Thermal Diffusivity During Water Bath Cooking of Breadfruit (Artocarpus communis)

Breadfruit is a fruit originated from South Pacific, and afterwards, it was widespread in the rest of Oceania. In Colombia, it is known as “Fruta de pan” and in-home use, breadfruit has been consumed cooked, fried or roasted. The optimisation of water bath cooking could reduce the energy cost of these operations; therefore, this study aimed to determine the thermal diffusivity during the process of boiling baking cylinders of breadfruit. The process was carried out using a temperature controlled water bath designed for this purpose at 95 °C. The method used for the determination of diffusivity was based on the analytical solution of the heat transfer by conduction equation written in cylindrical coordinates. We found a thermal diffusivity value of 6.45 10 m/s, which was within the range of values reported by other authors, although not in the product studied, since there were no previous studies on this matter for the breadfruit.


I. INTRODUCTION
Artocarpus communis, commonly known as breadfruit tree because of the "bread-like texture" of its edible fruits, is an equatorial lowland species of flowering tree in the mulberry family (Moraceae), which has about 50 genera and over 1000 species that grows best below elevations of 650 m [1]. A. communis is a fruit tree that is propagated by the root cuttings and the average age bearing the first crop is between 4 and 6 years. It produces its fruit up to 2-3 times in a year, and the number of fruits produced is very high. The fruit is aromatic, rich in latex and can weigh between 1 kg and 4 kg [2]. Breadfruit is a fruit originated from South Pacific, and afterwards, it was widespread in rest of Oceania. In Colombia, it is known as "Fruta de pan" and in-home use, breadfruit has been consumed cooked, fried or roasted [3], [4]. A. communis is edible and a valuable natural resource due to its high content of nutrients (84.2 % carbohydrates, 4.1 % proteins, 3.5 % fibre and 2.9 % minerals in dry weight) [5].
Water bath cooking is a wet cooking process, where the maximum water temperature is 100 °C at 1 bar, or under other pressure conditions [6]. The choice of cooking process parameters should represent an acceptable compromise between sensorial quality, microbiological quality and energy use [7]. Concerning the latter, optimising this type of process could reduce the energy cost of these operations, so it is of interest in engineering [8]. To control and optimise cooking operations, fundamental physical processes and physical properties must be analysed and estimated. Several studies have been devoted to studying the cooking process of some products, mostly meat and starchy products [6], [9]- [11].
There are no data related to the cooking of breadfruit; therefore, the objective of this work was to study the diffusivity during the cooking process of cylinders of A. communis from San Andrés Island (Colombia), looking for generating a calculation of thermal diffusivity.

A. Raw material
The breadfruit (A. communis) was obtained from the Central Market of San Andrés Island (Colombia) in a commercially mature state, healthy and free of fungi and weevil. The peel was removed and cut in the shape of a cylinder 10 cm long and 3 cm in diameter.

B. Cooking procedure
Cooking processing operations were carried out using a temperature controlled water bath built and designed at our laboratory for this purpose following the same specifications used by Ayadi et al., [6]. For this, J-type thermocouples were used to determine the time-temperature history of samples during cooking. One thermocouple was placed in the bath water to measure the process temperature. The second thermocouple was fixed at the geometric centre of the sample. The third one was placed just under the sample surface, at a maximum depth of 0.5 mm. At the beginning of the experiment, samples were placed in the water bath at 95 °C ± 1 °C. The water in the unit was not stirred during cooking. When the temperature at the centre of the sample had increased to the desired value (between 72 °C and 76 ºC), the sample was removed from the hot water. Temperature changes in the water bath (T ∞ ), sample centre (T C ) and on the sample surface (T S ) were monitored every 2 min. The water to sample ratio was 6 to 1.

C. Thermal diffusivity determination
To simplify the problem, the sample was modelled as a finite cylinder with radius R and length 2 L. The method used was based on the analytical solution of the conduction heat transfer equation written in cylindrical coordinates. This method was described and applied by Ayadi et al., [6] to measure the thermal diffusivity of salami and ham products prepared from turkey meat. The method assumes a constant sample surface temperature (T S ) and constant thermal diffusivity. If the initial temperature distribution (T 0 ) is uniform, the exact solution of the conduction heat transfer equation written in cylindrical coordinates is described by Equation (1).
If the sample is exposed to the temperature difference for a long time, Equation (1) can be reduced to the first term of the series. For m = n = 1, β m = π/2, β n = 2.405, J 1 (2.405) = 0.519, and at the centre of a cylindrical sample z = 0, r = 0 and J 0 (0) = 1.0. The temperature change at the central point of the sample in the case of long exposure to external temperatures is then given by Equation (2).
Then, thermal diffusivity can be calculated by using Equation (4). Equation (2) is linearly time-dependent and can be transformed to the equivalent linear form as Equation (3) [13]. e ( ) and on the ham products mperature of 7 of samples wa g plot scale) ob he process, fol t al., [9] in the d in the prese it has been re ntre of the This was s obtained [12], [13]