Note COOLING PARAMETERS FOR FRUITS AND VEGETABLES OF DIFFERENT SIZES IN A HYDROCOOLING SYSTEM

The cooling of fruits and vegetables in hydrocooling system can be a suitable technique. This work aimed to define cooling time for fruits and vegetables of different sizes, presenting practical indexes that could be used to estimate cooling time for produce with similar characteristics. Fruits (orange melonCucumis melo, mango-Mangifera indica, guava-Psidium guajava, orange-Citrus sinensis Osbeck, plumPrunus domestica, lime-Citrus limon, and acerola-Prunus cerasus) and vegetables (cucumber-Cucumis sativus, carrot-Daucus carota, and green bean-Phaseolus vulgaris), were cooled in a hydrocooling system at 1oC. The volume of fruits and vegetables ranged between 8.18 cm and 1,150.35 cm, and between 13.06 cm and 438.4 cm, respectively. Cooling time varied proportionally to produce volume (from 8.5 to 124 min for fruits, and from 1.5 to 55 min, for vegetables). The relationship between volume and time needed to cool fruits (from 1.03 min cm to 0.107 min cm) and vegetables (from 0.06 min cm -3 to 0.12 min cm) is an index that could be used to estimate cooling time for fruits and vegetables with similar dimensions as those presented in this work.


INTRODUCTION
In tropical climate, where mean annual temperatures can exceed 32ºC, fresh fruit losses reach up to 30%.Temperature plays a very important role in the preservation of recently-harvested products.To achieve a rapid and efficient decrease in product temperature, the use of fast-cooling systems is particularly important.
The use of systems utilizing cool water (hydrocooling) is an interesting technology, allowing high heat-transfer rates, which can result into three timesshorter cooling times in comparison with products cooled by forced air, or ten times, when products are placed in conventional or storage room (Mitchell, 1992;Talbot & Chau, 1991;Delgado & Wen Sun, 2001;Fraser & Otten, 1992;Teruel et al., 2002).
The objective of this work was to define cooling time for fruits and vegetables of different dimensions, establishing the relation between time and products volume, in order to present practical indexes that could be

Cooling experiments
Melon, mango, guava, orange, lime, acerola, and plum, with different sizes and an approximate spherical geometry, and cucumber, carrot, and green bean, with an approximate cylindrical geometry (Figure 1), all harvested in Campinas region, were submitted to cooling in an immersion-type cooling system with water circulation.The refrigeration system consisted of hermetic compressor, air condenser, thermostatic expansion valve, water tank, and circulation pump.The water tank (780 × 580 × 570 mm) had a 0.23 m 3 capacity and a water flow of 0.001 m 3 s -1 , insulated with fiberglass (5 cm) coated with asphalt membrane (Figure 2a).The water was treated with an active chlorine solution (150 mg L -1 ).The water temperature within the immersion tank was measured with thermocouples (1± 0.4ºC).
Fruit temperature was measured with type T thermocouples (Cu-Co, AWG # 28), calibrated to the nearest (± 0.15ºC).Thermocouples were inserted into the fruits according to the r/4 (spherical products) or r/3 (cylindrical products) relation ( Van der Sman, 2003).The longitudinal (rL) and transversal (rT) radius were measured with a caliper rule, and fruit sphericity was calculated based on the data (E= rT/rL).In vegetables, radius and length were measured.Before performing the experiments, the fruits and vegetables were weighed on digital scale.
The thermocouples were inserted into three products contained in the packaged (Figure 2b).The products (15 kg in each experiment), were packaged within a perforated, wire mesh package (450 × 300 × 300 mm) and transported in the first hours of the day to the laboratory.
A computerized data-acquisition system was used to acquire and monitor temperature data.The system consisted of an A/D converter, a signal conditioning card, and a microcomputer.Readings were stored into files for later analysis and processing.

Cooling time
Cooling time is a parameter that can be used to evaluate the efficiency of fast-cooling systems for commercial and/or research purposes.Two terms related to cooling time are considered, namely half cooling time (TAT 1/2 ), and seven-eight cooling time (TAT 7/8 ).This parameter (time) can be determined by the Dimensionless Temperature Rate (TAT) (ASHRAE, 1994;Mohsenin, 1980): where Tp is the temperature measured in the product during cooling, Ti is the initial temperature of the fruit, and Ta is the temperature in the cooling medium (water at 1ºC).
Figure 2 -a) Elements comprising the immersion refrigeration system.b) Position in the fruits where temperature was measured.

RESULTS AND DISCUSSION
Cooling of fruits Mean initial temperature of fruits was 25.1 ± 0.9ºC.Cooling time of all fruits varied proportionally with volume, temperatures dropping to 12.5ºC (TAT 1/2 ) and 3.2ºC (TAT 7/8 ) in time spans ranging from 8.5 min to 125 min.Larger fruits were cooled in time intervals 14 times longer than smaller fruits (Figure 3).Dincer et al. (1992) cooled 15 kg of peaches (r=2.3 cm), plums (r=1.85 cm), and pears (r = 2.8 cm) in a hydrocooling system at a temperature of 1ºC; cooling times were 17 min, 15 min, and 25 min, respectively.Pears reached the seven-eight cooling time in intervals 47% to 66% longer.Lime, orange, guava, and melon showed higher sphericity values (E = 1.01 to 1.06, with a slightly prolate geometry) than acerola and plum (E = 0.97 to 0.99, with a geometry tending to oblate shape), differently from mango (E = 1.12), which presented a more prolate geometry (Table 1).

Cooling of vegetables
The mean initial temperature of vegetables was 24.8 ± 0.4ºC, and the half cooling and seven-eight cooling times were attained when they reached a temperature of approximately 12.4ºC and 4.1ºC (Figure 4).Cucumber required 36 times longer time than green bean (37%) to reach 4ºC temperature, as a result of the difference in volume between both vegetables, which was 97% higher (Table 2).
the cooling time of products with similar characteristics to those presented in this work.
Figure 1-Fruits and vegetables hydrocooled at 1ºC

Table 2 -
Cooling time and cooling time/volume ratio of hydrocooled vegetables.

Table 1 -
Cooling time, cooling time/volume ratio and sphericity of fruits cooled with water at 1ºC.