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Preservation of beef using bacteriostatic chemicals and solar drying
S. K. Mbugua and E. G. Karuri
Simple potential meat-preservation methods that can be applied at either the household or the small-community level in rural areas were evaluated. Samples of beef treated with various bacteriostatic agents— brine, honey, glycerol, vinegar, hydrochloric acid, and ethanol—were dried for three days, either directly in the sun or in a solar drier box, and packaged and stored at ambient temperature for four weeks. The treated samples were compared with samples dried without other treatments and with frozen meat. The samples were evaluated for gas formation in the packages, odour, moisture, and discoloration. The water-activity level was measured, as were the extract release volume, the percentage of rehydration, and the pH, and microbial counts were determined. An organoleptic evaluation was carried out by nine panellists. The meat was preserved equally well by either direct sun drying or drying in a solar drier, provided it was protected from microbial and biochemical deterioration with bacteriostatic chemicals. Brine, glycerol, and vinegar gave equally effective protection. The dried meat tended to lose flavour compared with the frozen reference samples.
In Kenya meat from the major sources—cattle, sheep, goats, pigs, and poultry—is mainly marketed fresh through small-scale meat sellers, representing about 80% of the country's meat outlets, who lack refrigeration facilities [1]. Furthermore, the animals are slaughtered in poorly designed abattoirs, and the carcasses are heavily contaminated with bacteria as a result of unhygienic handling.
It is estimated that small-scale sellers incur more than 5% physical loss in meat for these reasons [1]. To minimize such losses, they have to gauge the market requirements and buy only as many carcasses as they can sell within 12 to 40 hours while keeping them in well-aerated rooms, protected from flies by wire gauze. At the Dagoretti market, a major abattoir for meat destined for the wholesale market in Nairobi, butchers who have not sold their carcasses by noon are forced to sell them at throw-away prices to wholesalers with refrigeration facilities.
In rural areas, where meat sells slowly because of the small number of customers, such losses are very high. In such regions protein is critically needed for better nutrition, but because of a lack of appropriate preservation methods, households find it difficult to slaughter animals either for home consumption or for sale to the local community.
The losses sustained when meat is stored at ambient temperatures are caused by weight loss due to dehydration and by bacterial spoilage. The latter is characterized by the development of foul odour, brown colour, black and green spots, gas formation in sealed packages, and drip formation. The most important bacteria in the spoilage of fresh meat are Pseudomonas sp., Lactobacillus sp., Acinetobacter sp., Alteromonas putrefacians, Enterobacter liquefaciens, and Yersinia enterocolitica, which produce hydrogen sulphide under anaerobic conditions [2, 3]. Spoilage due to pseudomonas and enterobacteriaceae occurs when the organism counts are more than 10(6)/cm² [2].
Traditional methods to control microbial spoilage, such as salting, smoking, and drying, are based on reducing the water activity of the meat and applying bacteriostatic substances such as formaldehyde and phenolic compounds in smoke. Some tribes in Kenya in the past used honey to preserve and produce special meat products such as rakuri. The preservation of fresh meat by microbe-inhibiting chemicals is well documented [4]. Low temperature, 2-4 °C, limits the spoilage of raw beef and veal by staphylococci and micrococci [3]. Creating an acidic pH between 6.5 and 5.3 with hydrochloric acid or lactic acid retards meat spoilage by yeasts and pseudomonas [3].
We evaluated simple potential meat-preservation methods that can be applied at either the household or the small-community level in rural areas.
Figure 1 (see Figure. 1. Design of the experiment comparing the effect on beef of preservation by freezing, solar drying alone without other treatment, and drying after treatment with a bacteriostatic agent) shows the experimental design followed in evaluating various bacteriostatic agents—brine, honey, glycerol, vinegar, hydrochloric acid, and chang'aa, a traditional distilled spirit containing about 32% ethanol (v/v) [5]—and meat-drying methods. Hydrochloric acid (MCI) is used in foods such as tomato sauce and neutralized to salt (NaCl) with sodium hydroxide (NaOH).
Samples of beef (200 g) from selected prime or retail cuts of the same steer were immersed in the preservative solutions for 12 hours at ambient temperature (21-25°C). Excess solution was allowed to drip off, and the samples were dried for three days, either directly in the sun or in a simple solar drier box (Figure. 2). They were then packed in polyethylene bags (200 gauge) and stored at ambient temperature for four weeks.
Evaluation of meat quality
Gas formation in the package, moisture formation, discoloration, and the development of odour were assessed visually and by smelling.
The weight loss of the samples was recorded during the storage period.
Water activity was measured with a Durotherm AwWert Messer (Germany).
Extract release volume (ERV) was measured according to the procedure of Wilson et al. [6].
The rehydration percentage was calculated on the basis of the weight gained through water uptake after immersion of meat samples in water at 50°C for three hours.
The pH was measured with a pH meter (Pye Unicam model 290 MK2) after macerating a sample and titrating it with 20 ml of distilled water.
Microbial counts
Total viable microbe counts were determined by mixing a 10-g meat sample with 90 ml of peptone water, plating using plate-count agar, and incubating the plates at 30 °C for three days. Yeasts and moulds were determined by plating using potato-dextrose agar and incubating at 25°C for five days. Lipolytic counts were established by the American Public Health Association method [7].
Organoleptic evaluation
Meat samples of 50 g were cut into pieces and fried in commercially available cooking fat (kimbo). Tap water (100 ml) was added to make a soup, facilitating distribution of the flavour. Nine panellists evaluated the cooked samples, using an eight-point ranking scale: 8, extremely acceptable; 7, very acceptable; 6, moderately acceptable; 5, slightly acceptable; 4, slightly unacceptable; 3, moderately unacceptable; 2, very unacceptable; and 1, extremely unacceptable. Meat preserved by deep freezing was used as a reference sample.
