Introduction the role of vacuum technology

It is well established that processing treatments for fruits and vegetables that are designed to preserve them in various forms (fresh, frozen, pasteurised or dried), have an effect on organoleptic qualities such as texture, colour or flavour. In reducing problems caused by deterioration that occur after harvest or during processing, there is a choice between:

• selecting more resistant varieties of raw material,

• adding corrective additives in the final preparation, or

• implementing 'minimal' physical treatments or novel technologies.

One of these new technologies contributing to the preservation of the original properties of fruit or vegetables is 'vacuum technology', which is also called 'vacuum infusion' or 'vacuum impregnation'.

Vacuum technology is considered to be a pretreatment for processed fruit or vegetables leading to improvement in their quality by active incorporation of functional ingredients in the product structure. Passive impregnation by common dipping of the plant products in a solution is usually used in the production of alcohol-based or candied fruits. The penetration of preservatives or humectant agents by soaking can be also required in dried products. However, the processing times of such treatments are long, extending from several hours to several days, and mass transport phenomena are mainly governed by molecular diffusion of the compounds present in aqueous solution. In contrast, vacuum infusion technology is based mainly upon rapid hydrodynamic mass transfer and consists of putting the food product under vacuum before the introduction of an impregnation solution. This allows, within a few seconds, the occluded air initially contained in the fruit or vegetable pores to be replaced by the impregnation solution, owing to the positive pressure differential which results when atmospheric conditions are restored. This treatment seems to adapt well to porous products and can be applied to whole or cut fruits and vegetables.

Vacuum technology was used for a long time in the treatment of various industrial materials such as wood, metal and so on. In the fruit or vegetables sector, vacuum impregnation was rarely studied in the past. It has received new interest for its potential to improve the organoleptic quality of foods and in the design of minimally processed products.

In the first part of this chapter, modelling of the mass transfer occurring during vacuum treatment and the following modification of the structural and physical properties of products will be described. The second part will highlight different known applications that allow improvement in the quality of stabilised products and/or the extension of their shelf-life.

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