Here are the results of my research. Most of what is here is from an 88 page "Literature Review of Microbiological Hazards Associated with Biltong". One hundred and seventeen research papers and reports were used in the review although biltong is not the direct topic of many of the studies as there is not that much material on biltong. It was a quite long and dense read so I've just bullet pointed the good bits. Most of it is copy and paste but I've edited a lot of it to make it more readable. It is important to note that the focus of this study is on the food safety aspect of making biltong and covers small scale commercial production but makes mention of home made biltong. I've even done the temperature conversions for the fine folk of America. I did this for myself but hopefully someone else finds it useful.
THE PUNCH LINE (for those not interested in the wall of text)
The ideal conditions for making biltong in a 6 day period:
- Temperature: 35°C (95°F). Commonly used range: 22-35°C (72-95°F)
- Humidity: 30%. Commonly used range: 30-50%
- Air speed: 3 m/s. Commonly used range: 2.5 to 3 m/s
- Target water activity for safety relative shelf stability 0.7 to 0.75 (Actual values of sold product vary greatly depending on consumer preference)
- The composition of biltong after drying is typically: moisture content 20-30%; salt 3-8%; pH 5.6-5.9; water activity 0.7 to 0.75.
- Nitrite is sometimes used as a color fixer only.
- Biltong's origins anecdotally stem from the Dutch who, whilst escaping from British rule in South Africa some 200 years ago, preserved meat by adding vinegar and spices and hung it from the back of ox wagons where it dried over 3 to 4 days.
- In its very simplest form, biltong has been spiced only with salt, black pepper and brown sugar (i.e. no vinegar).
- Traditionally in South Africa, drying of biltong was achieved by hanging the strips of meat on hooks and leaving them to ambient dry.
- Johannesburg Temps: Day Average(morning.min-afternoon.max) 9.9-22.4°C (50-72°F). Relative Humidity: Average(morning-afternoon) 69.9-40.5%
- Bloemfontein Temps: Day Average(morning.min-afternoon.max) 8.6-23.6°C (47-75°F). Relative Humidity: Average(morning-afternoon) 64.4-32.8%
- Most muscles in the carcass may be used but the large ones are most suitable.
- Both fresh and thawed meats can be used
- The meat strips cut from intact muscles are up to 400 mm long and 25 to 50 mm thick.
- The water activity of raw meat is around 0.98.
- Acidic marinades should used be at 0-4ºC (32-39°F)
- The air for drying biltong should be heated to around 35ºC (95°F)
- Drying to a water activity of 0.7 to 0.75 is advisable. Weight loss of the meat should be a good indicator of water activity.
- The meat is generally held in a marinade for 18 to 24 hours, traditionally at ambient temperature, but nowadays, this storage is more likely to be at 4ºC (39°F).
- One study found that pathogens were reduced by marination in vinegar for at least 7 hours.
- The use of nitrate or nitrite, often in the form of saltpetre is mentioned frequently as a source of colour enhancement.
- Important to note that spices are also a common form of pathogen introduction. Use good quality spices which have been stored properly.
- Few pathogenic microorganisms grow below a water activity (aw) of 0.90 and few microorganisms grow below aw = 0.75.
- Yeasts and moulds do not grow below a water activity of 0.60.
- Effective drying, to reduce water activity, relies on drying time and three inter-related process factors: air temperature, relative humidity, and speed.
- No microbial growth, or spore germination, was detected on biltong samples with a water activity of 0.7 or less. Allowing for an arbitrary safety margin, a water activity of 0.68 was regarded as the critical moisture content at, or below, which biltong could be kept for long periods.
- 35ºC (39°F) will enable a microbiologically stable product to be produced in around 6 days (144 hours). A lower air temperature would require a longer drying time and increase the microbiological hazard due to the slower drying rate.
- Data reported suggest an air speed around 2.5 to 3 m/s.
- Drying with air at 35ºC (39°F), 30% relative humidity and 3 m/s is suitable for making biltong within 6 days.
- Relative Humidity levels during drying process reported from other studies: 30-50%
- Temperatures during drying process reported from other studies: 22-35°C (72-95°F).
- The composition of biltong after drying is typically: moisture content (20 to 30%); salt (3 to 8%); pH (5.6 to 5.9); water activity (0.7 to 0.75).
- The use of weight loss during the drying of biltong could provide a useful indicator of water activity. Raw meat has a water content of around 75%. Reducing the weight of the meat by a factor between 3.75 and 2.5 during drying will produce a product with the required moisture content of around 20 to 30% and corresponding water activity around 0.7 to 0.75. In addition, the drying process should designed to decrease the water activity quickly to prevent microbial growth but not so quickly that case hardening occurs.
- No definitive shelf life for biltong was identified in the literature although “several months”, “very long” and “indefinite” were noted in the literature, all without the need for refrigeration.
- Many microflora identified and studied. No mention of Clostridium.
- Study catalogues all reported outbreaks of food borne illnesses relating to biltong, not a single case of botulism mentioned.
- The presence of organic acids, salt, and a lowered water activity achieved by drying are all controlling factors in the potential destruction of pathogens and also important in preventing microbial growth.
- The reduction in pathogen level increases as the water activity decreases and therefore it is important that water activity is reduced quickly. Growth studies have demonstrated that salt, presence of organic acids and spices are not in themselves inhibitory and therefore a hurdles approach to biltong manufacture is important.
Burfoot, D., Everis, L., Mulvey, L., Wood, A., & Campden, R. B. (2010). Literature Review of Microbiological Hazards Associated with Biltong and Similar Dried Meat Products. Food Standard Agency, London, 87.
Naidoo, K., & Lindsay, D. (2010). Survival of Listeria monocytogenes, and enterotoxin-producing Staphylococcus aureus and Staphylococcus pasteuri, during two types of biltong-manufacturing processes. Food control, 21(7), 1042-1050.
Taylor, M.B. (1976) Changes in microbial flora during biltong production. South African Food Review, 3(2) 120-121, 123.
Changes in moisture content, water activity, and salt content during the manufacture of biltong. Air temperature = 35ºC; relative humidity = 30%; speed = 3 m/s.
Changes in bacterial numbers during the manufacture of biltong. Air temperature = 35ºC; relative humidity = 30%; speed = 3 m/s