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EI.SEVIER PII: SO260-8774(97)00023-X 0260-8774197 $ I7.00 +lI.O(I

Determination of Freeze-drying Process Variables for Strawberries

Chokri Hammami & Fr6d&ic Ren6

Institut National de la Recherche Agronomique, INRA, Laboratoire de GCnie et Microhiologie des Prockdks Alimentaires, LGMPA, CBAI, Campus de l’INA-PG. 788.50.

Thiverval Grignon, France

(Received 26 November 1996; accepted 25 March 1997)

ABSTRACT

The production of high-quality freeze-dried strawheny pieces was studied by the response surface method (quadratic model). Working pressure (P) and heating plate temperature (T) were determined as the most important factors affecting the criteria of final product quality (appearance/shape, colour; texture, rehydration ratio). Freeze-drying experiments involving a thick layer of strawbeny pieces were conducted in different operating conditions according to a two-factor (F: T) experimental design. The su$ace response method allowed us to graphically determine optimal working conditions. By superimposing all quality criteria contour plots, the optimal conditions found were P = 30 Pa and T = 50°C. The freeze-drying time ranged fi-om 60 to 65 h. The experimental values for freeze-drying time, appearance and colour of freeze-dried strawberries obtained with these optimal conditions were very similar to those predicted tly the second-order su$ace response model. 0 1997 Elsevier Science Limited

a*, b* ali, '%j

D

NOTATION Chromatic coordinates Model coefficients Diameter (m) Degree of freedom Fisher t-test Luminance (5%) Total pressure (Pa) Determination coefficient Surface of internal condenser (m2) Surface of external condenser (m’) Surface of the plate (m’)

133

134 C. Hammami, E Rent

Student test Heating plate temperature (“C) Freeze-drying time (h) Glass transition temperature (“C) Energy (J) Model error

Subscripts

Fresh Freeze-dried Pulp Skin

INTRODUCTION

The preservation of biological products by reducing their water content can be achieved by several dehydration techniques. Among these methods, vacuum freeze- drying is considered as the reference process for manufacturing high-quality dehydrated products. This drying process involves a preliminary freezing of the products followed by placing them under reduced pressure ( ~300 Pa) with a suffi- cient heat supply to sublimate ice (2800 J per gram of ice).

Compared to classical dehydration techniques, the main advantages of the vacuum freeze-drying process are: (i) the preservation of most of the initial raw material properties such as shape, appearance, taste, colour, flavour, texture, bio- logical activity, etc. and (ii) the high rehydration capacity of the freeze-dried product.

Preliminary freezing of the product stiffens its structure and subsequently pre- vents solute and liquid motion during freeze-drying (Levine & Slade, 1989). During the formation of ice crystals, they grow and create a uniform network throughout the product that after sublimation yields a dense, spread and homogeneous porous matrix. Chemical and/or enzymatic reactions will thus be significantly limited and the phenomena of aroma loss and vitamin degradation will be reduced in compari- son to classical drying techniques (Simatos et al., 1974). The sublimation phenomenon (direct change from ice to vapour) explains why freeze-dried products are adulterated little or not at all and can rehydrate instantaneously. The poor quality and/or alterations of freeze-dried products that are sometimes encountered are generally linked to the quality of the raw material (nature and degree of ripeness) and to processing conditions (operating pressure, heating temperature, freezing rate, freeze-drying process control) (Genin & Rene, 1996).

Another important feature of the freeze-drying process is its high cost, for both capital (vacuum technology) and running costs. On an industrial scale, Flink (1977) considered that the running cost of freeze-drying processes is from four to five times higher than that of the spray-drying technique, and eight to ten times higher than that of the single-stage evaporator. The use of freeze-drying in food industries is then restricted to high added-value products such as coffee (tea and infusions), ingredients for ready-to-eat foods (vegetables, pasta, meat, fish, etc.) and several aromatic herbs.

Freeze-drying process variables for strawberries 135

A major problem with vacuum freeze-drying experiments is their duration (l-3 days). This is due to poor internal heat transfer inside the product and to low working pressures: the principal heat transfer phenomenon is radiation, since there is poor ambient convection and poor conduction between surfaces making contact under vacuum. This explains why most pilot scale studies have dealt with the understanding and improvement of heat transfer (RenC et al., 1993) to reduce freeze-drying time (and concomitant processing costs). Many systems have been tested, e.g. the use of new heat inputs such as microwave or infrared radiation (Le Loch, 1992), or pre-treatment of the product (grinding, partial dehydration, etc.). Other workers (Wolff & Gibert, 1988; Villaran et al., 1994) have studied the advant- ages of atmospheric pressure freeze-drying in a fluidized bed of starch granules or zeolite grains. Capital investment and operating costs were lower than those of vacuum freeze-drying (no vacuum pump and no condenser), but processing time was much longer and the quality of the freeze-dried products (appearance, colour, etc.) was inferior.

A literature search has indicated that the main advantage of the vacuum freeze- drying process, i.e. the quality of the freeze-dried product, was not a major goal of research work. The prime interest in fact was the influence of processing conditions (working pressure and heating temperature) on freeze-drying time and, to a lesser extent, the shelf-life of the freeze-dried products and the changes in some properties during storage (Roos, 1987; PtigkkGnen & Mattila, 1991). Significant progress was reached in the understanding of the phenomena occurring during freeze-drying (Monteiro-Marques et al., 1991; Rutledge et al., 1994; Genin & RenC, 1995), quality improvement of freeze-dried vegetables (Le Loch et al., 1992; Kompany & Ren6, 1993; Genin, 1995; RenC et al., 1996) or the monitoring and control of the vacuum freeze-drying process (Roy & Pikal, 1989; Rem? et al., 1994).

In some cases, improvement (energy saving and quality assessment) of classical dehydration techniques such as hot air drying, fluidized bed drying, vacuum drying, spray-drying, etc. (Mujumdar, 1987), as well as the development of new techniques such as ‘puffing’ (Guimard, 1994) or ‘DCtente InstantanCe ControlCe’ (Allaf, 1995) had given rise to cheaper products that can be used in place of freeze-dried products. As a result, the challenges and opportunities for the freeze-drying tech- nique require: (i) improvement of the quality of dehydrated products, and (ii) the processing of other products that are technically difficult to dehydrate at high temperature, e.g. because of their high sugar content.

The criterion of quality is becoming progressively more important for consumer choice. Thus, industrial products and ingredients must offer different convenient properties (taste, healthfulness, safety, etc.) that are close to those of fresh product. At the same time, new market demands are emerging that could concern freeze- dried products, for example dehydrated fruits to add to corn flakes, cereal bars, ice cream, or pastry making.

This study concerns the vacuum freeze-drying of a thick layer of strawberry pieces. The aim was to determine the influence of processing conditions (operating pressure and heating plate temperature) on both the quality of the final product and freeze-drying time. The criteria defined to assess the quality of the freeze-dried product are: appearance, colour, rehydration ratio and texture. The experiments were performed using a two-factor (P, T> experimental design. Optimal conditions for freeze-drying strawberries were determined by the use of the response surface method (RSM).

136 C. Hammami, I? Rent

MATERIALS AND METHODS

The experimental approach involved four principal steps:

(1) quality assessment of the fresh strawberries; (2) freeze-drying at various operating conditions according to the experimental

design; (3) quality assessment of the freeze-dried and/or rehydrated strawberries; (4) optimisation of freeze-drying process variables for strawberries, using the

response surface methodology (quadratic model), and finally validation.

In addition, the effect of the preliminary freezing temperature (-20°C -8O’C) on both final product quality and freeze-drying time is also studied at the optimal processing conditions.

It is to be noted that this study was carried out under conditions very similar to the industrial process: freeze-drying of a thick layer (about 3.5 cm high) of products previously cut and frozen in stainless steel trays (loading density about 18 kg/m*).

Fresh strawberries

To prevent introducing uncertainty in the analyses, resulting from heterogeneity of raw materials, a single batch of about 20 kg of Pujaro strawberries, Aquitaine variety (France) was used. Fruits were sorted (elimination of unripe and rotten fruit), cut into four pieces lengthwise, frozen as a thick layer (about 3.5 cm high) at -80°C in stainless steel trays (0.31 m long, 0.245 m wide, 45 lo-” m high, and 1.5 10-j m thick) and finally stored at this temperature. Each tray contained about 1.4 kg of fresh strawberry pieces, corresponding to a loading density of 18 kg/m*. Storage of all the products for the entire study at -80°C was based on published results (Cosio & Rene, 1996) showing the absence of significant variations of colour, texture and aroma when bananas were stored at this temperature for up to 2 months. The stability of volatile components of parsley stored at -65°C for more than 100 days was reported by Philippon et al. (1986). This can be explained by the theory of the glassy state and glass transition: to preserve the quality criteria of food products, it is necessary to maintain the non-frozen part of the product in an amorphous glassy state. This glassy state is characterised by the glass transition temperature (T,) below which the viscosity of the medium is extremely high. For temperatures lower than T,, diffusion phenomena are practically arrested, and the rates of chemical and biochemical reactions requiring contact between molecules are considerably reduced (Simatos et al., 1974; Genin & Rene, 1995). The glass transition temperature can be determined by differential scanning calorimetry (DSC), based on the variation of enthalpy per unit mass. Maltini & Giangiacomo (1976) and Levine & Slade (1989) reported a T, value for strawberries of about -35°C. This was confirmed as -34°C in this study.

Quality assessment

Colour The colour of fresh and freeze-dried strawberries (skin and pulp) was analysed with a spectrocolorimeter (Minolta CM-2002, Carrieres sur Seine, France). Diffused light from a xenon arc-lamp (D65 illumination) uniformly illuminates the surface of the

Freeze-drying process variables ,for strawberries 137

sample. Measurement of reflected light is expressed as the reflectance curve versus the wavelength (380-780 nm). This apparatus can also calculate tri-stimulus values and chromatic coordinates. The results are expressed with the L*, a*, h* colori- metric system, according to the International Commission of Illumination (CITE, 1986). In this system, a colour can be conventionally defined with three numerical parameters: sample luminance, L” (quantity of reflected light), and chromatic coordinates, a* (red-green axis) and b* (yellow-blue axis). For strawberry skin and pulp, each result is the arithmetic mean of two bilateral measurements (fruit cut in half). Forty strawberries were tested.

Penetration tests were performed with a texture analyser (LFRA, Stevens, St Albans, UK) to measure the firmness of the fresh and rehydrated strawberries. The probe used was a stainless steel cylinder 8 mm in diameter, moving at a constant speed of 1 mm/s. The strain-displacement curve enables the calculation of mechani- cal energy at rupture (area under the curve before the sample rupture). Preliminary tests with fresh strawberries about 10 mm high enabled maximal penetration dis- tance to be determined as 5 mm. The mechanical rupture of the strawberries was observed at about 3 mm (30% of penetration). The measurements were made in rhe equatorial plan of the fruit. The final value is the arithmetic mean of two bilateral measurements. Forty strawberries were tested.

Appearance The appearance of the freeze-dried products was assessed by visual comparison to a reference (fresh strawberry) as described elsewhere (Rent! rt al., 1996). Special attention was paid to shrinkage. The scoring system was:

1: none (product similar to fresh fruits) 2: poor 3: considerable (shrivelled).

Rehydration ratio The capacity of freeze-dried strawberries to rehydrate was measured by weighing about 1 g of freeze-dried product after soaking in distilled water (50°C) for different times. The rehydration coefficient (CR%) was defined as the ratio of the amount of water taken up over the total amount of water removed by freeze-drying. The maximum rehydration coefficient is obtained when soaking the product in warm water does not change its weight significantly.

Freeze-dryer

Trials were performed with a pilot scale freeze-dryer (SMHl5, Usifroid, Maurepas, France) which has been described elsewhere (RenC et al., 1993). It consisted of a cylindrical stainless steel enclosure (0.65 m long, 0.40 m diameter), hermetically sealed with a transparent perspex door (20 mm thick). This enclosure included a stainless steel plate (S, = 0.15 m2) that could be cooled or heated, an internal condenser (S,,c. = 0.15 m2) which could reach a temperature of - 6O”C, and a vacuum pump. The apparatus also included an external condenser (S, ,(. = 0.15 m’)

138 C. Hammami, l? Red

running at -55°C connected to the freeze-drying chamber by a pipe and separated by a valve. The pilot plant could thus operate with the two most often encountered configurations of the water vapour cold trap.

Instrumentation The total pressure inside the freeze-drying chamber was measured with a Baratron capacitive manometer, (MKS, Le Bourget, France). The measurement range was l-1400 Pa with an accuracy less than 0.25% of the read value. A controller unit (250D type, MKS Instruments, Sarcelles, France) enabled us to control operating pressure by injecting external dry air through a micro-valve. Partial pressure of the water vapour is measured with a hygrometer (3A, Panametrics, La Garenne- Colombes, France) placed in the freeze-drying chamber. The humidity measurement corresponded to the electrical capacitance of an alumina oxide condenser and is given as a dew point. The dew point scale ranged from -80°C to +2o”C, with an accuracy of about 1°C at 23°C. Partial pressure conversion was carried out according to a Clausius-Clapeyron type equation such as that of Goldblith et al. (1975). The temperatures of the heating plate and the condensers were measured with platinum resistance probes (PtlOO Heraeus, Fontenay Tresigny, France) with an accuracy of O.l”C. Each sensor signal was recorded with an industrial data collector @Mac 4000, Analog Device, Antony, France) connected to a personal computer (PC).

Freeze-drying of strawberries

A tray of frozen strawberries was placed on the freeze-dryer plate at an initial temperature of -60°C. The door was closed and the condensers and vacuum pump were turned on. The plate started heating (1”Cimin) when working pressure reached the operating value. This time reference is the beginning (t = 0) of each experiment. Kinetics of strawberry dehydration was monitored with a patented system (RenC et al., 1994). On the basis of partial pressure measurement, changes in water vapour flow can be calculated and the residual moisture content in the product estimated such as the freeze-drying end-point (Genin et al., 1996). The trial was stopped when partial pressure of water vapour inside the freeze-drying chamber reached the low constant value (about 5 Pa). Thus, the residual moisture content in the dried straw- berry was about 0.1 kg of water per kilogram of dry matter. The corresponding water activity is 0.10. In each test, the total amount of water removed was about 99% (grams of water removed per gram of initial water).

Experimental plan and analysis of factor effects

The experimental design was a uniform modified Doehlert plan with two factors: total enclosure pressure (P) and heating plate temperature (7). This experimental plan requires eight trials (Doehlert, 1970): six points uniformly spread over the experimental domain (Fig. l), with the central point in duplicate. The pressure and temperature ranges to investigate were determined according to the literature search and severat preliminary trials: 15-200 Pa for total pressure and +20 to +9O”C for heating plate temperature. It is to be noted that the experimental plan presents a non-equidistant distribution of pressure and temperature levels. Thus, the

Freeze-dtying process variables ,for strawhem’es 1 .?I

P (Pa)

Fig. 1. Uniform

I : l

20 55 90 T P-3

modified Doehlert experimental plan with two factors strawberry freeze-drying.

used for the study of

statistical analysis of factor effects could be done only after mathematical processing to calculate the corresponding orthogonal polynomials (Kobilinsky, 1988).

The response surface method is the most widely used analysis for these experi- mental designs (Giovanni, 1983; Mudahar et al., 1990; Guerrero et al., 1996). It is a linear multiple regression technique which consists in modelling the results by a second-order polynomial equation of the two factors (quadratic model). For each response variable (appearance, colour, texture, rehydration ratio, freeze-drying time), the linear, quadratic and simple interaction effects of pressure and tempera- ture are compared among themselves (Iwaniw & Mittal, 1990; Danzart, 1991). Any response variable (Y) can then be expressed as a function of the two independent factors (P, 7) and a measurement error I::

Y = aoo+u,,,P+ao, T+u&“+u,~J~+cI, ,P T+r: (1)

In this case, the coefficient a;j cannot be interpreted directly. As mentioned pre- viously, eqn (1) must be rewritten according to the orthogonal polynomials of P and T (Noel et al., 1991):

Y(P, T) = xo,,+x,,~A,(P)+x,,,B,(T)+ix,oA,(P)+x,,,B,(T)+~, ,A,(P)B,(T)+i: (2)

where x(~~, is the mean effect, r ,,, the linear effect of pressure, czoI the linear effect of temperature, r2,) the quadratic effect of pressure, xo2 the quadratic effect of temperature, x,, the simple interaction effect of pressure and temperature, and (Noel ct al., 1991):

A,(P)=0.016SlP - 1.77484 (3)

A,(P) = 0.00032P’ -0.06769P+2.48 196 (4)

B,(T)=@04367 T-2.40181 (5)

140 C. Hammami, IZ Rent

B,(T) = 0.0022 1 T2 -0.24222T+5.50641 (6)

Orthogonal polynomials and factor effects were calculated with Statgraphics Plus software for Windows.

For each response variable, the analysis of variance included the values of the estimated coefficients (coef), the standard error (std.err), the statistical Student t value (t.stat) and the associated probability P(t) of the model. P(t) values less than 0.05 for a 95% confidence interval indicate statistically significant non-zero coeffi- cients. Thus, variables with low probability levels contribute to the model, whereas the others can be neglected and eliminated from the model. This analysis also includes the Fisher test, F, and its associated probability, P(F), (overall model significance), and the determination coefficient, R2. Generally, P(F) must be lower than 0.10 and R2 greater than 0.90 in order to consider the model valid.

For each dependent variable, the quadratic models can be presented as response surfaces (3D) and contour plots (2D). Thus, by superimposing the contour plots of all response variables, optimal working conditions for processing high-quality freeze- dried strawberries was determined graphically. New experiments were then run with these operating conditions to validate the preceding analysis (appearance, colour, texture, rehydration ratio and freeze-drying time).

RESULTS AND DISCUSSION

Characterisation of fresh strawberries

The characterisations of fresh strawberries are presented in Table 1. The wide distribution of pulp colour values is due to the bundle zone situated in the internal part of the fruit (Fig. 2) but these values remain within the ranges found in the literature. The 9.8% dry matter content is consistent with the values reported by Skrede (1980) for 12 strawberry varieties (9.7-11.3%). The soluble solid content of 7.9% is slightly lower than the range of &l-10.9% found by the same author. The pH of 3.49 is included in the range 3.34-3.57 determined by Skrede (1980). Colori- metric measurements of fresh strawberries show that the pulp luminance (L,*) is much higher than that of the skin (L,*), a result previously reported by Sacks & Shaw (1994) for 47 strawberry genotype populations obtained from six different varieties. The values of the chromatic coordinates (a\* and h,*) are low in our case, but are consistent with a Pajuro strawberry that has a faint red colour.

The results obtained in the texture study are difficult to compare to those found in the literature. The principal reasons for this are the different systems used to express the results and the wide variety of measurement methods employed. In addition, product texture strongly depends on its degree of ripeness (Szczesniak & Smith, 1969) the variety of the strawberry and the measurement method (Skrede, 1982; Planton, 1993; Nunes et al., 1995).

Influence of processing conditions

The analysis of variance of the five response variables indicated that freeze-drying time, appearance and colour of freeze-dried strawberries are correlated with pro- cessing conditions (P(F) 10.05 and R’>0.96), but texture and rehydration capacity

Freeze-drying process vuriahles ,for strawberries 141

ratio are not (P(F) 20.50 and R’~0.50). In any case, operating pressure and heating plate temperature have a significant linear and/or quadratic effect, but their cross- linked effect is not significant at the 95% confidence level.

Freeze-drying time The results show only an effect (negative) of the first order of heating plate tem- perature on the freeze-drying time, at the 99% confidence level. There was no significant linear or quadratic effect of operating pressure at the 95% confidence level. Determination coefficient (R*) is about 0.99. The freeze-drying time model can then be expressed as the following:

ti = 60.07 - 9.29.B, (T)+c: (7)

The response surface in Fig. 3(A) confirms the strong effect of heating plate tem- perature on freeze-drying time. A temperature rise increases the heat flow transferred to the product and consequently increases the dehydration rate, Nevcr-

TABLE 1 Comparison of the Main Properties of the Fresh Pajaro Variety Aquituine Used in this Work with the Values Found in the Literature for Strawberry. The Standard Deviation Measure-

ments are Indicated in Par&theses

This work Literature

Dry matter content (5%) 9.8 (0.‘)) 0.6 to 12.3 0.7 to Il.3 9.2 to 1l.h

Reference

Simatos et al. (lY74) Skredc (1080) Skrede (I YX?)

PH 3.49 (0.04) 3.34 to 3.57 Skrede (1980) 3.45 to 3.67 Skrede ( 1082)

Soluble solids (5%) 7.0 (0.6) 8.1 to l0.Y 9.2 to I I .h

Skrede ( 1980) Skrede ( 1 YE)

Water activity Aw

Energy of rupture (10 ’ J)

Glass transition temperature r, (“C)

0.00 (0.005)

6.27 (O.YY)

-34.3 (1.Y) -34.1

-33.5 to -3Y.l

Maltini & Giangiacomo (lY76)

Levine & Sladc (IYXY)

Colour of skin surface

C’olour of pulp surface

Sacks & Shaw (iOY4) Nunes et al. (IYYS) Sacks & Shaw (lYY4) Nunes et al. (lYY.5) Sacks & Shaw (lYY4)

Sacks & Shaw ( lYY4) Sacks & Shaw ( IYY4) Sacks & Shaw (lYY4)

142 C. Hammami, E Rent

sepal

Fig. 2. Cross-section of a strawberry (from Szczesniak & Smith, 1969).

theless, the freeze-drying time of strawberries still remains longer than the dehydration time of any classical drying technique. This was also observed by Wolff (1988) for vials of milk, Le Loch (1992) for common mushroom and potato, Cosio (1997) for banana, and Genin (1995) for courgette, onion and parsley. These authors also showed that freeze-drying time at a given temperature is minimal for a working pressure of 100 Pa. Only Cosio (1997) did not find an effect of working pressure on freeze-drying time for banana slices.

It should be borne in mind that several authors have stressed the influence of freezing rate on freeze-drying time (King, 1971; Simatos et al., 1974) and on the quality of the final product (Lee & Salunkhe, 1967; Baumunk & Hondelmann, 1969). Freezing rate affects primarily the size of ice crystals and therefore the final porosity of the freeze-dried product. From the standard law of mass transfer through porous media, it can be deduced that the larger the pore size, the easier it will be to remove water vapour from the product. Thus, a slow freezing rate that allows the growth of large ice crystals should consequently lead to shorter freeze- drying times. Genin (1995) reported the effects of freezing rate on freeze-drying time for courgette, onion and parsley. He noted that this simple assumption is not true for all products and verified it only in the case of onion. He showed that texture, degree of ripeness and dry matter content are also variables that have a considerable effect on freeze-drying time. This parameter is also essential for the rehydration ratio and texture of the final product.

Other processing conditions can also affect freeze-drying time, such as the loading density, height of the product layer, specific surface of the product (cutting) and condenser capacity (temperature, surface). It is evident that the rate of dehydration increases when both the loading density and product layer thickness decrease. Mal- tini & Giangiacomo (1976) showed that the freeze-drying time for intact strawberries was 30% longer than for strawberry pieces. Le Loch (1992) showed that the freeze-drying time of a 3 cm thick layer of common mushroom slices was 1.5 times longer than for a 1.5 cm thick layer of slices.

In terms of the degree of ripeness state of the raw material, Genin & RenC (1996) showed that the freeze-drying time of courgette, whose skin luminance was lower than 41%, was 8% longer that of products whose skin luminance was higher than

Fretzt-‘-drying process variables for strawbenirs I -43

(l-ar*/ai*), ..:... : CC) (I-h*h*)s : .: L _i’ .‘. :

CD) j.... -:.._ : :

(l-hr*h*)p __ ‘. ;” 09

Fig. 3. Response surfaces of the criteria depending on operating pressure and heating plate temperature. (A) Freeze-drying time. (B) Shrinkage. (CD) Colour parameters of the skin. (E,F) Colour parameters of the pulp. The dark circles are the experimental measurements

and the quadratic model is the square surface.

144 C. Hammami, F Rent

41%. The luminance of courgette was correlated to its glass transition temperature, which in turn is linked to the degree of ripeness.

Appearance The results show that the phenomenon of strawberry shrinkage correlated primarily with operating pressure. Only the first-order coefficient of the pressure effect (posi- tive) is significant at the 98% confidence level. There was no significant linear or quadratic effect of heating plate temperature at the 95% confidence level. Determi- nation coefficient (Z?‘) is about 0.97. The shrinkage phenomenon model can then be expressed as follows:

Sh = 2.10+0.78.A,(P)+~ (8)

The response surface (Fig. 3B) shows that working pressure must be lower than 30-40 Pa in order to avoid strawberry shrinkage. Nevertheless, working pressure can be increased up to 50-60 Pa without excessive shrinkage of freeze-dried strawber- ries, because of its intricate polymeric structure. Thus, to avoid the shrinkage phenomenon during freeze-drying, the temperature of the frozen core must be lower than the initial melting temperature. Since the frozen core surface is at thermodynamic equilibrium (phase change), the partial pressure of water vapour in the surrounding atmosphere must be lower than the value corresponding to the initial melting temperature (Fig. 4). This explains why vegetables or milk, which present an initial melting temperature of about -15°C to -5°C (the corresponding partial pressures of water vapour are in the range of 200-400 Pa), can be freeze- dried even at 200 Pa (RenC ef al., 1993; Genin, 1995) while the fruits can be freeze-dried only at pressures lower than 100 Pa. This value corresponds to a tem- perature of -20°C and is in the range of initial melting points for fruits. Maltini & Giangiacomo (1976) showed that bananas and strawberries were perfectly rehydrated only if the freeze-drying pressure was lower than 80 Pa. Cosio (1997) also showed that the shrinkage phenomenon exists during the freeze-drying of banana slices at a working pressure of 200 Pa, but never appears at working pres- sures lower than 50 Pa.

Fig. 4. Phase

0 50 Temperature (“C)

diagmm for water and aqueous solutions. (P,: triple point triple point or eutectic point of an aqueous solution).

of pure water; P$

Freeze-drying process variables for strawberries 1 *as

Rehydration ratio The statistical analysis of the rehydration results shows that this criterion is indepen- dent of both operating pressure and heating plate temperature. The Fisher test probability is very high (P(F) > 0.50) and the determination coefficient is very low (R2 ~0.50). The same tendency had been observed by Cosio (1997) for the rehydra- tion of freeze-dried bananas. It is to be noted that the values of the rehydration ratio are not significantly different. These values range from 22% to 32% and are attained after soaking in heated water (50°C) for 5-20 min. The rehydration capa- city of the freeze-dried products obtained in this work was different from other published results. Thus, Carballido & Rubio (1970) and Simatos et al. (1974) reported that the rehydration ratio of strawberries (previously frozen at -25°C) was about 50% for warm water soaking times from 0.5 to 5 min. Besides the thickness of the product, its variety (origin and degree of ripeness) and dry matter content, differences in the rehydration ratio can be related to the initial freezing. As pre- viously mentioned, ice crystal size is governed by the freezing step (freezing rate and final temperature) and is the main parameter governing the rehydration stage. The influence of the freezing rate and thus ice crystal size on the rehydration of freeze- dried products has been extensively studied: Smithies (1962) showed that pieces of meat frozen in an acetone-dry ice mixture rehydrated much more slowly than those frozen at -20°C. The same tendency was observed for other foodstuffs such as asparagus (Spiess, 1964) prawns (Goldblith et al., 1964) peaches, apples and apri- cots (Lee & Salunkhe, 1967) strawberries (Baumunk & Hondelmann, 1969) bananas (Maia & Luh, 1970; Maltini & Giangiacomo, 1976) raspberries (Medas ok Simatos, 1971) carrots (Longan, 1973) passion fruit juice (Cal-Vidal & Falcone, 1985) and onions (Genin, 1995).

Texture Mechanical energy at 30% of penetration was determined for freeze-dried strawber- ries rehydrated in heated water at 50°C for 20 min. Statistical processing was carried out to determine the influence of processing conditions on texture, using the follow- ing variable:

(I -Wf/Wi) (9)

where W,- is the penetration energy of the rehydrated freeze-dried strawberry amd W, the penetration energy of the fresh product. As in the case of the rehydratmn ratio, no significant differences were found between the 30% energy penetration values. They ranged from 0.81 to 1.37 1OP’ J. Also, there was no significant infu- ence of the operating pressure of heating plate temperature on the texture of rehydrated strawberries. The associated probability of the Fisher test is very high [P(F) > 0.601. These results are similar to those of Cosio (1997). Texture values of the rehydrated strawberries were similar to those of thawed ones previously frozen at -80°C (0.87 10e3 JkO.15) and 85% of the mechanical resistance was lost after the freezing step.

In the case of some other products, however, there is a temperature effect on the texture of the freeze-dried product. Thus, meat and fish become hard at particular temperature conditions (Goldblith et al., 1964; MacKenzie & Luyet, 1967). Sterling & Shimazu (1961) observed a variation of carrot texture as a result of the tempera- ture rise during the process. More recently, Iwaniw & Mittal (1990) determined the optimal freeze-drying temperature profile for strawberries, leading to minimal

146 C. Hammami, E Rent

texture degradation. Within the experimental region, they determined two sets of temperature parameters:

(1) a heating plate temperature of 15°C and a temperature rise of 1.6”Umin; (2) a heating plate temperature of 45°C and a temperature rise of 0.4”C/min.

Heating plate temperature of 45°C is identical to the value recommended by Maltini & Giangiacomo (1976) to obtain freeze-dried strawberries without a cooked flavour.

In spite of this, the effect of freeze-drying conditions on the texture of the rehydrated products is negligible in comparison to the effect of the freezing step. Product texture is better preserved at high rates of freezing. The formation of small ice crystals limits the tissue destruction generally encountered during freezing. This was confirmed by the results of Fang et al. (1971) and Longan (1973) on mushrooms and carrots.

Colour The visual appreciation of strawberries freeze-dried under different processing con- ditions showed a slightly pronounced red colour compared to the fresh product, but this vanished after rehydration. The increased red colour of freeze-dried strawber- ries leads to an increase (10%) of a* and a decrease (15%) of b*. Nevertheless, these chromatic coordinate changes are comparable to frozen ones, but are more marked. Indeed, colour of fresh and dry fruits differ as water affects the appearance. Thus, this phenomenon of red colour increase can be attributed to both the freezing step and the water reduction effect.

Indeed, this colour change has consistently been observed in strawberries (skin and pulp) placed both at the top and at bottom of the layer. This phenomenon was also reported by Carballido & Rubio (1970) for strawberries freeze-dried at 25°C and 15 Pa. Wrolstad et al. (1970) have also shown that the red colour of strawberry was reinforced by freezing. This phenomenon is due to the modification of the form of one of the main strawberry dyes, pelargonidine-3-glucoside. The same phenom- enon was observed by Bengstsson & Bengstsson (1968) for uncooked beef meat: the sample frozen at a low freezing rate has a more pronounced red colour than the one frozen at higher freezing rates.

The more pronounced red colour of freeze-dried strawberries can also be explained by a concentration effect of the red pigments (anthocyans) in the dried product. According to Roudeillac & Veschambre (1987) the red colour of the Pajaro strawberry easily darkens.

Similarly, the pulp of strawberries freeze-dried at 64°C and 173 Pa presented a slight browning. In conditions of 8o”C/42 Pa and 89”C/132 Pa, the skin and the pulp of freeze-dried strawberries both presented considerable browning. This was restricted to fruits at the bottom of the layer (in contact with or near the heat transfer surface), and involved about 30% of the total quantity of fruit. Strawberry pulp is consistently more sensitive to this colour change than the skin. Differences in composition, particularly sugars, may explain this phenomenon.

To avoid variations due to the product rehydration (dissolution of red dyes), colour measurements were performed directly on the freeze-dried strawberries. Statistical processing of the results was carried out to determine the influence of processing conditions parameters, by comparison with the colour of the fresh product, with the following variables:

Freeze-drying process vuriahles for strawhenies 147

(1 -I!+*ILi*) (10)

(1 -Uf*lUi*) (11)

( 1 - bf*lbi*) (12)

The indices f and i correspond to the final (freeze-dried) and initial (fresh) straw- berry. When necessary, indices s and p have been added to differentiate product skin and the pulp. Since the variation coefficient of calorimetric measurements of fresh strawberries was about 15%, this value was used as the limit beyond which the difference is considered as statistically significant. The statistical colour results of strawberry dried in different processing conditions, showed that the colour param- eters are only dependent on the heating plate temperature.

It should be noted that the colour of strawberry situated at the top of the layer is not affected by the processing conditions. However, to obtain a homogeneous colour batch of freeze-dried strawberry, we have used those situated at the bottom of the layer.

Luminance (L *) Statistical analysis showed that the luminance of both the freeze-dried strawberry skin and pulp could not be explained by processing conditions. At the 95% confi- dence level, there was no effect of working pressure and heating plate temperature. Only the luminance value L* of freeze-dried strawberries increased slightly, but remained similar to that of the frozen product (L,* = 40.1% + 1.9% and L,,* = 63.3% k 2.3%). At higher working pressures (P > 108 Pa), we noted a slight decrease in the luminance value L* (skin and pulp). This phenomenon can be due to the pronounced shrinkage observed under these conditions. Similarly, at higher heating plate temperature (T > 60°C) we note also a slight decrease in the lumi- nance value L* (skin and pulp), which can be attributed to the appearance of the dark brown colour at the surface of the fruit, modifying its optical property of light reflection.

This parameter is defined as the light flux measured in a given direction and depends on the number and orientation of light-reflecting surfaces. As previously mentioned, a high rate of freezing leads to small, randomly oriented ice crystals. Thus, the luminance of a rapidly frozen product is higher than that of the same product frozen slowly. This had been discussed for instant coffee (Petersen et t21.,

1970) and bananas (Cosio, 1997) but the difference between the products freeze- dried at different rates disappeared after rehydration. The relationship between freezing rate and luminance of the freeze-dried product was also reported by Genin (1995) for courgette, onion and parsley.

Chromatic coordinates (a *, b *) Skin

Only the linear and quadratic effects of temperature had a significant effect on colour parameters at confidence levels higher than 97%. These effects were positive and consistent with the negative influence of temperature rise on strawberry colour. Determination coefficients (R’) are about 0.98. There was no significant linear or quadratic effect of operating pressure at the 95% confidence level. Colour param- eter models can be then expressed as follows:

( 1 - CIf*ICli*)~ = 0.142.B, (T)+O. 1 O9.B2( T)+r: (13)

148 C. Hammami, E Rent

(1 -bf*lbi*)s = 0.238+0.121~B,(T)+O. 106’B2(T)+c (14)

The corresponding response surfaces (Fig. 3C,D) show that calorimetric parameters (1 -af*/ai*)s and (1 -bf*/bi*)r were positive for high heating plate temperatures (T> 70°C). Browning of the skin occurred and led to a significant decrease of the chromatic coordinates (about 30% for us* and 50% for bS*). At low heating tem- peratures, parameter (1 -af*/Ui*)s was negative. This corresponds to the increase of the red colour previously discussed.

These temperature effects on colour (surface browning) is a well-known phenom- enon classically encountered during excessive heating of products that are already dried. Its origin is non-enzymatic browning reactions (Simatos et al., 1974; Cheftel et al., 1978) or Maillard reactions, that generate brown polymers on the surface of the strawberry.

Pulp Only the linear and quadratic effects of temperature had a significant effect on colour parameters at confidence levels higher than 93%. Determination coefficients (R2) are about 0.97. The plots of the response surfaces (Fig. 3E,F) show that the linear and quadratic effects of temperature on pulp colour were of the same order of magnitude, but positive for (1 -Uf*/Ui*)p and negative for (1 -bf*/bi*)p. There was no significant linear or quadratic effect of operating pressure at the 95% confidence level. Colour parameter models can be then expressed as follows:

(1 ~~~*/~~*)~~0.092+0.218~B~(T)+0~121~B~(T)+~~ (15)

(1 -hf*lbi*),=0.081 -0.209.B,(T)-O.O95~B~(T)+~: (16)

At elevated heating temperatures (T > 6O”C), browning of the pulp occurred and led to a significant decrease of the chromatic coordinate up* of about 40%, and to an increase of b,* of about 25%. The increase of b,* (yellow) is due to sugar browning reactions (caramelisation). The reinforcement of the red colour was also observed in strawberry pulp at a low heating temperature and corresponds to positive values of colour parameter (1 -at*/ai*)p and negative values for (1 - bf*/bi*),.

Determination and validation of optimal conditions

It must first be decided what ‘optimal’ refers to. Figure 5 summarises the results with respect to operating conditions. It is seen that the highest quality of freeze- dried strawberries cannot be obtained with the shortest processing time. Considering that the appearance of freeze-dried strawberries is the quality criterion by definition, operating pressure must be lower than 30-40 Pa to avoid shrinkage phenomena. Since the colour of strawberry skin (a\*, b,*) and pulp (up*, bp*) depends only on heating plate temperature, this parameter must be lower than 50-55”C, in order to avoid excessive changes of chromatic coordinates between freeze-dried and fresh products, and also to avoid browning of part of the product at the bottom of the layer.

Additional experiments (two trials) were conducted with a working pressure of 30 Pa and a heating plate temperature of 50°C to validate the above analysis. The strawberries used were from the same batch of Pajaro, previously frozen at -80°C in a thick layer 3.5 cm height. The experimental result values for appearance, colour and freeze-drying time are very close to those predicted by the model, to within 5-10%. The increased red colour phenomenon of freeze-dried strawberries was

Freeze-drying process variables for strawberries

t,(h)

,--rll-;”

( 1 - af*/ai*) I 90 4 a2

a0 Ql

m

slnhp 90, ”

8).

?I)’ 2

60’ 1

w

(l-W%*),

Fig. 5. Contour plots of (A) t ‘reczc-drying time: (B) shrinkage; (C,D) colour parameter?’ of the skin; (E,F) colour paramctcrs of the pulp.

150 C. Hammami, E Rem!

again shown. Results for the other criteria are similar: thus luminance of the skin and pulp were 39.9% (1.8) and 66.3% (4.8), the rehydration ratio was 23.6% (0.3) after soaking in water for 10 min, and the 30% penetration energy was 0.99 lo-” J (k0.13). In addition, neither off-flavour nor off-taste were detected in the freeze- dried and rehydrated strawberries.

Based on the results of Kompany & RenC (1993) on the retention of aroma in freeze-dried mushrooms, a freeze-drying trial at variable temperature was carried out. Since optimal pressure for freeze-drying strawberries is low (30 Pa), only heat- ing plate temperature was varied. The experiment started at 90°C until 50% of water was removed, then the temperature was reduced to 50°C until the end of the process. In this case, freeze-drying time is 13% shorter (54 h instead of 62 h) for an identical product quality.

Effect of the freezing rate on final quality and freeze-drying time

The previous part of this study showed the influence of processing parameters (heating plate temperature and working pressure) on both the quality of the freeze- dried strawberries and freeze-drying time. The response surface methodology used enabled the process to be optimised with respect to quality (appearance and colour) and duration. In this part of the study, we used Pujuro strawberries (Aquituine) previously frozen at -80°C to prevent variability of the results that could have arisen from changes in the raw material. We also examined the influence of prelimi- nary freezing on final product quality and freeze-drying time. The experiments were done under the optimal conditions previously determined (30 Pa and 50°C). At the time of these new experiments it was necessary to procure new fresh products. At that time of the year, available strawberries were Pujuro, but belonged to the Curpentrus variety. Their physico-chemical properties are comparable to the Aqui- tuine variety, only on the dry matter content which is slightly lower (5.5%&0.3%). Consequently, both soluble solids (4.7% *0.3%) and rupture energy (4.24 10-j J&-0.31) are lower. Two different preliminary freezing conditions were tested: -20°C and -80°C. The products were cut and placed on a stainless steel plate as in the first study. The differences between the freeze-dried products were examined for both quality criteria and freeze-drying time.

Preliminary freezing temperature had a significant effect only on energy at 30% of penetration for the rehydrated strawberries, which was three times lower for a freezing temperature of -20°C in comparison to -80°C. Since the rates of freezing were different, the mean size of the ice crystals at -20°C was larger than at -80°C and consequently cell structure was probably more damaged. Another point which may have the same effect is that the temperature of -20°C is higher than the initial melting temperature of strawberries (-34°C). Nevertheless, texture loss was always greater than 80%.

CONCLUSION

Processing conditions have an influence on both the quality criteria used to evaluate freeze-dried strawberries, and on freeze-drying time. The freezing step explains the instantaneous rehydration capacity of the product, and the texture loss due to the

Freeze-dying process variables for strawhem’es 151

cell wall damage. However, freezing rate has no significant effect on freeze-dried strawberry quality, nor on the freeze-drying time.

The use of an experimental plan and response surface methodology enabled us 1.0 first determine the effect of two operating parameters (working pressure and heat- ing plate temperature) on each criterion used to assess the quality of freeze-dried strawberry pieces. Secondly, we were able to determine working conditions that provided optimal quality. The values of optimal freeze-drying parameters are 30 Pa for working pressure and 50°C for heating plate temperature. Neither off-flavour nor off-taste were detected in the freeze-dried products. To remove 99% of the water content of the product, in conditions similar to those at the industrial scale (loading density of 18 kg/m2 and thick layer of 3.5 cm height), processing time was about 62 h. This freeze-drying time was reduced to 54 h, using variable heating plate temperature conditions, with an identical end-product quality.

In our experiments, neither the rehydration ratio nor texture depended on freeze- drying conditions. In fact, these two criteria are closely linked to the ultimate use of the freeze-dried product and cannot be considered as absolute criteria. Dehydrated fruits are intended primarily to be added to products such as corn flakes, cereal bars, ice cream, pastry sauces, etc.

It should be noted that the low rehydration ratio values ( ~32%) of freeze-dried strawberry found in this study is due to: size of the strawberry pieces (quarter of a fresh fruit) and the specific surface of the product (m’/g or m2/m”); the use of a thick layer that led to long processing times compared to those generally encoun- tered at the pilot scale (thin or monolayer of products; low quantities of products; very low loading densities); and the low value of the final residual moisture content of our products (from 0.1 to 0.2 kg/kg dry matter depending on the initial dry matter content).

These freeze-drying experiments also showed that the trials can be efficiently monitored on-line by a humidity sensor. It allows us to stop the trial at the appro- priate moment, and to avoid overheating of the product.

ACKNOWLEDGEMENTS

This work is part of the EC project No. AIR3-CT94-2254 (DG12 SSMA) entitled ‘A novel approach to preserve the intrinsic quality of fruits and vegetables in dry conservation processes. Process-quality relationships supported by macroscopic and microscopic parameters’.

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