- Email: [email protected]
Use of mild heat pre-treatments for quality retention of fresh-cut ‘Rocha’ pear
Postharvest Biology and Technology 30 (2003) 153 /160 www.elsevier.com/locate/postharvbio
Use of mild heat pre-treatments for quality retention of freshcut ‘Rocha’ pear Marta Abreu a, Sara Beira˜o-da-Costa a, Elsa M. Gonc¸alves a, Maria Luisa Beira˜o-da-Costa b, Margarida Molda˜o-Martins b,* a
UTPV/DTIA/Instituto Nacional de Engenharia e Tecnologia Industrial, Azinhaga dos Lameiros, Estrada do Pac¸o do Lumiar, 22, 1649-038 Lisbon, Portugal b CMIA/SCTA/DAIAT Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisbon, Portugal Received 16 December 2002; accepted 12 May 2003
Abstract The effect of mild heat pre-treatments on quality of fresh-cut ‘Rocha’ pear (Pyrus communis L.) was evaluated by response surface methodology (RSM) in different time/temperature combinations. Susceptibility to cut surface discoloration and changes in firmness, total soluble solids content (SSC) and pH of cut pears (quarters) were the measured dependent variables. The models developed for L *, a *, firmness and pH appeared adequate (R2 /0.72). RSM results also showed a significant lack of fit (P /0.05), for b * and SSC models. Firmness of the ‘Rocha’ pear quarters was preserved in the range of the mild heat pre-treatments used (36 /42 8C). An increase in firmness from an initial value of 27.2 to 70 N was observed whenever treatment temperature was greater than 45 8C. Treatment time did not have a significant (P /0.05) effect on firmness. To preserve the two significant colour parameters (L * and a *), treatments should be conducted in the range 36 /45 8C with treatment times greater than 40 min. Mild heat pretreatments produced an increase in pH values from 4.5 in raw material to 5.8 for samples exposed to high treatment intensities. In order to preserve the pH values, the best-suited conditions are those corresponding to treatments at temperatures lower than 45 8C for less than 150 min. Mild heat pre-treatments (35 /45 8C for 40 /150 min) were effective in avoiding the cut surface browning and preserving or increasing firmness of ‘Rocha’ pear quarters. When pre-treated fruit were stored for 7 days at 2 8C, no significant changes were detected in colour parameters, pH and SSC. However, firmness decreased approximately 30% from the 4th to the 7th day. # 2003 Elsevier B.V. All rights reserved. Keywords: ‘Rocha’ pear; Mild heat pre-treatments; Fresh-cut fruit; RSM; Quality attributes
1. Introduction * Corresponding author. Tel.: /351-21-3653547; fax: /35121-3653200. E-mail addresses: [email protected] (M. Abreu), [email protected] (M. Molda˜o-Martins).
‘Rocha’ pear (Pyrus communis L.) is the main pear cultivar produced in Portugal, and is important in Canada, Brazil, UK and France. ‘Rocha’ pear is classified as PDO (protected designation
0925-5214/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-5214(03)00105-4
154
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
and origin), which means that it corresponds to a traditional product produced under strict conditions and by law may be labelled as such. This cultivar has high storage potential, as whole fruit, supplying the market almost all year round, and is thus competitive with other cultivars. Fresh-cut or minimally processed fruit produce (peeled, cored and sliced) is an important developing class of food product because of attributes such as convenience and fresh-like quality (Gorny et al., 1998; Pretel et al., 1998; Pittia et al., 1999). In addition, this processing methodology may allow the utilization of smaller fruit, which are unsuitable for the fresh market. It is difficult to consistently ensure reasonable shelf-life in fresh-cut fruit, as many fruit must be processed at their optimal ripening quality and therefore are at the edge of senescence (Gorny and Kader, 1996). Another problem is the limited shelf-life, due to excessive tissue softening and surface browning after minimal processing (King and Bolin, 1989; Brecht, 1995; Gorny et al., 1998). Much research has focused on the use of postharvest chemical treatments such as the addition of antioxidants (ascorbic acid, erythrobate, sodium and potassium chloride), firming agents (calcium derivatives) and modified atmosphere packaging at reduced oxygen levels (Rosen and Kader, 1989; Sapers and Miller, 1998; Buta et al., 1999; Gorny and Kader, 1996; Pittia et al., 1999). Although some of these treatments were found to be effective, consumers are demanding a reduction in the overall use of chemicals on fresh products and alternative methodologies should be investigated to extend shelf-life of fresh-cut fruit. Heat treatments are generally effective in inhibiting enzymatic reactions and in reducing microbial levels, but are little used for stabilizing minimally processed fruit due to their negative effects on flavour, texture and fresh quality. On the other hand, the development of suitable mild heat pretreatments, which avoid the negative impacts, may be of great interest in producing minimally processed fruit with an increased shelf-life. This methodology has already been used to control postharvest decay and to improve storage uality in intact fruit by modification of
physiological and physicochemical characteristics and post-processing quality. Mild heat pretreatments have been shown to induce a firming effect on several processed vegetables such as potatoes, carrots and green beans (Bourne, 1987; Anderson et al., 1994; Greve et al., 1994; Aguilar et al., 1997). The influence of heat and mild heat pre-treatments on colour and texture of fruit pieces has been investigated (Mastrocola et al., 1995; Pittia et al., 1999). The results indicated that these treatments increased fresh-cut fruit quality by delaying the deterioration of physical, chemical and sensory properties. Apples heat-treated at 45 8C produced slices with less browning and firmer texture compared with non-treated apples (Kim et al., 1993). Nevertheless, the evaluation of these treatments with regard to other quality attributes such as microbiological stability, nutritional value and sensory characteristics is needed. Limited information is available on heat treatments applied to whole and fresh-cut pears. Pittia et al. (1999) studied the effect of low-intensity blanching treatments on enzymatic and microbiological stability on the fresh quality of ready-to-use pear cubes. The authors concluded that treatment at 95 8C for 3 min under aseptic conditions is efficient to enhance the stability of these products but with some reduction in the texture. In assessing the effect of treatments on quality attributes, the use of an adequate experimental design is particularly important. To study the interactions of two or more variables, response surface methodology (RSM) is a useful tool (Myers and Montgomery, 1995). RSM is a collection of mathematical and statistical techniques useful for modelling and analysis in complex process optimization obtaining a model topping formula with standard performance, and in addition, a full description of independent variables effective under the optimum conditions (Chow et al., 1988; Guillou and Floros, 1993; Montgomery, 1996). Several classes of treatment structures can be used as RSM experiments (Meilgaard et al., 1991). The most widely used class is very similar to a factorial experiment. This study evaluates the ability of mild heat pretreatments (time/temperature combinations), on
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
preserving cut ‘Rocha’ pear quality attributes, mainly colour and firmness, after treatments and during cold storage.
2. Material and methods 2.1. Plant material ‘Rocha’ pears were picked in August 2001 at an orchard located in the centre west of Portugal. Pears were stored for 3 months at 1 8C and 90% RH before testing. Fruit of uniform size (65 /70 g) and maturity stage (based on external colour and firmness) were selected. 2.2. Mild heat pre-treatments Fruit were subjected to different time/temperature treatments (according to the experimental design) by emerging them in various temperature-controlled water baths. After being treated, fruit were paper-dried, cooled and kept in cold storage (2 8C, 90% RH) for 24 h. Fruit were then peeled, cored and cut into quarters with sharpened knives and placed in perforated open plastic boxes, at 2 8C and 90% RH. 2.3. Experimental design A central composite rotatable treatment structure (CCRD) was used. Central composite experiments consist of three sets of experimental points (Meilgaard et al., 1991). The first set is a traditional factorial design with 2k points (k being the number of xi variables) with coded levels /1 and /1 for each of the k variables. These 2k points are the vertices of a k -dimensional cube centered at the origin of the coded system of reference. The distance of these points from origin is k1/2. The second set accounts for nonlinearity and is a star of 2k points coded as /a and /a on the axis of the system at a distance of 2k /4 from the origin. The third set also accounts for nonlinearity and consists of the central points, which are replicated to provide an estimate of the variance of experimental error. The central point, replicated several
155
times, makes it possible to estimate the lack of fit of the linear statistical model obtained as well as the pure error of the experiments (Montgomery, 1996). In the previous work, Abreu et al. (2001) concluded that mild heat pre-treatments retarded colour and texture degradation of cut ‘Rocha’ pear when compared with a control (non-pre-heated fruit). Combinations of time (3 /240 min) and temperature (40 /60 8C) were then tested. Based on those results, suitable extreme treatments were established for this study (Table 1).
2.4. Quality measurements Samples as identified in the experimental design were analyzed for colour, firmness, pH and % total soluble solids content (SSC). Surface colour was evaluated with a colorimeter (CR 300 Minolta, Osaka, Japan) by measuring L *, a*, b * parameters on the flat sides of each quarter. From these, chroma and hue angle were determined. A white tile (L * /97.10; a* /0.08; b* / 1.80) was used as reference. Ten quarters were analyzed for each condition set by the experimental design. Firmness was evaluated by performing a puncture test on the sides of the cubes prepared from each quarter using a TA-Hdi texture analyser (Stable Micro Systems) with a 50 kg load cell, equipped with a rounded 5 mm diameter flat-head steel probe. Firmness measurements were taken as the first peak force value obtained during the test to penetrate the fruit 7 mm at 1.5 mm/s. Mean values were calculated from results of 20 fruit quarters for each condition. A combined sample of juice extracted from 20 quarters from each treatment was used to evaluate SSC and pH. SSC was assessed with a table digital refractometer (DR-A1, ATAGO Co Ltd., Japan). pH was measured using a pH-meter (CRISON, GLP22). The same quality attributes were evaluated in the initial raw material, day 0, and over a 7-day storage period.
156
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
Table 1 Treatments corresponding to the experimental design showing real and coded values of the independent variables studied Coded independent variables
Decoded independent variables
Treatment temperature (8C)
Treatment time (min)
Treatment temperature (8C)
Treatment time (min)
/1 /1 /1 /1 1 1 1 1 /a /a 0 0 0 0 0 0
/1 /1 1 1 /1 /1 1 1 0 0 /a /a 0 0 0 0
34 34 34 34 46 46 46 46 31 48 40 40 40 40 40 40
52 52 158 158 52 52 158 158 105 105 30 180 105 105 105 105
2.5. Statistics Data were fitted to the second-order polynomials equations (1) to all dependent Y variables through a stepwise multiple regression analysis using ‘‘Statistica v. 5.0’’ software. Y b0 b1 T b11 T 2 b2 tb22 t2 b12 Tt
(1)
where bn are constant regression coefficients and T (temperature) and t (time) are independent variables.
may be due to the lack of fit by model obtained for b*. As L * and a * values are adequate, we used these parameters to assess the colour changes. The estimated partial regression coefficients for the quadratic models and the results of significance tests on the coefficients for L *, a *, firmness and pH are shown in Table 2. Both the studied independent variables seem to have an effect on pear quarter quality attributes, with treatment temperature being the most significant parameter (Table 2). 3.1. Effect of mild heat pre-treatments on texture
3. Results and discussion The average values found for the raw material quality attributes were: firmness, 27.29/5.1 N; L *, 72.729/1.65; a *, /0.389/0.10; b *, 21.409/1.53; pH 4.489/0.06 and SSC (%), 12.39/0.1. Eq. (1) was fitted to the experimental data and tested for adequacy and fitness by analysis of variance. Table 2 summarizes results of the analysis of variance for each of the dependent variables and their corresponding coefficients of multiple determination (r2). The model for SSC and for colour expressed in hue and chroma values showed a significant lack of fit (P /0.05). This
As can be seen in Fig. 1, firmness of the ‘Rocha’ pear quarters was preserved in the range of the mild heat pre-treatments (36 /42 8C). An increase in firmness from an initial value of 27.2 /70 N was observed when the treatment temperature was higher than 45 8C. Treatment time did not have a significant effect (P /0.05) on firmness, treatment temperature being the only independent variable accounting for textural changes. These results may be explained by pectin methylesterase (PE) activation. This enzyme has been demonstrated to have a high optimal activity temperature and may result in the release of
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
157
Table 2 Regression coefficients of the second-order polynomials for the six response variables Regression coefficients
Mean/inter Temperature (1L) Temperature (1Q) Time (2L) Time (2Q) 1L/2L r2 rAdj MS residual
Firmness
L*
a*
pH
244.367 11.633* 0.164* /0.139 0.001 /0.004
/156.106 11.822** /0.152** 0.320* 0.000 /0.009***
83.902 /4.357** 0.057** /0.118*** 0.000 0.003***
7.875 /0.194 0.003*** /0.003*** 0.000 0.000
0.72 0.60 30.15
0.97 0.95 3.46
0.98 0.96 0.33
0.88 0.82 0.10
L, linear; Q, quadratic; model on which T /treatment temperature; t /treatment time of the heat treatment effect is: Y /b0/b1T/ b11T2/b2t/b22t2/b12Tt . * P B/0.05. ** P B/0.001. *** P B/0.01.
also dependent on available Ca2 levels and so dipping in Ca-based solutions may produce better results. Several authors have reported this effect. Kim et al. (1993) studied the response to heat treatment of several apple cultivars and demonstrated a significant effect on firmness depending upon variety and treatment conditions. Of 11 cultivars studied, only ‘Golden Delicious’ and ‘Delicious’ showed a strong tolerance to heat treatment. Luna-Guzma´n et al. (1999) evaluated the effects of heat treatments and calcium chloride (CaCl2) on fresh-cut cantaloupe and concluded that the firming effect was provided by dipping in 2.5% CaCl2 with higher dip temperatures. These authors indicated that increased firmness may be due to temperature-induced diffusion rather than PE activation. Therefore, the mechanism involved needs further investigation. Fig. 1. Firmness of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
3.2. Effect of mild heat pre-treatments on colour
methoxyl groups from galacturonic acid residues of pectic substances. The pectin carboxyl groups formed are then available for complexing free cations, particularly endogenous calcium (Ca2) forming Ca-pectates, and so increasing rigidity of the middle lamella and cell wall. This increase is
The effect of mild heat pre-treatments on the colour of ‘Rocha’ pear quarters is shown in Figs. 2 and 3. The treatment conditions that were effective (P B/0.01) in preserving lightness (L *) were in the range 34 /45 8C regardless of the treatment time (Fig. 2). For these experimental conditions,
158
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
Fig. 2. L * values of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
range 36/45 8C for treatment times longer than 40 min. The observed beneficial results on colour parameters may be related to polyphenol oxidase (PPO) activity, the most important enzyme involved in oxidative browning reactions (Laurila et al., 1998), although the mechanisms that explain the colour preservation are not yet clearly understood. PPO enzyme is stable between pH 4 and 7 (Laurila et al., 1998) and in the range 35 /50 8C, being only completely inactivated at 80 8C (Va´mos-Vigya´zo´, 1981). Nevertheless, the length of the treatment may eventually have led to some change in enzyme activity. Results of this study are also in accordance with those reported by Kim et al. (1993) on minimally processed apple slices. These authors suggested that the kinetic characteristics of the enzyme in whole apples heated at the tested conditions might differ from those in vitro. Reduced browning could result from a change in internal oxygen and carbon dioxide concentrations, possibly induced by the treatment. On the other hand, phenolic contents (PPO substrates) may be related to browning susceptibility (Va´mosVigya´zo´, 1981; Robards et al., 1999). Some of them are heat sensitive and so it may be suggested that heat treatment induced changes in the substrate. The texture improvement observed, leading to a higher cellular integrity, may also result in less contact between substrates (phenolic compounds) and enzyme (PPO). Treatments conducted at temperatures higher than 45 8C enhanced cut surface discoloration. This effect may be due to increased tissue damage associated with cell death. 3.3. Effect of mild heat pre-treatments on pH
Fig. 3. a * values of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
L * values are not significantly different from those of the raw material (L * /72.72). Treatments that induced smaller changes (P B/ 0.01) in a * were those between 36 and 45 8C for treatment times longer than 40 min (Fig. 3). Therefore, to preserve the two significant colour parameters, treatments should be conducted in the
The effect of mild heat treatments on pH is shown in Fig. 4. To preserve the pH values, the best conditions are those corresponding to treatments at temperatures lower than 45 8C for less than 150 min. High treatment intensities may induce an increase in pH values from 4.5 (raw material value) to 5.8. This effect may partially be explained by the leaching of organic acids from the whole fruit during the heating treatment. Thus, the higher the treatment time/temperature combina-
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
159
Fig. 5. Changes in quality attributes during storage of the ‘Rocha’ pear quarters (pre-treated at 40 8C/105 min).
Fig. 4. pH of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
tion, the higher the increase in pH. Water uptake by the fruit from the water bath (pH 7.0 /8.0) probably also occurred and contributed to the pH increase. The effect on pH in response to heat treatments has been reported in apple slices and explained by the high respiration rate induced by the heat treatment, which resulted in consumption of organic acids (Kim et al., 1993). Most probably none of these theories alone can explain the phenomenon and more than one mechanism may be involved. 3.4. Changes in quality parameters during cold storage Quality parameters were evaluated over a 7-day period. Extension of protection seemed to have been effective during the storage period. In fact, the results showed that no significant effect (P / 0.05) was observed in colour parameters, pH and SSC. However, firmness (Max F) showed a decrease of about 30% from the 4th to the 7th day. Fig. 5 illustrates the observed results during the storage period for one of the mild heat pretreatment conditions that best preserved the quality attributes (40 8C/105 min) on the 4th and 7th
days compared with the initial raw material (day 0). Control sample values (without any pre-treatment) are not included in the figure because cut surface discoloration was observed a few minutes after minimal processing.
4. Conclusions Mild heat pre-treatments were effective in reducing or preserving the cut surface browning and preserving or increasing firmness of ‘Rocha’ pear quarters. The most suitable conditions were pretreatments conducted at 35/45 8C for 40/150 min. Cut-fruit quarters subjected to mild heat pretreatments when stored for 7 days at 2 8C did not show changes in cut surface colour, pH and SSC. However, firmness showed some decline. The results obtained in this work in applying a combination of mild heat pre-treatments and cold storage to ‘Rocha’ pear quarters are encouraging. This may be a methodology to maintain cut-fruit quality and to diminish decay. Further studies are needed to better understand the physiological and biochemical mechanisms contributing to colour preservation and the firming effects that were observed.
Acknowledgements Financial support was provided by AGRO 88.
160
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
References Abreu, M., Gonc¸alves, E.M., Vieira, J., Silva, C.L.M., 2001. Efeito dos tratamentos te´rmicos brandos na cor e textura de peˆra ‘Rocha’ minimamente procesada. In: Qualidade, Seguranc¸a, Inovac¸a˜o (Eds.), Proceedings 5 Encontro de Quı´mica de Alimentos, Porto, pp. 599 /602. Aguilar, C.N., Anzaldu´a-Morales, A., Talama´s, R., Gastelu´m, G., 1997. Low-temperature blanch improve textural quality of french-fries. J. Food Sci. 62, 568 /571. ¨ ste, R., 1994. Anderson, A., Gekas, V., Lind, I., Oliveira, F., .O Effect of preheating on potato and texture. Crit. Rev. Food Sci. Nutr. 34, 229 /251. Bourne, M.C., 1987. Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. J. Food Sci. 52, 667 /668, 690. Brecht, J.K., 1995. Physiology of lightly processed fruits and vegetables. HortScience 30, 18 /22. Buta, J.G., Moline, H.E., Spaulding, D.W., Wang, C.Y., 1999. Extending storage life of fresh-cut apples using natural products and their derivatives. J. Agric. Food Chem. 47, 1 / 6. Chow, E.T.S., Wei, L.S., DeVor, R.E., Steinberg, M.P., 1988. Performance of ingredients in a soybean whipped topping. J. Food Sci. 53, 1761 /1765. Gorny, J.R., Kader, A.A., 1996. Fresh-cut fruits products. In: Cantwell, M. (Ed.), Fresh-cut Products: Maintaining Quality and Safety. Postharvest Horticulture Series No. 10. Postharvest Outreach Program, University of California, Davis, CA, pp. 14-1 /14-11. Gorny, J.R., Gil, M.I., Kader, A.A., 1998. Postharvest physiology and quality maintenance of fresh-cut pears. Acta Hort. 464, 231 /236. Greve, L.C., Shackel, K.A., McArdle, R.N., Gohlke, J., Labavitch, J.M., 1994. The impact of heating on carrot firmness: the contribution of cellular turgor. J. Agric. Food Chem. 42, 2896 /2899. Guillou, A.A., Floros, J.D., 1993. Multiresponse optimization minimizes salt in natural cucumber fermentation and storage. J. Food Sci. 58, 1381 /1389.
Kim, D.M., Smith, N.L., Lee, C.Y., 1993. Apple cultivar variations in response to heat treatment and minimal processing. J. Food Sci. 58, 1111 /1124. King, A.D., Bolin, H.R., 1989. Physiological and microbiological storage stability of minimally processed fruits and vegetables. Food Technol. 45, 132 /135, 139. Laurila, E., Kervinen, R., Ahvenainen, R., 1998. The inhibition of enzymatic browning in minimally processed vegetables and fruits. Postharvest News Inf. 9, 53N /66N. Luna-Guzma´n, I., Cantwell, M., Barret, D.M., 1999. Fresh-cut cantaloupe: effects of CaCl2 dips and heat treatments on firmness and metabolic activity. Postharvest Biol. Technol. 17, 201 /213. Mastrocola, D., Pittia, P., Lerici, C.R., 1995. Quality of apple slices processed by combined techniques. J. Food Qual. 18, 133 /146. Meilgaard, M., Civille, G.V., Carr, B.T., 1991. Advanced statistical methods (Cap12). In: Sensory Evaluation Techniques, 2nd ed. CRC Press, Boca Raton, FL, pp. 275 /304. Montgomery, D.C., 1996. Introduction to Statistical Quality Control. Wiley, New York. Myers, R.H., Montgomery, D.C., 1995. Response Surface Methodology: Process and Product Optimization Using Designed Experiments. Wiley, New York. Pittia, P., Nicoli, M.C., Comi, G., Massini, R., 1999. Shelf-life extension of fresh-like ready-to-use pear cubes. J. Sci. Food Agric. 79, 955 /960. Pretel, M.T., Ferna´ndez, P.S., Romojaro, F., Martı´nez, A., 1998. The effect of modified atmosphere packaging on ready-to-eat oranges. Lebensm-Wiss u Technol. 31, 322 / 328. Robards, K., Prenzler, P.D., Tucker, G., Swatsitang, P., Glover, W., 1999. Phenolic compounds and their role in oxidative processes in fruits. Food Chem. 66, 401 /436. Rosen, J.C., Kader, A.A., 1989. Postharvest physiology and quality maintenance of sliced pear and strawberry fruits. J. Food Sci. 54, 656 /659. Sapers, G.M., Miller, R.L., 1998. Browning inhibition in freshcut pears. J. Food Sci. 63, 342 /346. Va´mos-Vigya´zo´, L., 1981. Polyphenol oxidase and peroxidase in fruits and vegetables. Crit. Rev. Food Sci. Nutr. 15, 49 / 127.
Use of mild heat pre-treatments for quality retention of freshcut ‘Rocha’ pear Marta Abreu a, Sara Beira˜o-da-Costa a, Elsa M. Gonc¸alves a, Maria Luisa Beira˜o-da-Costa b, Margarida Molda˜o-Martins b,* a
UTPV/DTIA/Instituto Nacional de Engenharia e Tecnologia Industrial, Azinhaga dos Lameiros, Estrada do Pac¸o do Lumiar, 22, 1649-038 Lisbon, Portugal b CMIA/SCTA/DAIAT Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisbon, Portugal Received 16 December 2002; accepted 12 May 2003
Abstract The effect of mild heat pre-treatments on quality of fresh-cut ‘Rocha’ pear (Pyrus communis L.) was evaluated by response surface methodology (RSM) in different time/temperature combinations. Susceptibility to cut surface discoloration and changes in firmness, total soluble solids content (SSC) and pH of cut pears (quarters) were the measured dependent variables. The models developed for L *, a *, firmness and pH appeared adequate (R2 /0.72). RSM results also showed a significant lack of fit (P /0.05), for b * and SSC models. Firmness of the ‘Rocha’ pear quarters was preserved in the range of the mild heat pre-treatments used (36 /42 8C). An increase in firmness from an initial value of 27.2 to 70 N was observed whenever treatment temperature was greater than 45 8C. Treatment time did not have a significant (P /0.05) effect on firmness. To preserve the two significant colour parameters (L * and a *), treatments should be conducted in the range 36 /45 8C with treatment times greater than 40 min. Mild heat pretreatments produced an increase in pH values from 4.5 in raw material to 5.8 for samples exposed to high treatment intensities. In order to preserve the pH values, the best-suited conditions are those corresponding to treatments at temperatures lower than 45 8C for less than 150 min. Mild heat pre-treatments (35 /45 8C for 40 /150 min) were effective in avoiding the cut surface browning and preserving or increasing firmness of ‘Rocha’ pear quarters. When pre-treated fruit were stored for 7 days at 2 8C, no significant changes were detected in colour parameters, pH and SSC. However, firmness decreased approximately 30% from the 4th to the 7th day. # 2003 Elsevier B.V. All rights reserved. Keywords: ‘Rocha’ pear; Mild heat pre-treatments; Fresh-cut fruit; RSM; Quality attributes
1. Introduction * Corresponding author. Tel.: /351-21-3653547; fax: /35121-3653200. E-mail addresses: [email protected] (M. Abreu), [email protected] (M. Molda˜o-Martins).
‘Rocha’ pear (Pyrus communis L.) is the main pear cultivar produced in Portugal, and is important in Canada, Brazil, UK and France. ‘Rocha’ pear is classified as PDO (protected designation
0925-5214/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-5214(03)00105-4
154
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
and origin), which means that it corresponds to a traditional product produced under strict conditions and by law may be labelled as such. This cultivar has high storage potential, as whole fruit, supplying the market almost all year round, and is thus competitive with other cultivars. Fresh-cut or minimally processed fruit produce (peeled, cored and sliced) is an important developing class of food product because of attributes such as convenience and fresh-like quality (Gorny et al., 1998; Pretel et al., 1998; Pittia et al., 1999). In addition, this processing methodology may allow the utilization of smaller fruit, which are unsuitable for the fresh market. It is difficult to consistently ensure reasonable shelf-life in fresh-cut fruit, as many fruit must be processed at their optimal ripening quality and therefore are at the edge of senescence (Gorny and Kader, 1996). Another problem is the limited shelf-life, due to excessive tissue softening and surface browning after minimal processing (King and Bolin, 1989; Brecht, 1995; Gorny et al., 1998). Much research has focused on the use of postharvest chemical treatments such as the addition of antioxidants (ascorbic acid, erythrobate, sodium and potassium chloride), firming agents (calcium derivatives) and modified atmosphere packaging at reduced oxygen levels (Rosen and Kader, 1989; Sapers and Miller, 1998; Buta et al., 1999; Gorny and Kader, 1996; Pittia et al., 1999). Although some of these treatments were found to be effective, consumers are demanding a reduction in the overall use of chemicals on fresh products and alternative methodologies should be investigated to extend shelf-life of fresh-cut fruit. Heat treatments are generally effective in inhibiting enzymatic reactions and in reducing microbial levels, but are little used for stabilizing minimally processed fruit due to their negative effects on flavour, texture and fresh quality. On the other hand, the development of suitable mild heat pretreatments, which avoid the negative impacts, may be of great interest in producing minimally processed fruit with an increased shelf-life. This methodology has already been used to control postharvest decay and to improve storage uality in intact fruit by modification of
physiological and physicochemical characteristics and post-processing quality. Mild heat pretreatments have been shown to induce a firming effect on several processed vegetables such as potatoes, carrots and green beans (Bourne, 1987; Anderson et al., 1994; Greve et al., 1994; Aguilar et al., 1997). The influence of heat and mild heat pre-treatments on colour and texture of fruit pieces has been investigated (Mastrocola et al., 1995; Pittia et al., 1999). The results indicated that these treatments increased fresh-cut fruit quality by delaying the deterioration of physical, chemical and sensory properties. Apples heat-treated at 45 8C produced slices with less browning and firmer texture compared with non-treated apples (Kim et al., 1993). Nevertheless, the evaluation of these treatments with regard to other quality attributes such as microbiological stability, nutritional value and sensory characteristics is needed. Limited information is available on heat treatments applied to whole and fresh-cut pears. Pittia et al. (1999) studied the effect of low-intensity blanching treatments on enzymatic and microbiological stability on the fresh quality of ready-to-use pear cubes. The authors concluded that treatment at 95 8C for 3 min under aseptic conditions is efficient to enhance the stability of these products but with some reduction in the texture. In assessing the effect of treatments on quality attributes, the use of an adequate experimental design is particularly important. To study the interactions of two or more variables, response surface methodology (RSM) is a useful tool (Myers and Montgomery, 1995). RSM is a collection of mathematical and statistical techniques useful for modelling and analysis in complex process optimization obtaining a model topping formula with standard performance, and in addition, a full description of independent variables effective under the optimum conditions (Chow et al., 1988; Guillou and Floros, 1993; Montgomery, 1996). Several classes of treatment structures can be used as RSM experiments (Meilgaard et al., 1991). The most widely used class is very similar to a factorial experiment. This study evaluates the ability of mild heat pretreatments (time/temperature combinations), on
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
preserving cut ‘Rocha’ pear quality attributes, mainly colour and firmness, after treatments and during cold storage.
2. Material and methods 2.1. Plant material ‘Rocha’ pears were picked in August 2001 at an orchard located in the centre west of Portugal. Pears were stored for 3 months at 1 8C and 90% RH before testing. Fruit of uniform size (65 /70 g) and maturity stage (based on external colour and firmness) were selected. 2.2. Mild heat pre-treatments Fruit were subjected to different time/temperature treatments (according to the experimental design) by emerging them in various temperature-controlled water baths. After being treated, fruit were paper-dried, cooled and kept in cold storage (2 8C, 90% RH) for 24 h. Fruit were then peeled, cored and cut into quarters with sharpened knives and placed in perforated open plastic boxes, at 2 8C and 90% RH. 2.3. Experimental design A central composite rotatable treatment structure (CCRD) was used. Central composite experiments consist of three sets of experimental points (Meilgaard et al., 1991). The first set is a traditional factorial design with 2k points (k being the number of xi variables) with coded levels /1 and /1 for each of the k variables. These 2k points are the vertices of a k -dimensional cube centered at the origin of the coded system of reference. The distance of these points from origin is k1/2. The second set accounts for nonlinearity and is a star of 2k points coded as /a and /a on the axis of the system at a distance of 2k /4 from the origin. The third set also accounts for nonlinearity and consists of the central points, which are replicated to provide an estimate of the variance of experimental error. The central point, replicated several
155
times, makes it possible to estimate the lack of fit of the linear statistical model obtained as well as the pure error of the experiments (Montgomery, 1996). In the previous work, Abreu et al. (2001) concluded that mild heat pre-treatments retarded colour and texture degradation of cut ‘Rocha’ pear when compared with a control (non-pre-heated fruit). Combinations of time (3 /240 min) and temperature (40 /60 8C) were then tested. Based on those results, suitable extreme treatments were established for this study (Table 1).
2.4. Quality measurements Samples as identified in the experimental design were analyzed for colour, firmness, pH and % total soluble solids content (SSC). Surface colour was evaluated with a colorimeter (CR 300 Minolta, Osaka, Japan) by measuring L *, a*, b * parameters on the flat sides of each quarter. From these, chroma and hue angle were determined. A white tile (L * /97.10; a* /0.08; b* / 1.80) was used as reference. Ten quarters were analyzed for each condition set by the experimental design. Firmness was evaluated by performing a puncture test on the sides of the cubes prepared from each quarter using a TA-Hdi texture analyser (Stable Micro Systems) with a 50 kg load cell, equipped with a rounded 5 mm diameter flat-head steel probe. Firmness measurements were taken as the first peak force value obtained during the test to penetrate the fruit 7 mm at 1.5 mm/s. Mean values were calculated from results of 20 fruit quarters for each condition. A combined sample of juice extracted from 20 quarters from each treatment was used to evaluate SSC and pH. SSC was assessed with a table digital refractometer (DR-A1, ATAGO Co Ltd., Japan). pH was measured using a pH-meter (CRISON, GLP22). The same quality attributes were evaluated in the initial raw material, day 0, and over a 7-day storage period.
156
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
Table 1 Treatments corresponding to the experimental design showing real and coded values of the independent variables studied Coded independent variables
Decoded independent variables
Treatment temperature (8C)
Treatment time (min)
Treatment temperature (8C)
Treatment time (min)
/1 /1 /1 /1 1 1 1 1 /a /a 0 0 0 0 0 0
/1 /1 1 1 /1 /1 1 1 0 0 /a /a 0 0 0 0
34 34 34 34 46 46 46 46 31 48 40 40 40 40 40 40
52 52 158 158 52 52 158 158 105 105 30 180 105 105 105 105
2.5. Statistics Data were fitted to the second-order polynomials equations (1) to all dependent Y variables through a stepwise multiple regression analysis using ‘‘Statistica v. 5.0’’ software. Y b0 b1 T b11 T 2 b2 tb22 t2 b12 Tt
(1)
where bn are constant regression coefficients and T (temperature) and t (time) are independent variables.
may be due to the lack of fit by model obtained for b*. As L * and a * values are adequate, we used these parameters to assess the colour changes. The estimated partial regression coefficients for the quadratic models and the results of significance tests on the coefficients for L *, a *, firmness and pH are shown in Table 2. Both the studied independent variables seem to have an effect on pear quarter quality attributes, with treatment temperature being the most significant parameter (Table 2). 3.1. Effect of mild heat pre-treatments on texture
3. Results and discussion The average values found for the raw material quality attributes were: firmness, 27.29/5.1 N; L *, 72.729/1.65; a *, /0.389/0.10; b *, 21.409/1.53; pH 4.489/0.06 and SSC (%), 12.39/0.1. Eq. (1) was fitted to the experimental data and tested for adequacy and fitness by analysis of variance. Table 2 summarizes results of the analysis of variance for each of the dependent variables and their corresponding coefficients of multiple determination (r2). The model for SSC and for colour expressed in hue and chroma values showed a significant lack of fit (P /0.05). This
As can be seen in Fig. 1, firmness of the ‘Rocha’ pear quarters was preserved in the range of the mild heat pre-treatments (36 /42 8C). An increase in firmness from an initial value of 27.2 /70 N was observed when the treatment temperature was higher than 45 8C. Treatment time did not have a significant effect (P /0.05) on firmness, treatment temperature being the only independent variable accounting for textural changes. These results may be explained by pectin methylesterase (PE) activation. This enzyme has been demonstrated to have a high optimal activity temperature and may result in the release of
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
157
Table 2 Regression coefficients of the second-order polynomials for the six response variables Regression coefficients
Mean/inter Temperature (1L) Temperature (1Q) Time (2L) Time (2Q) 1L/2L r2 rAdj MS residual
Firmness
L*
a*
pH
244.367 11.633* 0.164* /0.139 0.001 /0.004
/156.106 11.822** /0.152** 0.320* 0.000 /0.009***
83.902 /4.357** 0.057** /0.118*** 0.000 0.003***
7.875 /0.194 0.003*** /0.003*** 0.000 0.000
0.72 0.60 30.15
0.97 0.95 3.46
0.98 0.96 0.33
0.88 0.82 0.10
L, linear; Q, quadratic; model on which T /treatment temperature; t /treatment time of the heat treatment effect is: Y /b0/b1T/ b11T2/b2t/b22t2/b12Tt . * P B/0.05. ** P B/0.001. *** P B/0.01.
also dependent on available Ca2 levels and so dipping in Ca-based solutions may produce better results. Several authors have reported this effect. Kim et al. (1993) studied the response to heat treatment of several apple cultivars and demonstrated a significant effect on firmness depending upon variety and treatment conditions. Of 11 cultivars studied, only ‘Golden Delicious’ and ‘Delicious’ showed a strong tolerance to heat treatment. Luna-Guzma´n et al. (1999) evaluated the effects of heat treatments and calcium chloride (CaCl2) on fresh-cut cantaloupe and concluded that the firming effect was provided by dipping in 2.5% CaCl2 with higher dip temperatures. These authors indicated that increased firmness may be due to temperature-induced diffusion rather than PE activation. Therefore, the mechanism involved needs further investigation. Fig. 1. Firmness of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
3.2. Effect of mild heat pre-treatments on colour
methoxyl groups from galacturonic acid residues of pectic substances. The pectin carboxyl groups formed are then available for complexing free cations, particularly endogenous calcium (Ca2) forming Ca-pectates, and so increasing rigidity of the middle lamella and cell wall. This increase is
The effect of mild heat pre-treatments on the colour of ‘Rocha’ pear quarters is shown in Figs. 2 and 3. The treatment conditions that were effective (P B/0.01) in preserving lightness (L *) were in the range 34 /45 8C regardless of the treatment time (Fig. 2). For these experimental conditions,
158
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
Fig. 2. L * values of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
range 36/45 8C for treatment times longer than 40 min. The observed beneficial results on colour parameters may be related to polyphenol oxidase (PPO) activity, the most important enzyme involved in oxidative browning reactions (Laurila et al., 1998), although the mechanisms that explain the colour preservation are not yet clearly understood. PPO enzyme is stable between pH 4 and 7 (Laurila et al., 1998) and in the range 35 /50 8C, being only completely inactivated at 80 8C (Va´mos-Vigya´zo´, 1981). Nevertheless, the length of the treatment may eventually have led to some change in enzyme activity. Results of this study are also in accordance with those reported by Kim et al. (1993) on minimally processed apple slices. These authors suggested that the kinetic characteristics of the enzyme in whole apples heated at the tested conditions might differ from those in vitro. Reduced browning could result from a change in internal oxygen and carbon dioxide concentrations, possibly induced by the treatment. On the other hand, phenolic contents (PPO substrates) may be related to browning susceptibility (Va´mosVigya´zo´, 1981; Robards et al., 1999). Some of them are heat sensitive and so it may be suggested that heat treatment induced changes in the substrate. The texture improvement observed, leading to a higher cellular integrity, may also result in less contact between substrates (phenolic compounds) and enzyme (PPO). Treatments conducted at temperatures higher than 45 8C enhanced cut surface discoloration. This effect may be due to increased tissue damage associated with cell death. 3.3. Effect of mild heat pre-treatments on pH
Fig. 3. a * values of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
L * values are not significantly different from those of the raw material (L * /72.72). Treatments that induced smaller changes (P B/ 0.01) in a * were those between 36 and 45 8C for treatment times longer than 40 min (Fig. 3). Therefore, to preserve the two significant colour parameters, treatments should be conducted in the
The effect of mild heat treatments on pH is shown in Fig. 4. To preserve the pH values, the best conditions are those corresponding to treatments at temperatures lower than 45 8C for less than 150 min. High treatment intensities may induce an increase in pH values from 4.5 (raw material value) to 5.8. This effect may partially be explained by the leaching of organic acids from the whole fruit during the heating treatment. Thus, the higher the treatment time/temperature combina-
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
159
Fig. 5. Changes in quality attributes during storage of the ‘Rocha’ pear quarters (pre-treated at 40 8C/105 min).
Fig. 4. pH of the ‘Rocha’ pear quarters as a function of the temperature and treatment time.
tion, the higher the increase in pH. Water uptake by the fruit from the water bath (pH 7.0 /8.0) probably also occurred and contributed to the pH increase. The effect on pH in response to heat treatments has been reported in apple slices and explained by the high respiration rate induced by the heat treatment, which resulted in consumption of organic acids (Kim et al., 1993). Most probably none of these theories alone can explain the phenomenon and more than one mechanism may be involved. 3.4. Changes in quality parameters during cold storage Quality parameters were evaluated over a 7-day period. Extension of protection seemed to have been effective during the storage period. In fact, the results showed that no significant effect (P / 0.05) was observed in colour parameters, pH and SSC. However, firmness (Max F) showed a decrease of about 30% from the 4th to the 7th day. Fig. 5 illustrates the observed results during the storage period for one of the mild heat pretreatment conditions that best preserved the quality attributes (40 8C/105 min) on the 4th and 7th
days compared with the initial raw material (day 0). Control sample values (without any pre-treatment) are not included in the figure because cut surface discoloration was observed a few minutes after minimal processing.
4. Conclusions Mild heat pre-treatments were effective in reducing or preserving the cut surface browning and preserving or increasing firmness of ‘Rocha’ pear quarters. The most suitable conditions were pretreatments conducted at 35/45 8C for 40/150 min. Cut-fruit quarters subjected to mild heat pretreatments when stored for 7 days at 2 8C did not show changes in cut surface colour, pH and SSC. However, firmness showed some decline. The results obtained in this work in applying a combination of mild heat pre-treatments and cold storage to ‘Rocha’ pear quarters are encouraging. This may be a methodology to maintain cut-fruit quality and to diminish decay. Further studies are needed to better understand the physiological and biochemical mechanisms contributing to colour preservation and the firming effects that were observed.
Acknowledgements Financial support was provided by AGRO 88.
160
M. Abreu et al. / Postharvest Biology and Technology 30 (2003) 153 /160
References Abreu, M., Gonc¸alves, E.M., Vieira, J., Silva, C.L.M., 2001. Efeito dos tratamentos te´rmicos brandos na cor e textura de peˆra ‘Rocha’ minimamente procesada. In: Qualidade, Seguranc¸a, Inovac¸a˜o (Eds.), Proceedings 5 Encontro de Quı´mica de Alimentos, Porto, pp. 599 /602. Aguilar, C.N., Anzaldu´a-Morales, A., Talama´s, R., Gastelu´m, G., 1997. Low-temperature blanch improve textural quality of french-fries. J. Food Sci. 62, 568 /571. ¨ ste, R., 1994. Anderson, A., Gekas, V., Lind, I., Oliveira, F., .O Effect of preheating on potato and texture. Crit. Rev. Food Sci. Nutr. 34, 229 /251. Bourne, M.C., 1987. Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. J. Food Sci. 52, 667 /668, 690. Brecht, J.K., 1995. Physiology of lightly processed fruits and vegetables. HortScience 30, 18 /22. Buta, J.G., Moline, H.E., Spaulding, D.W., Wang, C.Y., 1999. Extending storage life of fresh-cut apples using natural products and their derivatives. J. Agric. Food Chem. 47, 1 / 6. Chow, E.T.S., Wei, L.S., DeVor, R.E., Steinberg, M.P., 1988. Performance of ingredients in a soybean whipped topping. J. Food Sci. 53, 1761 /1765. Gorny, J.R., Kader, A.A., 1996. Fresh-cut fruits products. In: Cantwell, M. (Ed.), Fresh-cut Products: Maintaining Quality and Safety. Postharvest Horticulture Series No. 10. Postharvest Outreach Program, University of California, Davis, CA, pp. 14-1 /14-11. Gorny, J.R., Gil, M.I., Kader, A.A., 1998. Postharvest physiology and quality maintenance of fresh-cut pears. Acta Hort. 464, 231 /236. Greve, L.C., Shackel, K.A., McArdle, R.N., Gohlke, J., Labavitch, J.M., 1994. The impact of heating on carrot firmness: the contribution of cellular turgor. J. Agric. Food Chem. 42, 2896 /2899. Guillou, A.A., Floros, J.D., 1993. Multiresponse optimization minimizes salt in natural cucumber fermentation and storage. J. Food Sci. 58, 1381 /1389.
Kim, D.M., Smith, N.L., Lee, C.Y., 1993. Apple cultivar variations in response to heat treatment and minimal processing. J. Food Sci. 58, 1111 /1124. King, A.D., Bolin, H.R., 1989. Physiological and microbiological storage stability of minimally processed fruits and vegetables. Food Technol. 45, 132 /135, 139. Laurila, E., Kervinen, R., Ahvenainen, R., 1998. The inhibition of enzymatic browning in minimally processed vegetables and fruits. Postharvest News Inf. 9, 53N /66N. Luna-Guzma´n, I., Cantwell, M., Barret, D.M., 1999. Fresh-cut cantaloupe: effects of CaCl2 dips and heat treatments on firmness and metabolic activity. Postharvest Biol. Technol. 17, 201 /213. Mastrocola, D., Pittia, P., Lerici, C.R., 1995. Quality of apple slices processed by combined techniques. J. Food Qual. 18, 133 /146. Meilgaard, M., Civille, G.V., Carr, B.T., 1991. Advanced statistical methods (Cap12). In: Sensory Evaluation Techniques, 2nd ed. CRC Press, Boca Raton, FL, pp. 275 /304. Montgomery, D.C., 1996. Introduction to Statistical Quality Control. Wiley, New York. Myers, R.H., Montgomery, D.C., 1995. Response Surface Methodology: Process and Product Optimization Using Designed Experiments. Wiley, New York. Pittia, P., Nicoli, M.C., Comi, G., Massini, R., 1999. Shelf-life extension of fresh-like ready-to-use pear cubes. J. Sci. Food Agric. 79, 955 /960. Pretel, M.T., Ferna´ndez, P.S., Romojaro, F., Martı´nez, A., 1998. The effect of modified atmosphere packaging on ready-to-eat oranges. Lebensm-Wiss u Technol. 31, 322 / 328. Robards, K., Prenzler, P.D., Tucker, G., Swatsitang, P., Glover, W., 1999. Phenolic compounds and their role in oxidative processes in fruits. Food Chem. 66, 401 /436. Rosen, J.C., Kader, A.A., 1989. Postharvest physiology and quality maintenance of sliced pear and strawberry fruits. J. Food Sci. 54, 656 /659. Sapers, G.M., Miller, R.L., 1998. Browning inhibition in freshcut pears. J. Food Sci. 63, 342 /346. Va´mos-Vigya´zo´, L., 1981. Polyphenol oxidase and peroxidase in fruits and vegetables. Crit. Rev. Food Sci. Nutr. 15, 49 / 127.