Modelling of drying kinetics and quality characteristics of solar dried beef

Abstract

Beef is highly perishable and its commercialization in the arid and semi-arid areas of Kenya is a challenge. Sun drying has always been used to preserve meat in these areas. However, there is a need to focus attention on improvement of quality of the drying process. This can be achieved through drying modelling and simulation as well as dried product quality characterization. This work aimed at investigating the solar tunnel dryer performance for dehydration of beef in one of the pastoral areas in Kenya. In this regard, appropriate models were used to establish beef desorption isotherms. Drying characteristics of fresh beef under controlled conditions (in a laboratory simulation dryer) were determined and related to drying kinetics under the varying atmospheric conditions (in a solar tunnel dryer). Assessment of mathematical models was done and moisture diffusivities established. The effect of drying methods on beef quality parameters as well as storage time and packaging type effects on solar dried beef samples were also determined. Laboratory analyses of the dried beef samples were done using standard procedures. The desorption isotherms of beef were sigmoid shaped, type II isotherms. There was a decrease in equilibrium moisture content (EMC) with temperature increase at constant water activity, an increase in EMC with increased water activity at constant temperature and a decrease in net isosteric heat of sorption with increased moisture content. The GAB and Oswin models fit desorption data best. Drying rates and drying times were lower for thicker beef slices and increased at higher temperatures in the cabinet dryer, whereas effective moisture diffusivity (Deff) was high at higher temperatures and increased sample thickness. There was a significant decrease (P ≤ 0.05) in L*, a*, b*and chroma/C* colour values with increased temperature and beef thickness. At 60 °C, the rehydration ratio (RR) of beef slices increased whereas the firmness decreased. Microbial numbers in beef dried at 30 °C and 40 °C were higher than that of fresh beef, whereas drying at 60 °C significantly (p ≤ 0.05) reduced the microbial load. The temperature profile along the drying chamber of the solar tunnel dryer increased with increased solar radiation and decreased continuously at high moisture content of beef. Whereas the beef drying processes occurred in the falling rate period, the drying rate was higher in the solar drier compared to open sun drying. The best prediction of beef drying characteristics was given by the Page model. Effective moisture diffusivity for the sun and solar dried samples varied from 1.775 x 10-10 to 2.282 10-10 m2/s. The L* and C* colour parameters, moisture content (%) and adhesiveness of solar dried samples were lower while RR, hardness (N), firmness (N), springiness (%) resilience and sensory scores were higher compared to sun dried beef. During storage, samples dried towards the centre of the drying chamber of the tunnel dryer were more stable to moisture changes, whereas those dried close to the ends were stable to microbial growth and fat oxidation. Deteriorative changes in dried beef during storage in Glass jars and aluminium foil packaging were significantly lower (p ≤ 0.05) compared to polyethylene and paper.