Use of vacuum impregnation in food salting process

Journal of Food Engineering 49 (2001) 141±151

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Use of vacuum impregnation in food salting process A. Chiralt *, P. Fito, J.M. Barat, A. Andres, C. Gonz alez-Martõnez, I. Escriche, M.M. Camacho Department of Food Technology, Universidad Polit ecnica de Valencia, P. Box 22012, 46071 Valencia, Spain Received 1 June 2000; accepted 1 December 2000

Abstract Salting is an ancient preservation method, usually used separately or in combination with other processes such as air drying and pH lowering. Traditional salting processes are divided into brining and dry salting, each of them speci®cally applied for particular products. In this work, the use of brine vacuum impregnation (BVI) instead of dry salting or brine immersion (BI) at atmospheric pressure is discussed. The in¯uence of di€erent process variables (length of vacuum pressure period, temperature, sample structure and dimensions) is analysed, in terms of kinetic data and process yields, for meat (ham and tasajo), ®sh (salmon and cod) and cheese (Manchego type cheese). In general, BVI processes imply a notable reduction of salting time, increasing the process yields in line with the greater values of the ratio salt gain to water loss. Likewise, samples lose natural gas or liquid phases entrapped in their structure and reach a ¯atter salt concentration pro®le than that obtained in the conventional salting methods. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum impregnation; Food salting; Meat; Fish; Cheese

1. Introduction Salting is a commonly used food preservation operation in meat, ®sh, dairy and some plant products, such as pickles and olives. In many cases a fermentation process follows salting operation contributing to the food preservation and developing its characteristic properties. During salting, two main ¯uxes occur: the uptake of NaCl and other possible curing compounds, and the loss of water and some internal soluble solids. The aim of this operation is to reduce product water activity …aw † in order to improve its microbial, chemical and biochemical stability, although speci®c role of salt ions has been described, such as their in¯uence on determined enzyme activity responsible for cheese ripening (Mulvihill & Fox, 1980; Delacroix-Buchet & Trossat, 1991). Salting also contributes to developing desirable characteristic ¯avour of the products. Salting process can be carried out by di€erent procedures, the most usual being dry salting and brining, or a combination of both methods. Likewise, salting is also combined with other operations, such as smoking,

*

Corresponding author. Fax: +34-96-3877369. E-mail address: [email protected] (A. Chiralt).

acidi®cation, or air drying, in order to obtain stable ®nal products. In these cases product stability is promoted by aw and pH reduction and by the action of competitive microorganisms. Salted products can be classi®ed in two groups, deeply and lightly salted, with very di€erent salting and consumption requirements. The former need to be desalted before consumption and their aw value are close to 0.75 since they have a liquid phase (di€erent ratio) saturated in NaCl. Among the deeply salted products can be found cod and Tasajo (traditional dry meat product from di€erent Latin-American countries). To reach the characteristic moisture and aw , products are equilibrated with dry salt or submitted to sun or air drying before salting. The lightly salted products (ham, cheese, sausages, olives, pickles, etc) are directly consumed. These are also fermented or ripened which enhances their preservation and gives particular properties. Salting processes of big food pieces are usually slow, and may take several days, due to the low values of salt di€usivity at the low temperatures required to assure food safety during the operation. This implies the management and waste recycling of great amounts of brine and the requirement of very big salting plants to achieve a reasonable production. Recently, the

0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 2 1 9 - 3

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A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

Nomenclature k0 , k1 M00 Mt0 p pc p1 p2 r V00 X X1 Xc

parameters in Eqs. (10) and (11) sample initial mass sample mass at time t pressure in the system capillary pressure vacuum pressure in the ®rst VI process step atmospheric pressure in the second VI process step compression ratio in the VI process sample initial volume sample volume fraction impregnated by the solution at the end of the VI process sample volume fraction impregnated by the solution at the end of the ®rst VI step sample volume fraction penetrated by the external liquid due to capillary forces

application of vacuum in salting process (brine vacuum impregnation: BVI) has been reported to reduce salting time in Manchego type cheeses (Chiralt & Fito, 1997) and ham for curing (Barat, Grau, Montero, Chiralt, & Fito, 1998b), while promoting a ¯atter salt distribution in the product. The salt uptake is accelerated by the coupling of hydrodynamic mechanisms (HDM) with the di€usional phenomena promoted by concentration gradients. HDM are due to the action of pressure gradients, due to capillary e€ect or imposed pressure changes (Fito, 1994; Fito & Pastor, 1994; Fito et al., 1994). When these gradients are provoked by applying vacuum pressure in a ®rst step of the process, they promote the out¯ow of internal gas or liquid and their ecient substitution by the external liquid, thus improving the gain of external solutes. In this work a review is presented of the di€erent variables a€ecting salting process kinetics and yield when vacuum impregnation is used, focusing on the results obtained for salting of meat products (tasajo and ham), ®sh (cod, salmon) and Manchego type cheese.

2. Process variables a€ecting salting by vacuum impregnation E€ectiveness of vacuum impregnation of food matrices is greatly a€ected by several factors such as all those concerning food microstructure (porosity, pore size and shape and kind of ¯uid, gas or liquid occupying the pores), ¯ow properties of the external ¯uid and operation variables: applied compression ratio, length of the vacuum period and temperature. The latter a€ect mechanical±structural properties of the porous matrix and ¯ow properties of the external ¯uid, as well as mass transport properties of the product due to concentration gradients. Mathematical model of vacuum impregnation in terms of the process operation is discussed below.

xit Yti yi zit zit; HDM c1 c ee q0 qb

mass fraction of component i in the sample at time t reduced driven force at time t, referred to component i mass fraction of the component i in the impregnating solution (brine) mass fraction of component i in the food liquid phase at time t of the process value of zit reached in the sample after VI with the external solution relative sample volume deformation at the end of the ®rst VI step relative sample volume deformation at the end of the VI process sample e€ective porosity density of the initial product density of brine

2.1. In¯uence of compression ratio. Mathematical modelling Food salting by brine immersion (BI) has been described as a di€usion process, but capillary mechanisms may also play an important role in the salt uptake in some products such as cheese (Geurts, Walstra, & Mulder, 1974). The capillary entry of the external brine in the product pores can contribute to the total salt gain when sub-atmospheric pressure is applied in the salting tank to quite an extent. Capillary penetration in a pore occurs coupled with the compression of the occluded internal gas. The volume fraction of the sample penetrated by the external liquid due to capillary forces …Xc † is a function of the capillary pressure …pc †, the pressure in system …p† and the e€ective porosity of the product …ee † (Fito, 1994). According to Eq. (1), the lower the pressure in system …p†, the greater the capillary penetration.   pc Xc ˆ e e : …1† p ‡ pc When atmospheric pressure …p2 † is restored in the tank at vacuum pressure …p1 †, the HDM acts, leading to a great penetration of external liquid. The volume fraction of the sample penetrated by the external liquid …X † has been modelled by Eq. (2) in sti€ matrices (Fito, 1994) as a function of the compression ratio r, given by Eq. (3), and product porosity. Depending on the capillary radius, the contribution of capillary pressure to the compression ratio may be negligible from a determined vacuum pressure (Fito, 1994). In this case, compression ratio can be estimated from the values of the vacuum pressure applied in the system and the atmospheric pressure. So, the greater the vacuum level applied in the ®rst step, the greater the impregnation degree when system is taken to normal pressure.

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

 X ˆ ee 1 rˆ

 1 ; r

…2†

p2 pc ‡ : p1 p1

…3†

In viscoelastic matrices such as found in most foods, pressure changes can promote sample deformations coupled with impregnation, in both the vacuum step where sample expansion occurs, and the atmospheric pressure step, or compression period (Fito, Andres, Chiralt, & Pardo, 1996). Sample compression may imply the partial collapse of the matrix pores, thus decreasing the e€ectiveness for capillary phenomena during mass transfer processes such as salting. So, compositional changes promoted by the HDM may occur coupled, to a greater or lesser degree, with deformation±relaxation phenomena (DRP), this depending on the microstructure and mechanical properties of the food solid matrix and ¯ow properties of the external liquid. The characteristic penetration and deformation times de®ne the relative advance of both HDM and DRP, and the ®nal equilibrium situation of the system (Chiralt et al., 1999a). Highly viscous external solution and small pore radius will be associated with high pressure drop during liquid penetration and so with a prevailing advance of deformation phenomenon. Fito et al. (1996) proposed Eq. (4) to describe equilibrium for the HDM coupled with DRP in vacuum impregnation operations of porous food. Eq. (4) may be used to calculate the values of ee , from experimental values of the ®nal relative impregnated sample volume (X), the ®nal relative sample volume deformation …c† and the relative sample volume deformation at the end of the vacuum step …c1 † (Fito et al., 1996). ee ˆ

…X

c†r ‡ c1 : r 1

…4†

143

When external pressure is constant, greatly deformed matrices may relax the mechanical energy stored in their elastic structural elements in line with a progressive impregnation of their pores (Fito, Chiralt, Barat, & Martõnez-Monz o, 2001). So, DRP also promote HDM without any changes in the external pressure. The mechanical relaxation level and subsequent sample volume recovery will depend on the viscoelastic properties of the matrix; the greater the elastic character, the higher the volume recovery and the coupled impregnation. On the basis of that described above, application of a vacuum step in brining processes will imply the substitution of a part of the sample initial gas and the free liquid phase in the structure for brine, in line with the HDM action promoted by capillary forces, external pressure changes and internal pressure gradients generated by structural e€ects. Table 1 schematically shows the di€erent phenomena occurring at the di€erent steps in the brining process. 2.2. In¯uence of brine concentration The salting brine concentration is an important process variable a€ecting the salting process. Brine concentration determines the salting driving force associated with di€usional mechanisms and thus process times. In this sense, saturated brines would be recommendable in order to accelerate the process. Likewise, in BVI process the overall NaCl mass fraction reached in the product liquid phase due to impregnation promoted by the vacuum pulse …zNaCl HDM †, without considering the coupled di€usional gain, can be calculated by applying Eq. (5) in terms of the characteristic penetration and deformation volume ratios (X and c), sample initial NaCl and water mass fractions …xNaCl ; xw0 † and the brine 0 density …qb † and NaCl mass fraction …y NaCl †. As deduced from Eq. (5), the greater the y NaCl value, the greater the

Table 1 Mass transport phenomena and structural changes occurring throughout food brining at the di€erent steps in vacuum impregnation processes Process step

Mass transport phenomena

Structural changes

Product immersion at atmospheric pressure

Capillary penetration of brine (brine penetration front: BPF) Salt and water di€usion in the product liquid phase near the sample surface

Changes in aqueous environment of product components (e.g. proteins) near the sample surface: conformational changes of biopolymers

Period at vacuum

Gas and free internal liquid ¯ow out Advance of the BPF due to the more intense capillary e€ects Development of salt-water concentration pro®les due to di€usion phenomena coupled with brine penetration

Expansion of the pores occupied by gas Progression of conformational changes of biopolymers and changes in their water bonding capacity (WBC), according to the developed salt concentration pro®le

Period at atmospheric pressure

Advance of brine penetration front due to compression Coupling with the di€usional transport of water and salt

Volume reduction of matrix pores: expulsion of free liquid phase can occur Progression of conformational changes of biopolymers and changes in their WBC, according to the developed salt concentration pro®les

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process e€ectiveness in terms of salt uptake by impregnation (Fito & Chiralt, 1997). zNaCl HDM ˆ

M00 xNaCl ‡ …1 ‡ c†V00 X qb y NaCl 0 : 0 w M0 x0 ‡ M00 xNaCl …1 ‡ c†V00 X qb 0

…5†

Nevertheless, the developed salt concentration pro®le at a determined overall salt uptake, will be di€erent depending on the brine concentration used. Di€erences in salt pro®le in the product liquid phase will be responsible for changes in water binding properties of the polymeric matrix (Table 1) at each distance from the sample surface and so for di€erences in the ®nal sample shape and volume. In meat products, water holding capacity during salting is greatly a€ected by NaCl concentration (Wilding, Hedges, & Lillford, 1986; O€er & Trinick, 1983). Till a critical NaCl level, muscle proteins show an open conformation where a great amount of water can be retained, but from this critical value, aggregation of myo®brillar proteins leads to water being expulsed from the structure and so to a great loss of sample weight and volume (O€er & Trinick, 1983). The structural e€ects related to the salt pro®les will be dependent on sample size. 2.3. In¯uence of the length of vacuum period in BVI process The required length of the vacuum period in a VI process is that necessary to achieve mechanical equilibrium inside the product (equal internal and external applied pressure), with the subsequent out¯ow of part of the internal gas and the free liquid taken along with it. In this case, the reestablishment of atmospheric pressure will lead to the substitution of the gas±liquid volume lost for the external liquid phase if no pore compression occurs. Kinetics of hydrodynamic gain of an external solution in the product pores is very fast as compared with di€usional transport, and is only dependent on the pressure drop during the liquid ¯ow determined by the liquid viscosity and pore diameter and tortuosity (Chiralt et al., 1999a). In plant tissue samples of about 2 cm characteristic dimension, with relatively wide intercellular spaces and elastic cellular arrangement, the necessary length of vacuum period in VI operations is in the order of 5 min and impregnation times with sugar syrups are in the order of the time required to achieve a stationary pressure in the tank after the valve is opened to restore atmospheric pressure (Chiralt et al., 1999a; Salvatori, Andres, Chiralt, & Fito, 1998). Nevertheless, in big pieces with small pores, much more time could be necessary to complete sample equilibration in mechanical terms because of diculties for gas release. Likewise, impregnation times can be lengthened due to the greater pressure drops and, in relatively soft matrices, to the compression of sample volume that, when it relaxes

prolongs the internal pressure gradients throughout time. In BVI process of meat, ®sh or cheese products, pores in the structure contain small gas phase volume entrapped in a free liquid phase, that makes the mechanical equilibration process in the vacuum period dicult due to the fact that it hinders the gas out ¯ow, mainly in big pieces. The length of this period could a€ect the impregnation level if no mechanical equilibrium is reached when restoring atmospheric pressure. On the other hand, if solid matrix is more viscous than elastic, compression step will lead to the pore collapse and no notable impregnation could occur. In this sense, the vacuum e€ect will be explained only in terms of the promotion of capillary action as predicted by Eq. (1), and not by a true complete impregnation in the second step. In salting process, the impregnation level will directly a€ect the sample weight loss and process yield at a determined salt concentration level reached in the product liquid phase. Therefore, the length of vacuum period in BVI process can a€ect this parameter. In Figs. 1(a) and (b), the development of sample weight loss …DM 0 † throughout salting time in meat (for tasajo) and cod ®llets (2.5 and 1 cm thick, respectively) can be observed for processes with di€erent vacuum period lengths. In general, the longer the vacuum time, the smaller the weight loss at a determined salting time, this being coherent with a greater impregnation level. Nevertheless, changes in this tendency may be observed from a determined length onwards (Fig. 1(b)). In meat ®llets, parallel linear relationships between DM 0 and the square root of time can be observed for 1 and 2 h of vacuum application. This seems to indicate that no notable differences in mass transfer rate behaviour were induced by vacuum pulse, except the amount of liquid impregnated by HDM, that was greater when vacuum was applied for 2 h. After 1 h vacuum, it is likely that mechanical equilibrium is not yet reached and so impregnation does not occur at the level de®ned by product porosity. In thicker cod ®llets, mass loss behaviour does not show parallel linear relationships for the di€erent lengths of vacuum application. The DM 0 vs. t05 straight lines slope increases when vacuum time decreases, which suggests that HDM could have a prolonged action in line with a sample volume relaxation after its compression when atmospheric pressure is restored. The notable low intercept of the straight line corresponding to 2 h vacuum could be due to a much greater expulsion of the free liquid phase during the vacuum step. 2.4. In¯uence of temperature Temperature a€ects not only the rate of di€usional phenomena but also the e€ectiveness of VI with brine. Viscoelastic properties of solid matrix change greatly

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

Fig. 1. Weight loss vs. time during meat (a) and cod (b) salting for di€erent lengths of vacuum period in the salting tank. Meat samples: 8  8  2:5 cm3 ®llets, cod samples: 2 cm thick ®llets.

with temperature, and a softening of the structure is observed in line with the temperature increase. Pressure changes in soft matrices will cause deformation more than impregnation e€ects, thereby decreasing the VI e€ectiveness. In pressed curd of Manchego cheese, both the loss and storage moduli sharply decrease from about 0°C, the product becoming softer and easier to deform by pressure changes (Andres, 1995). From a balance of the opposite e€ect of temperature on salt di€usion and on e€ectiveness of vacuum impregnation, a salting temperature of about 10°C as recommended for BVI of Manchego type curd (Chiralt & Fito, 1997). 2.5. In¯uence of sample microstructure Porosity of the solid matrix is the most relevant structural property involved in VI e€ectiveness. In plant tissues, porosity may be very high (20±30%) such as in

145

apple, egg plant or orange peel (Fito & Chiralt, 2000) and so VI can be highly e€ective. Nevertheless, curd, meat or ®sh tissues are much less porous and an important part of the matrix is occupied by free liquid phase that may be released from the matrix by pressure changes. In Manchego curd cylinders, experiments were conducted to evaluate the amount of liquid phase that can be released by applying vacuum (50 mbar). This amount (expressed as curd volume fraction) was 2.3% and 3.4%, respectively, before and after submitting curd to a VI operation with an isotonic solution. This indicates that about 3% of the curd volume may be replaced by brine in BVI process although the curd volume fraction occupied by gas is very small (less than 1%). Parallel studies using BVI in Manchego type curd carried out on curd cylinders of di€erent diameters and whole curd pieces, showed similar values of porosity and whey drainage capacity. The porosity development by e€ect of pressing and ageing and its in¯uence on BVI e€ectiveness was also reported (Gonz alezMartõnez, Fuentes, Chiralt, Andres, & Fito, 1999). The degree of compactness of the casein clusters increases according to the distance to the plunger press and to curd ageing time. The reduction of the curd extramicellar porosity provokes a decrease in the curd VI capability. Fig. 2 shows cryoSEM micrographs of Manchego type curd near to (Fig. 2(b)) and far from (Fig. 2(a)) the curd side directly in contact with the press plunger. Di€erences in the microstructure of casein clusters can be observed. In meat and ®sh products, the muscle structure and vascular network will be responsible for the VI response of muscle pieces during BVI. Muscle is constituted by ®bres (about 30 cm long cells), surrounded by a double membrane, sarcolemma. Individual ®bre is composed of a number of smaller longitudinal ordered units, the myo®brils, which are surrounded by a ¯uid the sarcoplasm, where di€erent structured bodies are present. A sheath of connective tissue (epymisium) surrounds the muscle as a whole, but also penetrates from its inner surface into the muscle, dividing it in bundles (perimysium). Perimysium contains nerves and blood vessels. From the perimysium, a thin framework of connective tissue penetrates further to envelop each individual ®bre (endomysium). Fig. 3 shows a cryoSEM micrograph in ®bre cross-section of cod muscle. The endomysium surrounding myo®brils can be observed as well as the myo®brillar structure. In the muscle structure, the gas phase is practically absent although blood vessels could intake gas during the animal slaughter and cutting may act as channels for HDM in VI operations. Nevertheless, the way in which muscle is impregnated by an external solution is not yet clari®ed. One consequence of the important role of food microstructure in BVI operations is a greater variability in

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Fig. 2. Pressed curd microstructure (cryoSEM micrograph) of Manchego type cheese far from (a) and near to (b) the curd side directly in contact with the press plunger during pressing.

3. E€ect of vacuum impregnation on food properties and process yield 3.1. Mass transfer properties

Fig. 3. CryoSEM micrograph of a cross-section of cod muscle ®bres (cells) showing myo®brils (MF) and endomysium or connective tissue (CT).

the ®nal salt content of the product than that obtained in conventional brining. This is explained due to the coupling of several phenomena throughout the salting process, all of which are a€ected by food structure: pore impregnation or partial collapse and di€usion in the food liquid phase whose volume fraction is a€ected by the impregnation level. When characteristic times of deformation and impregnation of the solid matrix are very similar, each one can occur to di€ering degrees with a notable repercussion on the salt transport behaviour. Likewise, in products where porosity can be greatly a€ected by process conditions, such as pressed curd, a careful control of these variables is necessary to assure a constant value of porosity that implies homogenous behaviour in BVI (Chiralt & Fito, 1997; Gonz alez-Martõnez et al., 1999; Gonz alez-Martõnez, 1999).

As commented on above, BVI may imply a modi®cation of the salting driving force at the beginning of the salting process while increasing the volume fraction of the product liquid phase (with the introduced brine) and eliminating the main part of gas phase in the pores. In this case, a decoupling of the action of HDM and pseudodi€usional mechanisms (PD) can be considered in order to model salting kinetics (Fito & Chiralt, 1997). This approach is based on the faster HDM kinetics as compared with the slow di€usional transport and will be applicable when vacuum pulse is applied for a very short period at the beginning of the process. Nevertheless, in many viscoelastic matrices impregnation±deformation and volume relaxation phenomena occur coupled and for long enough to overlap with PD mechanisms. In all cases, the application of vacuum will imply changes in the mass transfer behaviour of the product matrix. In general, as well as the hydrodynamic ¯uxes, di€usion in the food liquid phase is enhanced after VI (Fito & Chiralt, 2000). In modelling salting kinetics, de®nition of the process reduced driving force in terms of the NaCl mass fraction in the food liquid phase …zNaCl † has several advantages. Salt transport mainly occurs in the liquid phase and the zNaCl equilibrium value can be considered to be equal to NaCl the brine salt concentration …zNaCl ˆ ye † (Barat, Chire alt, & Fito, 1998a; Fuentes, 1999). Moreover, zNaCl is directly related with the product water activity and so, with the product stability. Eq. (6) gives the reduced driving force for salt and water concentration by con-

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

sidering both components as a binary system in the product liquid phase …Ytw ˆ YtNaCl †. The values of YtNaCl at each process time will be a function of a term dependent on sample response to VI …Yt;NaCl HDM † and a term dependent on concentration gradients …Yt;NaCl PD † as shown in Eq. (7). In the cases when the HDM action occurs very fast and decoupling of HDM and PD can be considered, the change in Y NaCl due to HDM occurs at near zero time …Yt;NaCl HDM; tˆ0 †, whereas PD mechanisms provoke changes in driving force progresNaCl sively throughout time …Yt;NaCl PD; t>0 †. The value of Yt; HDM; tˆ0 can be estimated by Eq. (8) if sample VI response is known through the zNaCl t; HDM value calculated with Eq. (5). Therefore, the values of Yt;NaCl PD; t>0 can be obtained by applying Eq. (9) resulting from the substitution of Eq. (8) in Eq. (7). This value can be expressed on the basis of a Fickian approach as a function of the di€usion coef®cient, and the product characteristic dimension for a determined geometry. Di€usion coecients obtained in this way re¯ect the changes in mass transport properties provoked by VI in the tissue. For highly porous products, VI greatly enhanced water and solute di€usion in the food matrix (Fito & Chiralt, 2000). Ytw ˆ YtNaCl ˆ

zNaCl t zNaCl 0

zNaCl zNaCl e ˆ tNaCl NaCl ze z0

NaCl YtNaCl ˆ Yt;NaCl HDM Yt; PD ;

Yt;NaCl HDM; tˆ0 ˆ

NaCl zNaCl HDM ‡ y ; NaCl z0 ‡ y NaCl

zNaCl ‡ y NaCl t ˆ Yt;NaCl PD; t>0 : NaCl zHDM ‡ y NaCl

y NaCl ; y NaCl

…6† …7† …8† …9†

In brining of meat, ®sh and cheese, previous studies (Chiralt, Fito, Gonz alez-Martõnez, & Andres, 1999b; Barat et al., 1998b) showed that in order for the VI to be e€ective, a long vacuum pulse is required. Therefore, high to be e€ective and so decoupling of HDM and PD is not feasible. In these cases, empirical approach (Eq. (10)) to modelling mass transfer kinetics has been applied, based on a simpli®ed Fickian equation with only one term of the integrated series solution for short times (Crank, 1975). In Eq. (10), the coecient k1 may be related with the e€ective di€usion coecient …De † and this can be estimated if sample characteristic dimension is known. The coecient k0 will, to some extent, quantify deviation from Fickian behaviour due to HDM action. Nevertheless, since HDM may act throughout the whole process time, the De coecient contains their contribution to mass transfer and not only di€usional contribution. YtNaCl ˆ k0 ‡ k1 t0:5 :

…10†

Fig. 4 shows ®tting of Eq. (10) to salting data of Manchego type cheese plane sheets (10 cm thick) in BI

147

Fig. 4. Changes in the salting driving force (Y) throughout salting time in di€erent brining processes of manchego type press curd: brine immersion at atmospheric pressure (BI), under constant vacuum conditions (BVI-t) and by applying vacuum for half an hour (BVI-0.5 h).

at atmospheric pressure and for BVI with di€erent length of the vacuum period, 0.5 h and for all salting time. The range of 1±Y values reached in the normal salted Manchego cheeses are marked in the plot. The required time to reach these salt levels is notably reduced when vacuum is applied, especially for processes carried constantly under vacuum conditions. This behaviour points out that a short vacuum pulse is not enough to complete sample impregnation probably because of the compression of the viscoelastic protein matrix of the curd when atmospheric pressure is restored (Gonz alezMartõnez, 1999; Chiralt et al., 1999b). Table 2 shows the values of De obtained by ®tting Eq. (10) to salting data of beef and salmon ®llets and Manchego type curd, in di€erent conditions. In all cases greater De values were obtained for BVI process than for BI when comparing processes carried out with the same values of the other involved variables. Di€erent degrees of e€ectiveness of VI may be obtained by varying length of the vacuum period, temperature and other process variables that can a€ect the product e€ective porosity. 3.2. NaCl concentration pro®les after brining Salt concentration pro®le developed in newly salted products by BI and BVI has been analysed for cheese (Andres, Panizzolo, Camacho, Chiralt, & Fito, 1997; Gonzalez-Martõnez, 1999), salmon (Bugue~ no, 2000) and ham for curing (Barat et al., 1998b). The deeper salt penetration induced by the HDM action has been observed through the comparative analysis of salt concentration pro®les developed in the products salted by BI and BVI. Fig. 5 shows the level lines for salt concentration (kg salt/kg solids) in a representative cheese sector (excluding 0.5 cm rind) obtained after salting and after 5 days ripening. In the plots, the cheese geometric centre has been taken as coordinate origin. The greater

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Table 2 Salt di€usivity values in the product liquid phase (water plus salt) obtained for di€erent products brined at atmospheric pressure (BI) and by applying vacuum for di€erent times (BVI)a Product

Beef ®llets, 2.5 cm thickc Salmon ®llets, 1 cm thick Cheese plane sheets, 10 cm thick

Brining process

BI

t1 (h)

Brining temperature …°C†

Brining time …t1 ‡ t2 † (h)

De  1010 …m2 =s†

k0 b

6 5 15 25 10

119 5 5 5 ±

0.139 1.61 2.45 3.93 ±

0.072 0.062 0.047 0.012 ±

10 10

±

4.8 ±

± ±

17

BVI De  1010 …m2 =s†

1 0.08 0.08 0.08 0.5 ± 17

0.174 2.36 3.15 4.15 7.3 ± 10.9

k0 0.13 0.101 0.056 0.074 ± ± ±

a

In BVI, process t1 is the length of the vacuum period and t2 the time at atmospheric pressure, t ˆ t1 ‡ t2 . Straight line intercept from the ®tting of Eq. (10) to the experimental data. c To obtain tasajo. b

Fig. 5. Salt pro®les in a representative sector of manchego type cheeses at 1 and 5 days of ripening for pieces salted by BI and BVI (2 h at vacuum). Salt content reached in both cases: 0.017 kg NaCl/kg curd (0.035 dry basis). Salt concentration expressed as kg NaCl/kg solids.

salt uptake in the internal zone of the sector for BVI sample can be observed. Likewise, at 5 days of ripening a much ¯atter salt pro®le is reached in BVI samples than in those BI treated. Di€erences in the same sense were previously observed in other studies on Manchego type cheese (Andres et al., 1997). These di€erences in salt distribution throughout the ®rst ripening step induced some small changes in the ripening pattern of the cheeses: less development of glycolysis, proteolysis and lipolysis in the external zone of BI samples due to the greater salt amount accumulated in this zone during this kind of salting procedure (Guamis et al., 1997; Pavia, Trujillo, Guamis, & Ferragut, 1999a). In the case of brining of whole hams for curing, both pieces brined for 15 days and also those treated by BVI

(9 days at 50 mbar plus 1 day at normal pressure) have been analysed as to the salt distribution in internal zones of the widest section of ham. Fig. 6 shows the salt mass

Fig. 6. Salt distribution in the internal parts (zones A±C in the widest ham cross-section) immediately after salting at 3°C for BVI (9 days at 50 mbar plus 1 day at atmospheric pressure) and for BI (15 days).

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

fraction in the ham liquid phase reached in each case at points A, B and C marked in the ham graph. A notably higher salt penetration was reached for BVI pieces, especially at point A which is on the opposite side to the fat layer (Barat et al., 1998a,b). In ham, this deeper salt uptake in the bone joint neighbouring zone is very important for product safety during curing.

149

obtaining a safer product due to the reduction of microbial growth. Likewise, oxidation of fatty ®sh such as herring, can be limited, as compared with those treated with dry salt, since BVI reduces the accessibility of oxygen to sample active points (Burgess, Cutting, Lovern, & Waterman, 1979). 3.4. Process yield

3.3. Product quality and stability Vacuum impregnation may imply di€erences in quality and stability parameters of salted products as compared with those brined at atmospheric pressure or salted with dry salt. In general, the salt required in the product liquid phase to assure further product stability is reached in BVI at higher moisture levels which may imply a juicier product. In cured products such as cheese or Spanish ham, the di€erent salt concentration pro®les in the ®rst ripening period may suppose small changes in the ripening pattern, as has been commented on above, texture (Pavia, Trujillo, Guamis, Capellas, & Ferragut, 1999b) and volatile pro®le (Escriche, Fuentes, Gonzalez, & Chiralt, 2001). Nevertheless, in practical terms, differences between production batches may be greater than those induced by the salting method. Vacuum is applied in the making of large blocks of Cheddar cheese in order to eliminate entrapped gas and whey, thus favouring pressing and elimination of mechanical openings (Irvine & Burnett, 1962; Reinbold, Hansen, Gale, & Ernstrom, 1993). When VI is used in cheese salting, a close structure is also obtained while mechanical eyes disappear and texture becomes smooth. This contributes to a lighter appearance of the cut surface (Pavia et al., 1999b). The obtained ¯atter salt pro®les also implied a slow rate of drying during the ®rst 20 days of ripening (Gonzalez, Martõnez-Navarrete, Chiralt, & Fito, 1998). In terms of stability, BVI contributes to the elimination of blood in meat products and whey in cheese, thus

Process yield is directly related with product weight loss during salting. Empirical equation (11) has been closely ®tted to experimental data from several products thus obtaining weight loss kinetics in terms of two constants (k00 and k10 ) (Fito & Chiralt, 1997). Such as has been commented on above for reduced driving force, the physical meaning of these constants can be attributed, respectively, as the contribution of HDM and PD to mass transfer, although these contributions cannot be found when both kinds of mechanisms are coupled. In Fig. 1, the close ®tting of Eq. (11) to experimental data from meat and cod ®llets can be observed. Di€erences in weight loss kinetics have previously been commented on in terms of VI e€ects. Table 3 gives constant values for several products salted under di€erent conditions. The most general pattern for kinetic constants when comparing BVI and BI process, carried out at constant values of other involved variables, is that both constants are greater in BVI than in BI. Therefore, when BVI is applied at a determined product salting level, a smaller weight loss is obtained. This is coherent with the greater ratio salt gain to water loss in the product due to the brine uptake by HDM action. Mt0

M00

M00

ˆ k00 ‡ k10 t0:5 :

…11†

In salmon brining at 10°C, till the commercial aw value in the product is reached …aw ˆ 0:965†, the weight loss obtained in a BVI (5 min vacuum pulse) was 3%, whereas 2% was obtained in BI at the same temperature.

Table 3 Kinetic constants (k00 and k10 ) for weight loss in BI and BVI process of di€erent products Product

Beef ®llets, 2.5 cm thickb Salmon ®llets, 1 cm thick Cod ®llets, 2 cm thick

a b

Brining process

t1 (h)

BI

Brining temperature …°C†

Brining time …t1 ‡ t2 † (h)

k10  104 …s 0:5 †

k00 a

6 5 15 25 4 4 4

119 5 5 5 ± 9 ±

)1.5 )5.9 )8.0 )9.5 ± )7.7 ±

)0.014 0.010 0.005 0.006 ± 0.012 ±

Straight line intercept from the ®tting of Eq. (11) to the experimental data. To obtain tasajo.

1 0.08 0.08 0.08 0.5 1 2

BVI k10  104 …s 0:5 †

k00

)2.0 )4.7 )6.7 )7.6 )6.3 )5.3 )3.6

0.027 0.0040 0.0007 0.0023 0.020 0.011 )0.019

150

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151

This value was associated with a greater moisture content (63%) in the product obtained by BVI than in that obtained by BI (62%). The required salting times were 80 and 50 min, respectively, for BI and BVI (Bugue~ no, 2000). 4. Conclusions By using vacuum impregnation techniques in salting processes of porous food, faster salting kinetics can be obtained with a more even salt distribution in the product and with increased process yields. Nevertheless, careful control should be taken with process variables, especially with those a€ecting the sample impregnation level, in order to ensure a homogeneous salting level. For several studied products, quality parameters of BI and BVI products range in the same order and, in some cases, some bene®cial safety or quality aspects might be observed in BVI products. Acknowledgements Authors thank the Comisi on Interministerial de Ciencia y Tecnologõa (Spain), FAIR Program (E.U, DGXII) and CYTED program for the ®nancial support. References Andres, A. (1995). Impregnaci on a vacõo en alimentos porosos. Aplicaci on al salado de quesos. Ph.D. Thesis, Universidad Politecnica de Valencia, Spain. Andres, A., Panizzolo, L., Camacho, M. M., Chiralt, A., & Fito, P. (1997). Distribution of salt in Manchego type cheese after brining. In R. Jowitt (Ed.), Engineering and food at ICEF 7 (A, pp. 133± 136). Sheeld: Academic Press. Barat, J. M., Chiralt, A., & Fito, P. (1998a). Equilibrium in cellular food osmotic solution systems as related to structure. Journal of Food Science, 63(5), 836±840. Barat, J. M., Grau, R., Montero, A., Chiralt, A., & Fito, P. (1998b). Feasibility of brining of ham for curing. In A. Diestre, & J. M. Monfort (Eds.), Meat consumption and culture (Vol. II, pp. 970± 971). Institute of Food Agricultural and Technology (IRTA) and EUROCARNE. Bugue~ no, G. (2000). Salado-ahumado de salm on (Salmo salar) por impregnaci on a vacõo. In¯uencia del envasado en la calidad. Ph.D. Thesis, Universidad Politecnica de Valencia, Spain. Burgess, G., Cutting, C., Lovern, J., & Waterman, J. (1979). El pescado y las industrias derivadas de la pesca. Zaragoza: Acribia. Crank, J. (1975). The mathematics of di€usion. London: Oxford University Press. Chiralt, A., & Fito, P. (1997). Salting of Manchego type cheese by vacuum impregnation. In P. Fito, E. Ortega, & G. Barbosa (Eds.), Food engineering 2000 (pp. 214±230). New York: Chapman & Hall. Chiralt, A., Fito, P., Andres, A., Barat, J. M., Martõnez-Monz o, J., & Martõnez-Navarrete, N. (1999a). Vacuum impregnation: a tool in minimally processing of foods. In F. A. R. Oliveira, & J. C. Oliveira (Eds.), Processing of foods: quality optimization and process assessment (pp. 341±356). Boca Rat on: CRC Press.

Chiralt, A., Fito, P., Gonzalez-Martõnez, C., & Andres, A. (1999b). Salt transport in pressed curd as a€ected by brine vacuum impregnation. In G. Barbosa-Canovas, & S. P. Lombardo (Eds.), Proceedings of the Sixth conference of Food Engineering (CoFE '99) (pp. 106±111). AICHE. Delacroix-Buchet, A., & Trossat, P. (1991). Proteolyse et texture des fromage a pate cuite pressee. I. In¯uence de l'activite de l'eau. Lait, 71, 299±311. Escriche, I., Fuentes, C., Gonzalez, C., & Chiralt, A. (2001). Development of medium volatility compounds in Manchego type cheese as a€ected by salt content and salting method. Journal of Food Composition and Analysis, 13, 827±836. Fito, P. (1994). Modelling of vacuum osmotic dehydration of foods. Journal of Food Engineering, 22, 313±318. Fito, P., Andres, A., Chiralt, A., & Pardo, P. (1996). Coupling of hydrodynamic mechanism and deformation relaxation phenomena during vacuum treatments in solid porous-liquid systems. Journal of Food Engineering, 27, 229±240. Fito, P., & Pastor, R. (1994). Non-di€usional mechanism occurring during vacuum osmotic dehydration. Journal of Food Engineering, 21, 513±519. Fito, P., & Chiralt, A. (1997). An approach to the modelling of solid food-liquid operations: application to osmotic dehydration. In P. Fito, E. Ortega, & G. Barbosa (Eds.), Food Engineering 2000 (pp. 231±252). New York: Chapman & Hall. Fito, P., & Chiralt, A. (2000). Vacuum impregnation of plant tissues. In S. M. Alzamora, M. S. Tapia, & A. L opez-Malo (Eds.), Design of minimal processing technologies for fruit and vegetables (pp. 189± 201). Maryland: Aspen Publishers. Fito, P., Chiralt, A., Barat, J. M., & Martõnez-Monz o, J. (2001). Mass transport and deformation relaxation phenomena in plant tissues. In J. Welti-Chanes, G. Barbosa-Canovas, & J. M. Aguilera (Eds.), Engineering and food for the 21st century. Lancaster: Technomic Publishing Co., Inc. (in press). Fito, P., Chiralt, A., Serra, J., Mata, M., Pastor, R., Andres, A., & Quan, X. (1994). An alternating ¯ow pocedure to improve liquid exchanges. In Food products and equipment for carrying out said procedure. European patent 94500071.9. Fuentes, C. (1999). Salado de quesos tipo Manchego: Aplicaci on de la impregnaci on a vacõo y evoluci on de los compuesto volatiles durante la maduraci on. Ph.D. Thesis, Universidad Politecnica de Valencia, Spain. Geurts, T. J., Walstra, P., & Mulder, H. (1974). Transport of salt and water during salting of cheese. 1. Analysis of the processes involved. Netherlands Milk and Dairy Journal, 28, 102±129. Gonzalez, Ch., Martõnez-Navarrete, N., Chiralt, A., & Fito, P. (1998). Drying behaviour of Manchego type cheese throughout ripening. In C. B. Akritidis, D. Marinos-Kouris, & G. D. Saravacos (Eds.), Drying '98 Vol. B (pp. 1251±1258). Thessaloniki: Ziti Editions. Gonzalez-Martõnez, C., Fuentes, Cl., Chiralt, A., Andres, A., & Fito, P. (1999). E€ectiveness of vacuum impregnation brining of Manchego type curd. International Dairy Journal, 3, 143±148. Gonzalez-Martõnez, C. (1999). Salado de quesos tipo Manchego por impregnaci on a vacõo e in¯uencia en la maduraci on. Ph.D. Thesis, Universidad Politecnica de Valencia, Spain. Guamis, B., Trujillo, J. A., Ferragut, V., Chiralt, A., Andres, A., & Fito, P. (1997). Ripening control of Manchego type Ewe's cheese salted by brine vacuum impregnation. International Dairy Journal, 7, 185±192. Irvine, R. O., & Burnett, K. A. (1962). E€ects of vacuum treating on the textured qualities of Cheddar cheese. Canadian Dairy and Ice Cream Journal, 41, 24±28. Mulvihill, D. M., & Fox, P. F. (1980). Proteolysis of as -casein by chymosin in dilute NaCl solutions and in Cheddar cheese. Irish Journal of Food Science and Technology, 4, 13±23.

A. Chiralt et al. / Journal of Food Engineering 49 (2001) 141±151 O€er, G., & Trinick, J. (1983). On the mechanism of water holding in meat: the swelling and shrinking of myo®brils. Meat Science, 8, 245±281. Pavia, M., Trujillo, A. J., Guamis, B., & Ferragut, V. (1999a). E€ects of ripening time and salting method on glycolysis in Manchego type cheese. Milchwissenschaft, 54, 379±381. Pavia, M., Trujillo, A. J., Guamis, B., Capellas, M., & Ferragut, V. (1999b). Changes in microstructural, textural and color characteristics during ripening of Manchego type cheese salted by brine vacuum impregnation. International Dairy Journal, 9, 91±98.

151

Reinbold, R. S., Hansen, C. L., Gale, C. M., & Ernstrom, C. A. (1993). Pressure and temperature during vacuum treatments of 290kilogram stirred-curd Cheddar cheese blocks. Journal Dairy Science, 6, 909±913. Salvatori, D., Andres, A., Chiralt, A., & Fito, P. (1998). The response of some properties of fruits to vacuum impregnation. Journal of Food Process Engineering, 21, 59±73. Wilding, P., Hedges, N., & Lillford, P. J. (1986). Salt-induced swelling of meat: the e€ect of storage time, pH, ion-type and concentration. Meat Science, 18, 55±75.