Principles mass transfer and product behaviour 1821 Mass transfer

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The most complete description of the mass transfer phenomena occurring during vacuum infusion is generally found in studies dealing with mass exchange in the osmotic dehydration of fruit pieces immersed in concentrated solutions. These two techniques, vacuum application and prolonged immersion of plant products in hypertonic solutions, can be easily coupled (Shi and Fito, 1994; Shi et al., 1995). Without encroaching on the specific field of osmotic dehydration, it seems very important to underline the close link which can exist between the two techniques. In these soaking processes, the use of vacuum forces accelerates the penetration of aqueous solution compared with the apparently slow molecular diffusion process that is predominant in the osmotic process.

When a vacuum pulse is applied, trapped gases are expanded and partially removed from the food matrix. After restoring atmospheric pressure, a positive pressure differential results which allows penetration of the liquid into the free voids in the structure until internal and external pressure equilibrium is reached. The time taken to reach a vacuum usually depends on the efficiency of the vacuum system (pump, closed volume of apparatus, etc.) and only lasts at best for a few seconds. In most cases, products have to be maintained under vacuum for a few minutes to ensure good extraction of internal gases, but this step could be unnecessary if degassing is completed during the pressure drop. At the end of the treatment, vacuum release is generally obtained instantly.

Fito and Pastor (1994) and Fito (1994) gave a clear description of the mass transfer phenomena observed in vacuum technology. The mass transfer occurring during the vacuum treatment is referred to as the 'hydrodynamic mechanism' (HDM). Intercellular spaces in plant products are described as a whole by elementary cylindrical pores occupied by an ideal gas undergoing isothermal compression (Fig. 18.1).

Solidl

= Liquid

Solidl

= Liquid

(c)

Fig. 18.1 Main stages during vacuum infusion of porous food immersed in a liquid. The situation in an elementary ideal pore (adapted from Fito, 1994); (a) the capillary effect under normal pressure, (b) degassing under vacuum conditions, (c) capillary effect under reduced pressure, (d) HDM at restored normal pressure.

Fig. 18.1 Main stages during vacuum infusion of porous food immersed in a liquid. The situation in an elementary ideal pore (adapted from Fito, 1994); (a) the capillary effect under normal pressure, (b) degassing under vacuum conditions, (c) capillary effect under reduced pressure, (d) HDM at restored normal pressure.

The penetration of solution into the ideal rigid pores breaks down into two stages. First, the pores fill by capillary action in the first part of the treatment that corresponds to atmospheric immersion and vacuum application. Secondly, when restoring normal pressure, the resulting driving force induces liquid flow in the pores. The quantity of external liquid transferred can be almost as great as the available void space in the food structure. The impregnated sample volume fraction (X), usually measured by a gravimetric method, has been modelled on the basis of the HDM and the Hagen Poiseuille equation. X is a function of the product effective product porosity (ee) and the apparent compression rate (r = P2P P1 is the applied vacuum pressure, P2 is the restored atmospheric pressure). Thus, Fito and colleagues established that the simple expression of the volume fraction occupied by the liquid in the fruit or vegetable product after vacuum infusion is:

Capillary pressure is not considered in this simplified expression, because it appears to be negligible with respect to the driving force imposed on the system when the work is carried out at sufficiently low pressure (lower than 600mbar according to Fito, 1994). Effective porosity is expressed a priori as the percentage of sample volume initially occupied by the gases (Calbo and Sommer, 1987), but is defined more precisely as the sample volume fraction available for an HDM mechanism; this parameter is thus determined from an experimental procedure by calculating the slope of the linear function given by adjusting the X versus 1 - 1/r curve (Fito, 1994; Del Valle et al., 1998). In the case of fruit and vegetables, the porosity values were found to be extremely variable depending on the raw materials, for example average ee values are 0.20 for apple and 0.05 for apricot. These variations in porosity can explain the observed variations in weight gain measured in fruit or vegetable pieces after vacuum impregnation step carried out under equivalent experimental conditions (Table 18.1). Moreover, the effective porosity will depends not only on the type of fruit or vegetable, but also on their variety and their maturity (Del Valle et al., 1998; Sousa et al., 1998).

Table 18.1 Weight gain of various fruits and vegetables after vacuum infusion in water at 20°C (50mbar, 1min) and some indicative effective porosity values from different literature sources

(d = diameter, t =

thickness)

Weight gain (%)

Effective porosity

Apple, Granny

Slice (d = 3 cm, t

= 0.5 cm)

32

0.18-0.252-4

Smith

Banana

Slice (d = 2.5 cm,

t = 0.5 cm)

17

0.08-0.311,4,5

Cherry

Whole

1

-

Citrus peel

Slice (d = 3 cm)

57

-

Kiwi

Slice (d = 4 cm, t

= 0.5 cm)

2

0.0053

Mango

Slice (d = 3 cm, t

= 0.5 cm)

9

0.03-0.1513

Orange

Segment

3

-

Pear

Slice (d = 3 cm, t

= 0.5 cm)

24

0.141

Pineapple

Slice (d = 3 cm, t

= 0.5 cm)

5

0.051

Strawberry

Half

10

0.03-0.113,4

Button

Slice (t = 0.5 cm)

66

-

mushroom

Carrot

Slice (d = 2.5 cm,

t = 0.5 cm)

6

-

Chicory

Leaf

19

-

Courgette

Slice (d = 4cm, t

= 0.5 cm)

43

-

Eggplant

Slice (d = 5 cm, t

= 0.5 cm)

180

-

Onion

Slice (d = 3 cm, t

= 0.5 cm)

10

-

Potato

Slice (d = 4 cm, t

= 0.5 cm)

3

-

Red pepper

Slice (d = 2 cm, t

= 0.5 cm)

13

-

Spinach

Leaf

43

-

Turnip

Slice (d = 4 cm, t

= 0.5 cm)

5

-

Basil

Leaf

58

-

Mint

Leaf

50

-

'Fito (1994), 2Del Valle et al. (1998), 3Salvatori et al. (1998), 4Fito et al. (1996), 5Sousa et al. (1998).

'Fito (1994), 2Del Valle et al. (1998), 3Salvatori et al. (1998), 4Fito et al. (1996), 5Sousa et al. (1998).

Viscosity (mPa s)

Fig. 18.2 Effect of viscosity on the weight gain of apple slices (diameter 20mm, thickness 8 mm) after vacuum infusion at 20°C in water and different pectin solutions. Vacuum treatment conditions are 50mbar for 1min 15 s.

Viscosity (mPa s)

Fig. 18.2 Effect of viscosity on the weight gain of apple slices (diameter 20mm, thickness 8 mm) after vacuum infusion at 20°C in water and different pectin solutions. Vacuum treatment conditions are 50mbar for 1min 15 s.

Equation [18.1], which is derived from capillary flow theory, is not adapted in the case of the infiltration of non-Newtonian liquids or high viscosity solutions. Figure 18.2, reporting results obtained in our laboratory, shows the effect of the solution viscosity - adjusted using low methylated (LM) pectin - on the weight gain of apple slices chosen as a model fruit after vacuum treatment. The decrease in values for weight gain with viscosity indicated that the hydrodynamic mass transfer was limited and could not be predicted from the previous model. Even if applied vacuum pressure represents the main control factor of the process, the composition and the concentration of the aqueous solutions used are potential variables that modify the liquid intake in porous fruits or vegetables. The influence of viscosity (as mentioned above) and the interaction between hypertonic solutions and plant products (osmotic and HDM coupled phenomena are discussed above and in sections 18.3-18.7) appear to be substantial governing factors.

The other variables upon which the vacuum process depends were not studied much systematically, that is temperature of impregnation solution, time to achieve vacuum, time maintained under vacuum, time to restore atmospheric pressure. The time to achieve vacuum and time to restore atmospheric pressure were not noted to have any substantial effect in the literature, whereas the time the vacuum was maintained had no consequence on the HDM transfer beyond a few minutes

(2min suggested by Fito and Pastor, 1994). The few existing data concerning the effect of temperature showed that just a slight variation in mass transfer rate was induced (Hoover and Miller, 1975). In practice, temperature conditions are limited when nearing the liquid boiling point under vacuum, for example near 46°C for water at 100mbar. Finally, the temperature effect on liquid viscosity or food matrix plasticity is certainly suggested to play a role in vacuum technology.

18.2.2 Modifications to structural and physical properties

Several authors (Fito et al., 1996; Sousa et al., 1998; Salvatori et al., 1998) reported that the HDM mechanism is accompanied by deformation of the food matrix which influences the final liquid uptake and affects the mechanical properties of the product after treatment. The deformation phenomenon corresponds first to an extension of the internal occluded air volume inside the product when degassing at the time the vacuum is created, and secondly to a partial retraction in pore volume caused by structure relaxation at the time of return to atmospheric pressure. As a function of the viscoelastic properties of the internal structure and the cohesive forces in plant cellular tissue, the deformation-relaxation phenomenon could induce irreversible effects, involving in some cases rigidity loss caused by embrittlement or rupture in the cell wall junctions, possibly accompanied by juice loss. Generally, this phenomenon, correlated with the pressure driving forces and perhaps with the time during which they operate, results in an increase in effective porosity values and enhances the quantity of infused liquid in the product with the detrimental effect of a moderate loss of firmness.

From microscopic observations of kiwi fruit before and after vacuum treatment with glucose solutions, Muntada et al. (1998) noticed that the size of the cells in the infused plant tissue and their arrangement were preserved even if ruptures in the cellular walls were observed. This was in agreement with the previous work of Bolin and Huxsoll (1987) on apple, which showed that vacuum impregnation causes the rupture of a non-negligible number of cells. As it will be emphasised hereafter, this food structure damage may be masked by the reinforcement of the cell wall structure by calcium or by strengthening of the wall with gelling agents or other solutes, which could improve texture of the processed products still more. The paradox of the vacuum technique becomes clear when considering both the microscopic observations and the negative effect of the deformation-relaxation phenomenon: the moderate loss of integrity as a consequence of the vacuum treatment can be compensated to a large extent by the active role of the transferred solutes.

Thanks to the incorporation of functional agents with vacuum technology it is possible to modify the physical or physicochemical properties of fruit and vegetables. The work of Martinez-Monzo et al. (1998a) seems to be a particularly representative example, providing promising prospects for the development of a pretreatment that will modify the initial composition of porous fruit, making it more resistant to damage caused by the freezing-thawing process. The infusion of cryoprotectants (low molecular weight solutes) or cryostabilisers (high molecular weight solutes) into apple pieces before freezing did not modify significantly the measured glass transition temperatures, but when concentrated cryoprotectant solutions were used, a notable reduction in freezable water was obtained. The reduction in freezable water content should contribute to decrease the damage produced by ice crystals because of the reduction in their volume fraction. After impregnation with modified grape must as the chosen cryoprotec-tant, cryo-scanning electron microscopy observations of the cellular structure of apple showed that the formation of ice crystals was similar in intercellular space and inside the vacuole, without detecting any apparent disturbances in the cell (size, shape and intracellular arrangement). With a cryostabiliser like high methylated (HM) pectin, in the first instance the penetration of viscous pectin solution was not complete, leaving empty intercellular spaces. Secondly a difference between the ice crystals in inter- and intracellular spaces respectively was observed. The presence of pectin could nevertheless increase the stability thanks to modification of the ice crystals in the intercellular spaces and reinforcement of the structure by intercellular bridges formed from polysaccharide gel.

In considering the changes in thermal properties (thermal conductivity, thermal diffusivity and specific heat), Martinez-Monzo et al. (2000) indicated first that the vacuum impregnation treatment applied to apple could increase (up to 24%) thermal conductivity considerably. This result was mainly justified by replacement of the inner gases by liquid in the fruit pores, which reduces the thermal resistance commonly related to the void fraction. The increase in thermal conductivity is consequently proportional to the fruit porosity, the quantity of transferred solution and the osmotic pressure of the solution. The specific heat was not modified in the case of isotonic solutions whereas thermal diffusivity only increased slightly (2-4% higher). In addition, when the concentration of the impregnating solution increased and became hypertonic, the increase of conductivity and diffusivity values was less significant because the aqueous fraction tended to decrease in the product. For the highest concentrations, this even led to a reduction in thermal diffusivity and specific heat up to values below the initial ones. Finally, Martinez-Monzo et al. (2000) established predictive equations for the thermal parameters of infused products. The thermal parameters measured for apple were estimated with reasonable accuracy by the equations. The proposed models could be adjusted to other high moisture foods and could be used to evaluate the potential advantage to the effectiveness of heat transfer when vacuum treatments are applied prior to thermal processing.

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