VACUUM IMPREGNATION TECHNOLOGY

An innovation in osmotic dehydration of fruits and vegetables

Manjunath J Shetty and Vinayaka H P

Dehydration, by partial removal of water to reduce water activity, has been widely used to extend shelflife of fruits and vegetables. Dehydrated fruits and vegetables can be used as a food ingredient in many products, have also been added to cereals, granola bars, baked goods and mixes, and can even be eaten out of hand. Traditional air-drying method consumes intensively high energy and causes significant loss of flavour and nutrients because of the high heat exposure.

Osmotic dehydration, being a simple process has achieved greater attention in recent years as an effective method for preservation of fruits and vegetables since they have largest number of pores to make favourable condition for osmotic dehydration. It facilitates processing of tropical fruits and vegetables such as banana, sapota, pineapple, mango, and leafy vegetables etc. with retention of initial fruit and vegetables characteristics viz., colour, aroma and nutritional compounds. A further development in osmotic dehydration of foods is going on and vacuum impregnation (VI) is the newly developed one. VI of a porous product consists of exchanging the internal gas or liquid occluded in open pores to an external liquid phase due to the action of hydrodynamic mechanisms (HDM) promoted by pressure changes.

Vacuum impregnation (VI) is considered as a useful technique to quickly introduce external liquids in the porous structures of animal and plant tissues, in a controlled way. As consequence some mass transfer processes (as dewatering) are improved and also some changes in food composition may be produced. VI has broad applications in fruit and vegetable processing and provides many unique advantages.

Vacuum impregnation and other osmotic treatments

Three kinds of osmotic treatments have been defined, depending on the pressure applied on the system; OD (osmotic dehydration at atmospheric pressure, VOD (osmotic dehydration at vacuum pressure), and PVOD (pulsed vacuum osmotic dehydration)

In PVOD, VI with the osmotic solutiontakes place during the first 5–10 min of process by the actionof a vacuum pulse, causes a fast compositional change in theproduct that will affect the osmotic driving force and masstransfer kinetics. The most commonor familiar osmotic treatment is osmotic dehydration (OD).It is necessary to clarify this technology before moving to the discussion about VI.

Osmotic Dehydration (OD)

Process of water removal by immersion of water containing cellular solid in a concentrated aqueous solution.

Two major simultaneous counter current flows 1.Water diffusion into the solution 2.Solute diffusion into the food Major driving force is osmotic pressure

Osmotic Dehydration under Vacuum (VOD)

Osmotic dehydration in presence of vacuum can intensifies capillary flow and favours the mass transfer rate. Intercellular tissue gas gets replaced by the solution on vacuum release. It reduces processing time and pressure gradient results in rapid Hydro-Dynamic Mechanism (HDM).

Pulsed Vacuum OD (PVOD)

VI treatment is carried out with the osmotic solution at the beginning of OD process. It is a short vacuum treatment is followed for long time holding in atmospheric pressure.

Vacuum Impregnation (VI)

Vacuum impregnation (VI) is a nonthermal, non-destructive treatment that aims to modify the composition of food material by partially removing water and air, impregnating it with physiologically active compounds without affecting the material’s structural integrity.

Dehydration involves the capillary flow and mass transfer. VI simply intensifies

the rate of capillary flow and mass transfer i.e. rate of dehydration. When porous structure is immersed in water then there is fast mass transfer and is termed as hydro dynamic mechanism (HDM). This involves the inflow of the external liquid throughout the capillary pores, controlled by the expansion/compression of the internal gas. When low pressure is imposed in solid liquid system followed by atmospheric pressure, hydro dynamic forces built up and responsible for VI processes for porous products.

Principle of vacuum impregnation

Hydro Dynamic Mechanism (HDM): A fast mass transfer mechanism which occurs when porous structures are immersed in a liquid phase. This involves the inflow of the external liquid throughout the capillary pores, controlled by the expansion/ compression of the internal gas.

Deformation Relaxation Phenomena (DRP): Changes in pressure promote deformation (that could increase volume) of the product due to viscoelastic properties of its solid matrix called DRP. DRP is affected by food microstructure and mechanical properties.

Factors affecting the VI process

External factors:

1.Vacuum pressure:

•More effective ranged between 50 to 600 mbar •It produces the pressure gradient •Promote hydrodynamic mechanism •High Water Loss (WL) rate can be obtained in low pressure system. •Porosity increase with decreased pressure

2. Vacuum time and Relaxation time:

•Both vacuum time and relaxationtime affects the impregnation •Both are varies with structure of products •The vacuum time varies approximately between 2 to 60 minutes, relaxation time varies between few minutes to several hours.

3. Solution characteristics:

•Types of external solution •Solubility of solutes in solution •Concentration of solution •Viscosity of the solution •Solution to sample ratio •Temperature Internal factors: •Internal structure •Porosity •Shape and size of sample

Fig 1: Vacuum impregnation system

The response of many fruits and vegetables to the VI processing with respect to deformation and impregnation has been characterized mathematically and experimentally. Impregnated sample volume fraction, sample relative volume of deformation, and effective porosity strongly depend on raw material characteristics (porosity, size, and shape), and VI conditions (type and concentration of solution, vacuum level, and time). Texture, total acids, and colour are among those physicochemical properties most affected by VI as a result of change in product density, especially in highly porous samples. The texture quality of VI processed products is significantly related to the type of VI solutions used. Thermal conductivity and diffusivity are greatly dependent on product composition and structure. VI processing promotes changes in product composition and structure, hence leading to modification in thermal properties, especially thermal conductivity of highly porous matrices. Porosity, pore size, and distribution in relation with the direction of heat flow, impregnating solution composition, and operation parameters strongly affect the changes of thermal properties and VI with isotonic solutions increases thermal conductivity because of the gas replacement, but only causes slight changes in thermal diffusivity due to the simultaneous density increase.

Problems in Vacuum impregnation technology

•Technical challenges •Control of mass transfer rate •Reuse of solution

Authors are from Department of Postharvest Technology, College of Horticulture, UHS campus, GKVK post, Bengaluru-65