4. RESULTS AND DISCUSSION
4.4 Quality Attributes
4.4.5 Microbiological safety
109 between samples of all thicknesses were significantly different. The interaction between the drying system and the product thickness or the marinating duration, as well as between the product thickness and the marinating duration was not statistically significant (p0.05) for product rehydration. The three-way interaction between the drying system, product thickness and the marinating duration was also statistically not significant (p0.05).
In summary, the sample thicknesses had a significant effect on the rehydration rates of biltong while the drying system used as well as different the marinating durations assayed, gave comparable rehydration behaviour of biltong products. The rehydration values in this study were, however, low compared to those reported in studies by Nathakaranakule et al.
(2007) that ranged from 57.17% to 86.66%. Differences in rehydration rates between their study and the present study may be attributed to differences in the drying method used and the inherent biological constitution of the products under investigation. In their study, they rehydrated chicken meat that was dried using superheated steam and heat pump-assisted superheated steam, and this implies a product of lower fibre density compared to that of beef, and a drying system that operated at higher temperatures (above 100oC) compared to those of the present study.
4.4.5 Microbiological safety
110 Table 4.8 Presumptive most probable number (MPN) counts of fresh beef and biltong samples that were dried under the convective air dryer, high wavelength (HWL) and low wavelength infrared heaters
Sample MPN/g Log MPN SE Log MPN
Fresh beef
Convective dried biltong HWL infrared dried biltong LWL infrared dried biltong
440,802 7,029 4,077 2,952
5.644 3.847 3.610 3.470
0.2563 0.2517 0.2460 0.2433
*Turbidity observations in incubated tubes made after 24 hours – presumptive
Table 4.9 Conclusive most probable number (MPN) counts of fresh beef and biltong samples that were dried under the convective air dryer, high wavelength (HWL) and low wavelength (LWL) infrared heaters
Sample MPN/g Log MPN SE Log MPN
Fresh beef
Convective dried biltong HWL infrared dried biltong LWL infrared dried biltong
1,163,403 70,353 24,300 7,020
6.066 4.847 4.386 3.846
0.2325 0.2517 0.1898 0.2410
**Turbidity observations in incubated tubes made after 48 hours – conclusive
MPN counts in food samples analyse the total viable count (TVC) of bacterial cells that are able to grow and multiply in the food. MPN counts in this study showed a general reduction in microbial counts as the processing temperature increased. It can be concluded that samples that were dried under the convective air dryer and the HWL infrared heater had high proportions of slow-growing coliforms due to over five-fold increase in the total bacterial cells (MPN/g) between the presumptive and confirmed counts. Samples dried under the LWL infrared heater had the highest microbial log reduction of 2.2.
Nortjé et al. (2005) and Naidoo (2010) showed that commercial biltong products have TVC counts of between 6-7 logs/g. Therefore, MPN counts in the present study were 1.153 to 3.315 logs lower than those reported in their studies indicating good microbial levels, especially for samples that were dried under the LWL infrared heater. Raji (2006) also investigated the microbial safety of dried, sliced beef (kilishi) sold in the Ilorin metropolis and reported total microbial counts ranging from log 4.38 to 4.54 CFU/g.
In another study by Naidoo and Lindsay (2010) where the microbial safety of biltong from three commercial retail outlets in Johannesburg was investigated, total counts ranging from 6-7 logs/g were reported. The overall quality of biltong processed under both the convective
111 and infrared conditions was therefore remarkably good in terms microbial contamination when compared to that of commercially-produced biltong products.
Identification and isolation of E. coli
E. coli is an indicator organism in food products that can be used to assess the sanitary and overall microbial safety levels in their production (Gill et al., 1996; Cassin et al., 1998; Reij and den Aantrekker, 2004; Naidoo and Lindsay, 2010). Table 4.10 presents the results of Eosin methylene blue (EMB) culture plates that were used to detect the presence of E. coli in the fresh beef slices and dry samples. EMB plates of fresh beef samples did not show the presence of E. coli. The Australian standards would, therefore, categorize the freshly sliced beef as of excellent microbial quality due to the undetectable levels of E. coli in these samples.
Table 4.10 Eosin methylene blue (EMB) culture plate results for fresh beef and biltong samples that were dried under the convective air dryer, high wavelength (HWL) and low wavelength (LWL) infrared heaters
Sample Presence of E-coli in 10-1 ST1 broth tubes Tube 1 Tube 2 Tube 3 Tube 4 Tube 5
E-coli plate count (Log CFU/g) Fresh beef
Convective dried biltong HWL infrared dried biltong LWL infrared dried biltong
– – – – – + + + + + + + + – + – – – – –
Not detected 4.045
1.893
Not detected The convective air dried samples had high numbers of E. coli colonies indicated by the presence of nucleated and dark colonies with a metallic green sheen. The colonies (plate count) in this case were log 4.045 CFU/g .This number was well in excess of 102 CFU/g acceptability threshold for E. coli in RTE meat products (Commission of European Communities, 2005). There were E. coli colonies in four of the five EMB plates for samples dried under the HWL infrared heater. The number of E. coli colonies in the plates was log 1.893 CFU/g and therefore, passed the acceptability threshold. No E. coli colonies or any other colonies were detected in EMB plates for biltong products that were dried under the LWL infrared heater.
The microbial hurdles used in the handling of fresh beef as well as in the process of making biltong effectively suppressed the microbial populations in the meat. RTE meats should be
112 processed to temperatures that exceed 70oC as this is known to be the thermal lethality threshold that effectively sterilizes E. coli (Seyer et al., 2003; Dierschke et al., 2010). From Figure 4.2 and Figure 4.3, it can be observed that the HWL infrared heater did not exceed the lethality temperature of 70oC known to eliminate E. coli. Similarly, samples that were dried in the convective air system did not reach the lethality temperature as they dried at an air temperature of about 25oC (the convective air dryer set-point temperature)
The product temperatures in the case of the LWL infrared heater exceeded 70oC as can be seen in Figure 4.4 and Figure 4.5 and this explains why the LWL infrared heater effectively sterilized E. coli as the core temperature of samples dried in this dryer exceeded the lethality temperature. This therefore, implies that LWL infrared dried samples had excellent microbial quality that guaranteed microbiological safety to consumers and better shelf-life of the dried products compared to samples dried under the other drying systems.
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