Application of basic drying theory to determine drying mass constants of selected seafoods

The drying of seafood could ensure increase in life span, minimization of storage space, ease of transportation, etc of the product. It is therefore, necessary to dry and know when to stop drying at an appropriate moisture content that would minimize associated challenges in promoting food security. In this study, selected sea foods, such as prawn and oyster were obtained and then dried to determine their drying mass constants using model equation developed based on basic drying principle. The value of the constants obtained would provide data bank to facilitates prediction of mass of the drying material at given time that would correspond to any desired moisture content, provided the initial moisture content of the material is known. The plots of moisture content against drying time, drying rate against drying time and moisture content versus [Mo] [Mt] ⁄ were carried out. It was observed that increase in size of the drying material increased the drying time. Moreover, the drying rate curve resembles a typical drying curve. Hence, the experimental values obtained are reliable. Technical analysis showed that the drying mass constants for prawn and oyster are 0.2950 ± 0.0119 and 0.2453 ± 0.0476, respectively.


Introduction
The essence of drying food materials is to remove moisture to increase its life span, and promotes food security. The materials when dried would become lighter and smaller; and would encourage ease of transporting them for commercial purpose. In addition to these advantages, the drying of sea food would minimize storage space and packaging challenges; keep the material to be unique, nutritious and tasty. Some major sea foods of economic importance include prawn, fish and oysters. In Nigeria, prawn is locally known as 'abu ekem' in Ibibio, 'oporo' in Igbo and 'ede' in Yoruba [1,2,3,4,5]. In Nigeria, oyster is locally known as 'nkop' in Ibibio and 'gigei' in Yoruba. It contains high quality protein and has short shelf life and favourable flavour [2,6,7]. Since seafoods are highly perishable, it is necessary to preserve and handle them through effective equipment and techniques such as bed drying, drum drying, micro wave-vacuum drying, freeze drying, convective drying, spray drying, combined thermal hybrid drying, shelf drying, sun drying, infrared radiation drying, oven drying and commercial food dehydration. The application of each technique and equipment may have its own disadvantages. The drying temperature that was found to be favourable for shrimp was 700C [8,9]. This temperature could be used in drying other seafoods. However, oven drying method could be widely accepted because it is cost effective and can accommodate large volume of samples at the same time; and so would produce higher throughout, high accurate and precise result. The moisture content of a material subjected to drying could be calculated based on data obtained from experimental runs.
Based on the model Equation 1.6 and 1.7, the F value could be referred to as drying mass constant. In this study, the drying mass constants of some seafoods were determined in order to provide a data bank that could be used with empirical Equation 1.6 and 1.7 developed from the basic drying principle to predict easily at a given time the mass of the drying material that would correspond to any desired moisture content, once the initial mass is known.

Materials and Equipment
The materials and equipment that were used are oven, weighing balance, prawn and oyster.

Sourcing of Materials
The fresh samples of sea food were sourced from a local beach market in Itu, Akwa Ibom State, Nigeria.

Preparation of Sample
Prior to drying, the individual samples were cleaned, sorted and grouped into three different sizes namely: large, medium, and small size samples. The samples were dried in a hot air conventional oven.

Determination of Moisture Content (MC)
The selected seafoods were dried at 70°C. Drying of the samples was done in triplicate. The reduction in weight of the drying samples was monitored at an interval of 1 hour until constant mass was achieved. The digital electronic weighing balance with 0.01g precision was employed in measuring the mass of the samples.

Experimental Procedure
The sample moisture content was calculated using Equation     The drying curve obtained is similar to an ideal drying curve. This indicates that the experimental data gotten are reasonably valid; and can be used for analysis to obtain empirical relations or predicting moisture content wet basis and dry basis for the fresh sample used in this work as was discussed by [10]. The rate of drying was noted and observed for all the prawn sizes decreased with increase in drying time as shown in Figures 3 and 4. There was an increase in drying rate at the initial drying periods and later decreased with increase in drying period. The   The drying mass constants of the small, medium, large and bulk quantities of prawn are 0.2819, 0.3044, 0.3000, and 0.2950, respectively. The standard deviation for the sizes from the bulk quantity was obtained as 0.0119. This suggests that the drying mass constant values for the small, medium, large sizes are close to the values of the drying mass constant of the bulk prawn; and the drying mass constant of the bulk can serve as a representative for the other sizes. The graph has a coefficient of determination (R 2 ) of 1 which suggests a good quality fit. This may imply that the equation is reliable for moisture content prediction.  Each sample size moisture content increased as drying period increased. It is seen that the moisture content is independent of sample size. Figures 9 and 10 show the drying rate of the sample sizes and bulk sample, respectively.

Figure 9
Drying rate against drying time for the different size ranges of oyster

Conclusion
The drying time of the samples increased with increase in size. The drying mass constant based on model Equation 1.6 were obtained for prawn and oyster; and could be used to forecast their masses at any given time with respect to any desired moisture content during drying.