Elsevier

Journal of Food Engineering

Volume 49, Issue 4, September 2001, Pages 297-302
Journal of Food Engineering

Vacuum impregnation for development of new dehydrated products

https://doi.org/10.1016/S0260-8774(00)00226-0Get rights and content

Abstract

Vacuum impregnation (VI) of structured foods implies the partial release of gas from pores and its substitution by an external liquid. Therefore, important changes in physicochemical and structural properties take place in the food and these affect its behavior in drying operations (air-drying (AD) and/or osmotic dehydration (OD)). The adequate control of VI prior to dehydration may be used as a tool both to improve mass transfer and to develop engineered products. In order to evaluate this alternative, the effectiveness of VI as a tool in porous matrix formulation is analyzed. Likewise, its influence on some physical and transport properties of the plant tissue and the relevant changes induced in osmotic and convective drying processes are discussed, since these are probably the most interesting alternative processes to lengthen the impregnated product shelf-life. Improved yield of some dehydration processes, such as fruit candyin, when VI is applied at the beginning, is also discussed in terms of the cell network relaxation mechanism, responsible for hydrodynamic tissue impregnation.

Introduction

Vacuum impregnation (VI) of a porous product consists of exchanging the internal gas or liquid occluded in open pores for an external liquid phase, due to the action of hydrodynamic mechanisms (HDM) promoted by pressure changes (Fito, 1994; Fito & Pastor, 1994). The operation is carried out in two steps after the product immersion in the tank containing the liquid phase. In the first step, vacuum pressure (p1∼50–100 mbar) is imposed on the system for a short time (t1) in the closed tank, thus promoting the expansion and outflow of the product internal gas. The releasing of the gas takes the product pore native liquid with it. In the second step the atmospheric pressure (p2) is restored in the tank for a time (t2) and compression leads to a great volume reduction of the remaining gas in the pores and so to the subsequent in flow of the external liquid in the porous structure. Compression can also reduce the pore size depending on the mechanical resistance of the solid matrix.

The volume fraction of the initial sample (X) impregnated by the external liquid when mechanical equilibrium was achieved in the sample has been modeled (Eq. (1)) as a function of the compression ratio r (Eq. (2)), sample effective porosity (εe), and sample volume deformations at the end of the process (γ) and the vacuum step (γ1), as described in Eq. (2) (Fito, Andrés, Chiralt, & Pardo, 1996). If γ=γ1=0, Eq. (1) gives the relationship for VI of non-deformed products (Fito, 1994). In practical terms, sample deformations in VI are seen to be negligible for a great number of fruits (Salvatori, Andrés, Chiralt, & Fito, 1998; Chiralt et al., 1999).r=p2+pcp1,εe(r−1)=(X−γ)r+γ1.The aim of this paper is to discuss how VI may modify composition of porous food, and therefore its effectiveness as a tool in porous matrix formulation, its influence on some physical and transport properties of the plant tissue and the relevant changes induced in osmotic and convective drying processes, which are probably the most interesting alternative processes to lengthen the impregnated product shelf-life. Improved yield of some dehydration processes, such as fruit candy, when VI is applied at the beginning is also discussed. Special emphasis is placed on the behavior of apple samples because of their high porosity and so their suitability for VI processes.

Section snippets

VI process for the formulation of porous matrices

VI operation can allow us to incorporate any ingredients in a porous product in order to adapt its composition to certain stability or quality requirements, in a quick and simple way. Structured foods such as fruit and vegetables have a great amount of pores (intercellular spaces) which are occupied by gas, or native liquid to quite an extent, and which offer the possibility of being impregnated by a determined solution thereby improving composition by adding specific/selected solutes:

VI effects on physical properties of fruit

The physical property which is most clearly affected by VI is the product density. Since air in the pores is replaced by liquid, density increases, thus affecting other related properties such as thermal conductivity which is greatly affected, especially when highly porous. Changes are dependent on total porosity and pore distribution in relation with the direction of heat flow and impregnating solution composition (Martı́nez-Monzó et al., 2000; Barat, Martı́nez-Monzó, Alvarruiz, Chiralt, &

Mass transport properties in osmotic and convective dehydration

Mass transport properties, such as the effective diffusion coefficient of the plant tissue in osmotic dehydration (OD) processes are also modified by previous VI pretreatment. This modification has been analyzed in apple tissue VI with isotonic solutions in order to avoid any change in the value of the process driven force (Martı́nez-Monzó, Martı́nez-Navarrete, Chiralt, & Fito, 1998b). The values of the effective diffusion coefficient (De) in the product liquid phase (PLP), sugar gain (ΔMs) and

Impregnation in long-term osmotic processes. Fruit candying

Impregnation of the fruit pores due to HDMs has been seen to occur without external pressure changes when the cellular tissue remains immersed in a liquid phase for a long time (e.g., syrup canned and candied fruits). This was explained in terms of the capillary forces, pressure and temperature fluctuations in the system and relaxation phenomena of the shrunk cellular matrix when hypertonic solutions were used in the treatments (Fito et al., 2000; Barat et al., 1998, Barat et al., 1999). It has

Acknowledgements

We thank the Comision Interministerial de Ciencia y Tecnología, the UE (STD3 programme) and the CYTED Program for their financial support.

References (20)

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