4. RESULTS AND DISCUSSION
4.2 Drying Characteristics
4.2.3 Drying time
65
66 Figure 4.8 shows the drying curves of 5 mm thick samples that were marinated for different durations before being dried under the convective air dryer. These curves are representative of the typical shape of the curves of biltong slices of other thicknesses that were marinated for different durations then dried using the convective dryer (see Appendix C). Clearly, it can be seen from Figure 4.8 that the 6-hour marinated samples dried faster (95 hours) than the 12- hour marinated samples (180 hours) and the 24-hour marinated samples (195 hours).
Products of other thicknesses had their drying times increase with increasing marinating duration.
Figure 4.8 Changes in moisture content with drying time, when the marinating duration is varied for samples dried under the convective air dryer
From Figure 4.8, it can be seen that doubling the marinating time from 6 to 12 hours increases the drying time by 89%, while doubling it from 12 hours to 24 hours increases it only by 8%.
It can therefore, be concluded that under the given experimental conditions, an increase in the product thickness (Figure 4.7) as well as the marinating (Figure 4.8) duration results in an increase in the drying time for biltong products.
Figure 4.9 presents the drying curves of 5, 10 and 15 mm thick samples, previously marinated for 6 hours then dried under the LWL infrared heater. These curves represent the typical
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 50 100 150 200
Moisture content (decimal, wb)
Time (hours)
5 mm 6 hours 5 mm 12 hours 5 mm 24 hours
67 shape of the curves of samples that were marinated for 12, and 24 hours then dried under the same conditions using the LWL infrared heater.
Figure 4.9 Changes in moisture content with drying time for the high wavelength (HWL) infrared drying of 6 hour marinated biltong slices.
The drying time for samples dried under the HWL infrared heater ranged from 16 hours to 36 hours. The drying time increased with increasing product thickness just like the case of products dried under the convective dryer, although the drying time was significantly (p0.05) shorter when compared to the drying time of the samples that were dried under the convective air dryer.
The 5 mm thick biltong slices (see Figure 4.9) had shorter drying times compared to the 10 and 15 mm thick biltong slices. The drying time increased by 37% when sample thickness was increased from 5 to 10 mm, while increasing by only 13% when the product thickness was increased from 10 to 15 mm.
Figure 4.10 depicts the drying curves of 5 mm thick samples previously marinated for 6, 12 and 24 hours then dried under the HWL infrared heater. Drying curves of 10 and 15 mm thick samples that were marinated for 6, 12 and 24 hours, then dried under the HWL infrared heater as shown in Appendix C). The drying times of the samples dried under the HWL infrared heater generally increased with increasing marinating times. Although Figure 4.10
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0 5 10 15 20 25 30
Moisture content (decimal, wb)
Time (hours)
5 mm10 mm 15 mm
68 represents only 5 mm thick slices, this trend was observed for all slice thicknesses dried under the HWL infrared heater. In addition, the difference in drying time due to marinating duration (Figure 4.10) is not as distinct as the difference due to product thickness (Figure 4.9). It can also be observed that the curves for the 24 hour and 12-hour marinated samples are close to each other.
Figure 4.10 Changes in moisture content with drying time, when the marinating duration is varied for samples dried under the high wavelength (HWL) infrared heater The samples that were dried under the LWL infrared heater had drying times ranging from 5.25 hours to 10.25 hours and was significantly (p0.05) shorter, when compared to those of the HWL infrared heater (16-36 hours) and those of the convective air dryer (95-230 hours).
Figure 4.11 shows the drying curves of biltong slices of different thicknesses previously marinated for 6 hours, then dried under the LWL infrared heater. These curves are typical shapes of the drying curves of samples of different thickness that were marinated for 12 and 24 hour durations then dried using the LWL infrared heater (see Appendix C).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 5 10 15 20 25
Moisture content (decimal, wb)
Time (hours)
5 mm 6 hours 5 mm 12 hours 5 mm 24 hours
69 Figure 4.11 Changes in moisture content with drying time, when the slice thickness is varied
for the low wavelength (LWL) infrared dried samples
It can be clearly seen that the 15 mm thick slices had drying durations that were longer compared to those of 10 and 5 mm thick slices. This trend was observed for all marinating times of samples dried under the LWL infrared heater (see Appendix C). It can be deduced (from Figure 4.11) that when the product thickness is increased from 5 to 10 mm, the drying time increased by 22% and when the thickness was further increased to 15 mm, the drying time increases by 24%.
Figure 4.12 shows the drying curves of the 6-hour marinated, 5 mm thick samples that were dried under the LWL infrared heater and is a typical representation of the general drying trends for samples dried under the LWL infrared heater. Increasing marinating durations for the 5 mm thick, LWL infrared dried samples, generally reduced the drying time. This trend was also true for samples of other thickness and marinating that were dried using the LWL infrared heater (Appendix C). It can also be deduced from Figure 4.12 that, as the marinating time is doubled from 6 to 12 hours, the drying time reduces by 2 hours while doubling it from 12 to 24 hours reduced the drying time by 0.5 hours.
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0 2 4 6 8 10 12
Moisture content (decimal, wb)
Time (hours)
5 mm10 mm 15 mm
70 Figure 4.12 Changes in moisture content with drying time, when the marinating duration is
varied for samples dried under the low wavelength (LWL) infrared heater Effects of the drying systems on product’s drying time
Comparison of the drying times of samples dried under different drying systems show that the LWL infrared heater was the fastest drying system. On the other hand, biltong slices dried under the convective air drying system had the longest drying times of all marinating conditions and product thicknesses, for the drying systems under this study.
The trend depicted by these results is corroborated by Wang and Shi (1999), where beef samples of different thicknesses influenced their drying time, with thinner samples giving shorter drying times due to their lower internal resistance to mass transfer and a higher internal heat generation rate. There is, however, a critical thickness size in their research where increasing thickness did not alter the drying times. This was not encountered in the present research probably due to differences in experimental conditions.
The drying times increased with increasing marinating time for samples that were dried under the convective air drying system and the HWL infrared heater while the converse was true for those dried under the LWL infrared heater. This phenomenon may be attributed to the differences in the peak wavelength of the energy from the HWL infrared heater when compared to the LWL infrared heater. Spicing is known to decrease the drying time of meat
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0 2 4 6 8
Moisture content (decimal, wb)
Time (hours)
5 mm 6 hours 5 mm 12 hours 5 mm 24 hours
71 products, while salting increases the drying time (Chabbouh et al., 2013). However, with high drying temperatures, an increase in drying rate with increasing salting level is known to occur in salted food products, hence a decrease in the drying time (Lewicki, 2004).
A three-factor ANOVA was used to evaluate the effect of the different pre-treatment conditions and the drying systems on the drying time of biltong. It showed that there was a significant (p0.05) difference in drying time of different drying systems while minor differences were attributable to the product thickness and degree of marination.
The drying times for samples that were dried under the convective air drying system were 84.6% and 95.6% longer than the drying times for the samples that were dried under the HWL and LWL infrared heaters, respectively.