Infrared drying of biltong : effect of pre-treatment and drying conditions on the drying characteristics and product quality.
In this study, the drying kinetics of biltong was investigated under infrared and convective drying systems, with two product pre-treatment conditions namely, slice thickness and marinating duration. The effect of these treatments on the quality and microbiological food safety of biltong was investigated. In addition, the study sought to establish and compare the energy efficiencies of the infrared drying systems under investigation. The drying of biltong was conducted under two infrared drying conditions (2.5 μm and 3.5 μm peak wavelengths) and a convective drying condition that mimicked conventional and commercial drying conditions at a temperature of 25°C and 60% relative humidity. The two infrared heating systems were set so that they had the same intensity of 4777 W.mˉ² at the product surface, but different peak wavelengths (2.5 μm and 3.5 μm). The marinated products were dried from a moisture content of 73.99% ± 1.46% wet basis (wb) to the commercial quality requirement of 20% ± 1% wb. Samples had a product thickness of 5, 10 or 15 mm, and had been marinated for durations of 6, 12 or 24 hours prior to drying. The experiments were arranged in a complete randomized block design. The drying rate, product temperature, quality attributes, such as colour, texture, shrinkage, rehydration, as well as total viable bacterial counts were measured, along with the infrared power consumption during the drying process. It was observed that the infrared heater with a peak wavelength of 2.5 μm dried the products faster, when compared to the one with a peak wavelength of 3.5 μm at all times, even when the infrared intensity of both heaters on the products was the same. The convective drying system recorded lower drying rates and longer drying durations compared to the infrared heating systems. Increasing product thicknesses increased the drying times and reduced the drying rates for samples dried under the 2.5 μm peak infrared heater and the convective drying system. Increasing the duration of marinating, reduced the drying rates for samples dried under the convective air dryer and increased the drying rates for samples that were dried under the 2.5 μm peak infrared heater. The 3.5 μm peak infrared heater recorded varied results, but generally had the 10 mm thick and 12-hour marinated samples having higher drying rates for all samples dried under this system. The drying system used significantly (p≤0.05) affected the drying times, while the pre-treatment conditions had no significant (p≥0.05) effect on the drying times. The core temperature increased with decreasing product thicknesses and increasing marinating duration for the 2.5 μm peak infrared heater. However, the 3.5 μm peak infrared heater product core temperature did not follow the same trend. This system recorded lower specific energy consumption (SEC) values, compared to the 2.5 μm peak infrared heater. Out of the five drying models tested, the drying kinetics of biltong was best described by the approximation of diffusion model (ADM) on the basis of the model’s high coefficient of determination (R²) and low root mean square error (RMSE) values. The sample lightness (L*) of dried samples was significantly (p≤0.05) influenced by the drying system. The convective air drying system recorded lower total colour difference (ΔE) values compared to the infrared systems. The L* a* b* colour parameters decreased after marinating fresh beef slices, and decreased even further after the drying process. Textural analysis showed lower hardness and puncture values for samples dried under the 2.5 μm peak infrared heater compared to the other drying systems. Samples dried under the 3.5 μm peak infrared heater recorded the highest shrinkage coefficient and rehydration rates. The three drying systems achieved at least a 2-log reduction in the total viable bacterial count, with the 2.5 μm peak infrared heater having the lowest most probable number (MPN) count of 7,020. The infrared drying systems produced biltong that had acceptable plate counts of E-coli less than 10² CFU.g⁻¹. However, the convective air drying system did not meet this acceptability threshold for the safe consumption of biltong. The samples that were dried under the 2.5 μm peak infrared heater recorded high drying rates, good microbiological safety and textural attributes compared to the convective and 3.5 μm peak infrared drying systems. However, this system had consumed more energy compared to the 3.5 μm peak infrared heater that produced biltong with superior shrinkage coefficient and rehydration characteristics as well as colour attributes that closely compared to those of samples dried under the convective air drying system. The study recommends both the 2.5 and 3.5 μm peak infrared heaters for the best texture of biltong, improved drying characteristics and enhanced microbial food safety of the product.