for the mass transfer should emerge as follows diffusion through the thickness of the sheet associated with the convection into the liquid. Finally, the parameters of main importance for a polymer package in contact with a liquid food are the diffusivity and the coefficient of convection. The diffusivity is concentration-dependent, as for example the case of highly plasticised polyvinylchloride where the plasticiser concentration may reach up to 50% of the polymer, but in the present case of the low concentration of the additives distributed in the polymer of the packaging -which are necessary to provide its qualities - the diffusivity can be considered as constant. 

Plate heat exchangers (PHEs) are the most common form of compact heat exchanger, but their area densities are inferior to other types discussed in this chapter. There are several forms of construction, mainly associated with the methods used to seal the edges of the plates - gaskets, braze, welding or laser welding. The PHE is widely used in the liquid food processing industry, where the use of... 

Research at Leeds University in the UK (Dunn, 2007) that studied the pouring of liquids onto heated or cooled spinning discs and collection of them at the periphery suggested that the phenomenon could provide the food industry with more efficient and shorter mixing and processing times. It was demonstrated that mixing and... 

Dfuription and Principle This process is the application ol mild heat for a specified time to a liquid, food, or beverage. Pasteurization destroys molds, yeasts, and the non-spore forming banana. The lower the lempsrature, the longer the application bme, and vice versa, for example, milk can be pasteurized at 143 "f for 3D min, or 162°F I72° l ter 15 seconds. Time and temperature vary with the type of food. Foods are rapidly cooled following the treatment. 

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