Effect of thermal processing on the texture of canned apricots

Abstract

The effect of thermal processing on the texture of canned apricots was studied by using the main cultivar canned in Greece, Bebecou. The test temperature ranged from 82 to 95 °C. The loss of hardness was tested immediately after processing and after 3 months storage. The z-value was 16.7 °C and the energy of activation 116.5 kJ mol⁻¹. Some restoration in the hardness was found after 3 months storage, which might be attributable to the absorption of sucrose by the fruit.

Introduction

Texture is considered as one of the most important quality attributes of fresh or processed fruit and vegetables. Thermal processing of these products to the level necessary to achieve commercial sterility causes considerable softening of the tissue. It is reported that they retain only 3–16% of the fresh material firmness (Huang & Bourne., 1983).

Huang & Bourne (1983) used two apparent first-order kinetic mechanisms to describe thermal softening of vegetables. One contributing to the initial high rate of softening (mechanism I) and the other responsible for softening during prolonged heating times (mechanism II). Leonard et al. (1986) suggested that the z-value is about 33 °C for most reactions related to quality characteristics. Z and E (activation energy) values for the loss of texture because of heat processing have been reported for different products: for artichoke hearts, z=27 °C (Rodrigo et al., 1992); for apple, Anantheswaran et al. (1985) reported z-values of 24.8 and 42.28 °C and E-values of 110.9 and 65.3 kJ mol⁻¹, respectively, for the two varieties tested, and Lazos et al. (1996) reported values of z=25–99 °C and E=27.6–92.6 kJ mol⁻¹; for peach, Billis (1988) reported two z-values, z₁=24 °C for the initial heating and z₂=74 °C for prolonged heating.

Varoquaux et al. (1986) studied the effect of blanching on the texture of apricot and reported a z-value of 24.9 °C and an activation energy of 96.6 kJ mol⁻¹. Apricot appears to be particularly susceptible to softening problems on canning. There are two theories on the mechanism of the extensive softening of apricot on canning: (i) a chelation hypothesis (French et al., 1989) and (ii) an acid hydrolysis hypothesis (Claypool, 1974). No work has been conducted on the kinetics of the texture loss in apricots because of heat processing.

In the present work, the kinetics of apricot texture loss during heating was studied. The data obtained from this work will be useful to canners in order to optimize the thermal process, resulting in a better texture of the final product.

Materials and methods

Raw material

Fruits of the `Bebeco' cultivar were obtained at the right stage of maturity from a commercial orchard in the area of Korinth, and stored at 0–2 °C until processed.

Thermal treatment

The fruit was peeled by immersion in a boiling solution of 4% NaOH for 40 s and washed thoroughly under running water. A sample was kept for the measurement of the texture of the peeled product. About 200 g were transferred to each half kilo (300 × 400) can.

The cans were filled with hot 27 Brix syrup and placed in a water bath exhauster (95 °C) until the syrup temperature reached 75 °C, and then the cans were closed. The heat processing was done in a water bath pasteurizer whose temperature was controlled by a thermostat to ±0.5 °C. The temperature of the syrup in the can was monitored by placing thermocouples in two of the test cans, using thermocouples and fittings from Ecklund Ltd (Florida, USA). At predetermined time intervals, four cans were removed and cooled immediately in a cold water bath. The contents of two of the cans were used for measuring the texture immediately after processing and the other two were stored at room temperature in order to measure texture after 3 months. Experiments were done at 82, 84, 85, 88, 89, 90, 91, 93 and 95 °C.

Texture measurements

From every apricot piece in the can, one or two pieces of 1.5 × 1.5 cm were cut. Sixteen pieces weighing about 50 g were placed in one level in a standard 10-blade Kramer shear-press cell of 67.5 × 67.5 mm sample area (Food Technology Corporation, Maryland, USA) and the maximum force required for the blades to extrude the fruit through the slots was recorded. The test was repeated with the contents of the second can and the average maximum force calculated.

Texture loss

The logarithm of the average maximum force was plotted against time for each test temperature, and the D-values (time necessary at each temperature for the hardness to be reduced by 90% or one log cycle) was calculated from these graphs. The z-values for loss of texture were obtained by plotting log D_T against temperature.

Results and discussion

The loss of hardness because of thermal processing at 84 °C is shown in Fig. 1, the other eight temperatures showed similar plots. The D_T values for all test temperatures are shown in Table 1. The z-values obtained from the plots of D_T against temperature (Fig. 2) are also shown in Table 1. From the Arrhenius plot (Fig. 3), the apparent activation energy was calculated to be 116.5 kJ mol⁻¹.

The z-values, about 16.7 °C, for the apricot cultivar Bebeco, are much lower than those reported for other fruits and vegetables, these were normally 24 °C or more. The Bebeco cultivar is one of the few apricot cultivars that can withstand thermal processing (Mallidis, 1992) and retain acceptable texture. Bebeco is characterized by a low ratio of citric/malic acid and the changes recorded are consistent with the chelation hypothesis suggested by French et al. (1989).

Figure 1 shows that some restoration in the hardness of the fruit occurred after 3 months storage. This might be because of the absorption of sucrose by the fruit or because of the hardness of the syrup water. Apostolopoulos & Brennan (1994) observed that an increase in the sugar concentration and `hardness' amount of calcium in the processing liquid resulted in an increase in the firmness of canned peaches.

The results from this work should help canners optimize their thermal treatment of apricots.

References