Abstract

This study was conducted to investigate and compare the thermal properties of two varieties: Tympanotonus fuscatus and Pachymelania aurita of Periwinkle in Nigeria. The thermal properties, namely; specific heat capacity,C_p, thermal conductivity, k, thermal diffusity,α, thermal absorptivity, γ and thermal effusivity, ∈ of Periwinkle samples were determined. The mean thermal conductivity of T. fuscatus was found to be 0.085±0.00015 W/m.K at temperatures of 308 – 373 K, while that of P. aurita was 0.0952±0.00056 W/m.K at the same temperature range.The average specific heat capacity value of T. fuscatus was found to be 2403.663±3.4379 J/kg.K at temperature of 308 – 373 K lower than that of P. aurita having a mean value of 2832.314±1.7385 J/kg.K at the same temperature. The mean thermal diffisivity of T. fuscatus was found to be 2.6553×10^-8 m²/s while that of P. aurita was found to be 5.6790×10^-8 m²/s. The average values for thermal absorptivity and effusivity of T. fuscatus and P. aurita were obtained as 81.085m^-1; 525.084 W.s^1/2/m²K and 55.441 m^-1; 396.952 W.s^1/2/m²K, respectively. The results shown that average thermal absorptivity and effusivity values were higher in T. fuscatus than P. aurita. A Tukey pairwise comparison analysis carried out on the mean values of these thermal properties of T. fuscatus and P. aurita revealed that there is statistically significant difference at α<5% between the thermal properties of the two varieties of periwinkle samples. These data would help us in predicting and controlling the heat flux during the design of periwinkle processing equipment.

1. Introduction

Periwinkle meat is a potential source of good-quality proteins and minerals like calcium, potassium, iron and phosphorus and some vitamins. It contains most of the essential amino acids in adequate amount for human nutrition. They are used in the preparation of indigenous traditional dishes such as “edikang ikong”, “ekpang nkukwo”, “afia efere” and “afang” soup among others by the Efik and Ibibio ethnic groups in Nigeria. The periwinkle shells are potential sources of calcium for animal feeds. The powdered periwinkle shell with its high CaCO₃ content can be utilized as energizer during pack carburization of low carbon steel. The periwinkle shell strength and hardness makes it an excellent material for building construction and ornament decorations ^{1, 2, 3, 4}.

The thermo physical properties of foods, are important for modeling and optimization of processes involving heating and cooling. Thermal properties data are required in engineering and process design. The properties used in a mathematical model of heat transfer are usually thermal conductivity (k), specific heat (C_p), density (ρ) and diffusivity (α) ^{5, 6}, where k is in (W/mºC), C_p in (J/kg°C), in (kg/m³) and in (m²/s).Specific heat and thermal conductivity of foodstuff are essential in designing and operating thermal processing units ⁷.These properties are highly dependent on temperature, phase change during freezing, and composition, especially fat and moisture, thermal conductivity in particular has significant dependence on tissue structure ^{8, 9, 10}. According to ⁷ the thermal conductivity of shucked oyster increased from 0.577 to 0.677 W/mºC as temperature increased from 0 to 50°C measured by a line heat source thermal conductivity probe while its specific heat increased from 3.795 to 4.047kJ/kg°C when temperature was raised from 10 to 50°C. The objective of this work was to determine the thermal properties of two varieties of periwinkle (Tympanotonus fuscatus and Pachymelania aurita) relevant for the development of their processing equipment.

2. Materials and Methods

Fifteen kilogram (15kg) each of two varieties of periwinkle, namely Tympanotonus fuscatus and Pachymelania aurita, were purchased from the Itu waterfront market Akwa Ibom State, Nigeria. The periwinkle samples were washed, cleaned and graded and, then taken to the laboratory for analysis (Figure 1 and Figure 2).

In determining the thermal properties of periwinkle meat for the two varieties, a substantial quantity of the sample each was de-shelled dried and blended into powder of particle size of 1.0 mm and was then compressed into a mould to form a cube of dimension 50mm 50mm. This was carried out in triplicates (Figure 3 and Figure 4).

The Guarded Hot Plate located in Department of Physics, Akwa State University, Ikot Akpaden Mkpat Enin, Nigeria, was used to determine the thermal conductivity of periwinkle in accordance with ASTM C177-13 and ASTM C1044-16.The standard guarded hot plate method is based on the steady-state longitudinal heat flow principle which determines the thermal conductivity of the material by applying Fourier’s law. The experimental setup consisted of placing the sample material on one-sided mode to measure the heat flux for 48hrs and recorded.

The thermal conductivity (k) of the periwinkle samples was evaluated using the expression from ASTM C1044-16.

(1)

Where is the heat flow = 1.4W, is the cross sectional area perpendicular to the heat flow (m²), is distance between temperature sensors (m) and is the different between upper and lower temperature values (K).

The experiment was conducted using the Differential Scanning Calorimeter (DSC 2 –Mettler Toledo) located in Department of Polymer Technology, Yaba College of Technology, Lagos, Nigeria, based on ASTM E1269-11. The sample was weighed into the sample pan and sapphire was measured into the reference pan. Aluminum standard 40µL was used. Heating was done from 35 to 100°C at 10K/min. Three scans were made: one for the sample, one for a standard (reference), and the third for the empty sample pan (holder).

The specific heat capacity of the periwinkle samples was determined using the expression from ASTM E1269-11.

(2)

is the specific heat capacity of the periwinkle samples, J/(g.K)

is the specific heat capacity of the reference samples, J/(g.K)

is the vertical displacement between the sample holder and the sample DSC thermograph at a given temperature, mW

is the vertical displacement between the sample holder and the reference DSC thermograph at a given temperature, mW

is the mass of periwinkle samples, mg. is the mass of reference(sapphire) standard, mg.

Statistical parameters such as standard deviation, coefficient of variance, mean, maximum and minimum values were used to analyze the thermal properties data of periwinkle samples.ANOVA was carried out to determine the significance and the effect among the two varieties of periwinkle. Turkey parwise comparison test was also used to check the difference in means of the responses for the two varieties of periwinkle using Minitab 17.0 software.

3. Results and Discussion

Table 1 and Table 2 present experimental and calculated values for the thermal properties of the two varieties of periwinkle meat samples which include specific heat capacity, C_p, thermal conductivity, thermal diffusity, thermal absorptivity, and thermal effusivity, Comparatively, T. fuscatus with a mean bulk density value of 1338.0451.4052 kg/m³ had a mean thermal conductivity of 0.0850.00015 W/m.K at temperatures of 308 – 373 K which is slightly lower than that of P. aurita at the same temperature range which was found to be 0.09520.00056 W/m.K with a mean bulk density value of 588.090.3134 kg/m³. Thermal conductivity is dependent strongly on temperature, structure of the material and moisture content ⁹.

The average specific heat capacity value of T. fuscatus was found to be 2403.6633.4379 J/kg.K at temperature of 308 – 373 K lower than that of P. aurita having a mean value of 2832.3141.7385 J/kg.K at the same temperature. ^{7, 9, 11} reported the average specific heat capacity values of fresh Loco meat, Shrimp and Oyster meats to be 2.96130.0887;3.6300.06 and 3.8343 KJ/kg°C at temperature of 0 – 30°C respectively. The mean thermal diffusivity of T. fuscatus was found to be 2.655310^-8 m²/s while that of P. aurita was found to be 5.679010^-8 m²/s. The average values for thermal absorptivity and effusivity of T. fuscatus and P. aurita were obtained as 81.085 m^-1; 525.084 W.s^1/2/m²K and 55.441 m^-1 ; 396.952 W.s^1/2/m²K, respectively. The results shown that average thermal absorptivity and effusivity values were higher in T. fuscatus than P. aurita. Thermal conductivity is used to predict and control the heat flux during food processing such as cooking, frying, freezing, sterilization, drying or pasteurization, thermal diffusivity quantifies a material's ability to conduct heat relative to its ability to store heat transfer, thermal absorptivity entails the quantity of heat penetration to the periwinkle meat samples while effusivity measures the ability of periwinkle meat to exchange thermal energy ^{12, 13}. Variation of thermal properties of T. fuscatus and P. aurita varieties of periwinkle showed P. aurita having a higher than T .fuscatus while and were higher in T. fuscatus than P. aurita (Figure 5). The two periwinkle varieties showed no apparent transition peak, this could be possibly due to exothermic nature of periwinkle samples tested as presented by the DSC thermograph (Figure 6).

ANOVA carried out on the data reported for thermal properties of T. fuscatus and P. aurita varieties of periwinkle meat samples showed that mean values reported were significant (p < 0.05) (Supplementary Table 1), also a pairwise comparison analysis carried out on the mean values of the thermal properties of T. fuscatus and P. aurita revealed that there is statistically significant difference at α<5% between the thermal properties of the two varieties of periwinkle meat sample.

4. Conclusion

This study investigated some thermal properties of two periwinkle varieties (T. fuscatus and P. aurita). The following conclusions were reached from the results of the study:

a) P. aurita had a slightly higher mean thermal conductivity value than T. aurita at the same temperature range.

b) The average specific heat capacity value of P. aurita was higher than that of P. aurita.

c) The results also shown that average thermal absorptivity and effusivity values were higher in T. fuscatus than P. aurita.

d) The two periwinkle varieties showed no apparent transition peak

e) The thermal properties of the two periwinkle varieties were significant on all the parameters investigated at p< 0.05.

References