Effect of processing on the texture and sensory attributes of raspberry (cv. Heritage) and blackberry (cv. Thornfree)

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Abstract

The effect of pre-treatments (CaCl2, low methoxyl pectin (LMP), and combined solutions) on objective texture parameters of fresh raspberries and blackberries, as well as the effect of freezing method and thawing mode on the same objective texture parameters and sensory attributes of both berries was studied. Following, the effect of complete freezing process combining the best pre-treatments with the best freezing/thawing conditions found for each fruit was investigated. Four instrumental tests, Kramer Shear Cell (KSC), back extrusion, compression and multiple penetration were used to measure fruit texture objectively, whereas a descriptive quantitative method was used to carry out fruit sensory analysis. For calcium and LMP pre-treatments applied separately, texture parameters were significantly higher in samples treated at the highest concentrations (100 mM of CaCl2 for both fruits and 0.3% and 3% of LMP for raspberry and blackberry, respectively), while for the combined pre-treatments, the highest texture parameters were found in the samples treated with CaCl2 (100 mM) and LMP (0.1%) in the case of raspberries and CaCl2 (100 mM) and LMP (3%) in the case of blackberries, compared to fresh controls. Objective texture parameters evidenced as blackberry fruit was more susceptible than raspberry fruit to the effect of pre-treatments. The maximum force of the multiple penetration tests was the parameter that best reflects the changes occurring in the turgidity and firmness of the fruits as a result of the pre-treatments. Raspberries frozen at 3.3 °C min−1 and thawed at +5 °C and blackberries frozen at 2.2 °C min−1 and thawed at room temperature had higher texture parameters. For the raspberry, panellists hardly distinguished between the sensory firmness of fresh control and those of raspberries frozen and thawed under the above cited conditions, corroborating the effect positive of freezing at 3.3 °C min−1 in the fruit texture. For the blackberry, panellists did not detect a significant difference between the sensory firmness of frozen/thawed fruit and those of fresh control, thus confirming that the blackberry structure is more resistant to freezing process. In both fruits, over the complete process objective texture parameters were again highest in the samples treated with 100 mM CaCl2 (applied separately or combined with LMP), suggesting that the pre-treatment with CaCl2 (100 mM) can be sufficient to prevent loss of firmness in frozen raspberries and blackberries. Panellists did not detect the effect of firmness caused by this pre-treatment in the frozen raspberry, but panellists scored the firmness of the samples pre-treated with CaCl2 (100 mM) higher than those of the untreated control in the frozen blackberry. For the raspberry, none of the correlations between objective and sensory attributes of texture was significant, whereas for the blackberry firmness presented a higher and significant correlation with compression maximum force.

Introduction

Texture is one of the organoleptic quality attributes appreciated by the consumer of fruits, and the major contributor to the texture of fresh fruits is cell turgor pressure. Many studies have focused on turgor pressure (Alvarez et al., 2000, Brusewitz et al., 1989, Lin and Pitt, 1986, Ramana and Taylor, 1994) as the factor that preponderantly controls the stiffness of liquid-filled cells. Turgidity is in fact the characteristic largely responsible for the final degree of firmness in cherries (Alonso, Canet, & Rodríguez, 1994). In delicately textured fruits, freezing causes a loss of turgidity and firmness (Alonso, Rodríguez, & Canet, 1995). During freezing, ice begins to form in the extra-cellular medium and progresses via the cytoplasm when the cell membrane loses permeability (Alonso, Canet, & Rodríguez, 1997). Decompartmentalization caused by ice crystals prevents the return of water to the intracellular medium during thawing, causing loss of turgidity and hence of fruit texture (Grout et al., 1989, Reid, 1993). The freezing rate determines the size of ice crystals that form, and this in turn determines the degree of structural damage, which produces a change in the rheological parameters of the tissue (Alvarez & Canet, 1997). A number of authors have noted that fast freezing rates positively affect the texture of fruits, including cherries (Alonso et al., 1994), blueberries and blackberries (Martí & Aguilera, 1991), raspberries (Crivelli and Senesi, 1990, Reynoso and Michelis, 1994) and strawberries (Delgado & Rubiolo, 2005).

Also, in order to enhance fruit and vegetable firmness and attenuate any freezing-induced loss of texture, thermal or calcium pre-treatments (Alonso et al., 1995, Canet and Hill, 1987, Fuchigami et al., 1995) have been proposed that preserve firmness. The effect of calcium on vegetable texture has been attributed to the formation of calcium bridges between galacturonic acid residues. McFeeters and Armstrong (1984) suggested that the textural effects of calcium were a result of calcium binding at sites other than pectic carboxyl groups. Application of calcium treatments increased firmness and pectinesterase activity in frozen sweet cherries (Alonso, Canet, & Rodríguez, 1993). In calcium-treated cherries, a decrease in the degree of pectin esterification has been reported, and an increase in both the EDTA-soluble pectin fraction and the Ca2+ cation content in the cell wall (Alonso et al., 1995). The calcium–pectin complex formed acts as an intracellular cement to give firmness to vegetable tissues. Independent of the formation of Ca-pectates, the degree of esterification has been shown to play an important role in the rigidity and configuration of pectic substances whereby the stiffness of the pectic macromolecule increases with increasing methoxyl content (Hudson & Buescher, 1986). Thus, firm tissue seems to be associated with the formation of insoluble and methylated pectic substances. Addition of low methoxyl pectin was undertaken to test the hypothesis that increasing the degree of esterification of pectic substances may facilitate formation of divalent bridges between residues of galacturonic acid attached to adjacent pectic chains in presence of Ca2+ cations.

The term “firmness of fruit” is commonly used to describe a parameter assessed by means of empirical mechanical tests and understood as an attribute that ought to be maintained during storage and processing (Alvarez, Canet, & Tortosa, 1997). Firmness, interpreted as a mechanical response intrinsic to the fruit structure, is influenced by the stage of physiological development, degree of ripeness, damage and identification, fibrousness and turgidity (Alonso et al., 1994). A variety of methods have been used to evaluate these characteristics, and various interpretations have been made of mechanical parameters. Santerre, Cash, and Iezzoni (1991) designed a multiple stoning cell (five units) for use on texturometers. Comparing these results with the results from a test using the Kramer Shear Cell, they concluded that a multiple test was required to assess the texture of cherries.

The objectives of the current study were: (1) to determine the effect of the pre-treatment with calcium, low methyl pectin and combined solutions on the texture of fresh raspberries and blackberries; (2) to investigate the effect of freezing and thawing rates on the texture and sensory attributes of both fruits and to find the best conditions for each one; (3) to determine the effect of the pre-treatment with calcium, low methyl pectin and combined solutions on the texture and sensory attributes of frozen/thawed raspberries and blackberries; and (4) to find the mechanical parameters that best express the firmness of both fresh and frozen fruits.

Section snippets

Plant material

Raspberry fruits (Rubus idaeus L. cv. Heritage) and blackberry fruits (Rubus fruticosus L. cv. Thornfree) were obtained from commercial orchards in the region of Cova da Beira (Covilhã, Portugal), and harvested at the commercial maturity stage. Both fruits were collected manually and brought to the Estação Agronómica Nacional (Oeiras) within 12 h after harvest via refrigerated transport. On arrival, fruits were stored in a chamber at 2 °C and 90% relative humidity and were kept there for a

Effect of protective pre-treatments

Table 2 shows average values of the different texture parameters in the calcium pre-treatment at the concentrations chosen for each fruit. In the case of raspberry, the pre-treatment significantly affected the values of KSC, back extrusion and multiple penetration maximum forces (FKSC, FE, FMP), and of compression and multiple penetration slopes (SC, SMP) (P  0.01). The texture parameter values were highest in the samples treated with 100 mM of CaCl2, which with the exception of the back

Conclusions

A pre-treatment with CaCl2 (100 mM), applied independently or combined with LMP, should be applied in raspberry and blackberry fruits in order to prevent loss of firmness induced by freezing and thawing processes. Objective texture parameters evidenced as blackberry fruit was more susceptible than raspberry fruit to the effect of pre-treatments. Multiple penetration maximum force (FMP) may be the most suitable parameter for assessing the firmness of both fresh and pre-treated fruits without

Acknowledgements

The authors wish to thank the Instituto Nacional de Investigação Agrária (INIA Proj Piddac 117/97), the Consejo Superior Investigaciones Científicas (CICyT, Project ALI98-1055) (Spain) and Junta Nacional de Investigacão Científica y Tecnológica de Portugal (Proc. no. 423/CSIC), which financially supported this cooperative research.

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