3. MATERIALS AND METHODS
3.3 Drying Experiments
Three drying methods were investigated in this research namely; convective air drying and infrared drying using two infrared heaters with 2.5 µm and 3.5 µm peak wavelengths, but both heaters set at an infrared intensity of 4777 W.m-2 at the product surface. In total, three drying treatments were evaluated.
3.3.1 Conventional drying Drying equipment
Convective air drying was carried out in a forced-air mechanical oven (prolab, PRIS, South Africa). The oven is 0.95 m x 0.65 m x 0.77 m (LxWxH) in size. It is connected to a 220VAC outlet and has a temperature control unit with a resolution of 0.1oC and an accuracy of
±0.5oC. Its operating temperature range is 0oC to 220oC.
Hot air drying procedure
Drying runs were conducted in a forced-air mechanical oven (prolab, PRIS, South Africa) at an air temperature of 25oC and approximately 60% RH. The drying unit controller maintained the e uipm ent’s interior temperature at an accuracy of ±0.5oC from its set point temperature.
The dryer was run idle for two hours prior to drying in order to stabilize it.
A drying run involved placing 6-hour marinated samples in the convective dryer. These were comprised of 3 pieces of 5 mm thickness, 3 pieces of 10 mm thickness and 3 pieces of 15 mm thickness. Three drying runs were carried out for each sample group marinated for a different length of time.
The weight loss of each sample was monitored at regular intervals by quickly removing the sample from the drying unit, weighing it and putting it back in the dryer (within 10 seconds).
The door of the drying unit was closed after removing the sample to limit temperature fluctuations in its interior. Weighing was done using an electric balance (CQT 202, Adam Core, USA). The samples were removed from the drying unit when they reached the target moisture level of 20% ± 1% wb and stored in labelled ziplock polythene bags (Victoria packaging, Pietermaritzburg, South Africa) in order to prevent water adsorption or desorption, prior to the quality analysis tests. This moisture content was determined based on preliminary investigations of the moisture level of commercial biltong found in different retail outlets.
44 Each sample was put in its ziplock bag and stored at room temperature. Quality analysis tests were carried out for each drying run immediately after all the samples reached the target moisture level.
3.3.2 Infrared drying Heater characteristics
Two infrared heaters were used to investigate the effects of different infrared peak emission characteristics on the drying kinetics of biltong. Model QF-121210 (Omega, UK) had a surface temperature of 538oC at the peak emission wavelength of 3.5 µm while model QC- 121240 (Omega, UK) had a surface temperature of 871oC at the peak emission wavelength of 2.5 µm. The heaters had aluminized steel casings for maximum strength, mounting studs and equal dimensions. The heaters also had similar response times of 7–8 minutes. Model QF- 121210 had an emitting surface made of black quartz ceramic cloth surface with an output wavelength range of 2.5 to 6 microns while model QC-121240 had an emitting face made of high purity fused quartz glass with an output wavelength range of 2.5 to 6 microns. The infrared spectral characteristics and response times for the heaters are shown in Figures 3.2–
3.5.
Figure 3.2 The response time of model QC-121240 (Low wavelength-LWL) infrared heater (Omega, 2013)
45 Figure 3.3 The emission characteristics of model QC-121240 (Low wavelength-LWL)
infrared heater (Omega, 2013)
Figure 3.4 The response time of model QF-121210 (High wavelength-HWL) infrared heater (Omega, 2013)
46 Figure 3.5 The emission characteristics of model QF-121210 (High wavelength-HWL)
infrared heater (Omega, 2013)
The heaters were connected to 220V AC outlet with the QF-121210 model (High wavelength infrared heater-HWL) drawing 1440 Watts while the QC-121240 (Low wavelength infrared heater-LWL) model drew 5760 Watts on a dual voltage connection.
Description of experimental infrared drying system
Figure 3.6 presents a sketch of the infrared drying rig with the infrared heater mounted on it.
The heaters shared a drying rig designed to accommodate one infrared heater (Part A in Figure 3.6) at a time. The drying tray (E) was made of food grade stainless steel wire grill mounted on a vertically sliding frame for height adjustment (D). The heaters were mounted at the top of the rig on a mounting frame with the radiating surface facing downward (B). The drying rig was open on its four sides, to allow free convective air movement over the products.
The heater’s electrical outlet was first connected to a power meter (DEM0 5S G, Acorp, South Africa) that recorded the power used in kWh by each heater for each drying run. The low wavelength infrared heater (LWL) used 2.5 mm electrical leads, while the high wavelength infrared heater (HWL) used 1.5mm electrical leads
47 Figure 3.6 A sketch of the assembled infrared drying rig depicting the relative positions of the infrared heater (A), the radiating surface (B), thermocouple (F) placement in biltong sample (C) that is placed on a vertically adjustable (D) drying tray (E) , and the data logger (G) connected to the thermocouples
Infrared drying procedure
Each drying run used the same protocol as that of convective air dried samples. Samples were put in the drying tray 8 minutes after switching on the heaters to ensure that drying occurred at the heater’s maximum temperature. Each of the samples had its temperature monitored by a K-type thermocouple (TC-TT-KI-24, Omega, UK) placed at the approximate centre of the samples (illustrated by Part F in Figure 3.6). The thermocouples were connected to a data logger (OM-DAQ-USB-2401, Omega, UK), (part G in Figure 3.6) that automatically recorded the product temperature at its approximate centre point every 30 minutes as shown.
Samples were placed on the drying tray in a way that they covered up the entire surface without touching the adjacent sample, for maximum interception of the radiation from the heaters. The drying tray was also set perpendicular to the radiating heater surface and had the same surface area as the heaters (0.3 by 0.3 m). Moisture loss for each of the samples was monitored at suitable intervals in the same manner as for samples dried under the convective air dryer. The drying process was terminated when the sample reached the target moisture level of 20% ± 1% wb. They were then removed from the dryer and each of them stored in a
B B
E A
C D
F G
48 labelled Ziploc bag, waiting for quality analysis tests that commenced immediately after they cooled down to room temperature. This process was repeated for each of the three marinating levels, and for each heater, after separately setting the infrared intensity of each heater to 4777 W.m-2 at the product surface by adjusting the distance between the drying platform and the radiating surface. An infrared power meter (LS122, Shenzhen Lishang Technology Co.
Ltd, China) was used to achieve this. This intensity of 4777 W.m-2 was selected based on preliminary tests carried out on the heaters and information gathered from literature (Krishnamurthy et al., 2008; Jun et al., 2011).
3.4 Quality Analysis