Contents - Previous - Next

This is the old United Nations University website. Visit the new site at http://unu.edu



Preservation of beef using bacteriostatic chemicals and solar drying


S. K. Mbugua and E. G. Karuri

 


Results and discussion


Treatment with acids and ethanol

Table 1 shows the spoilage characteristics of meat dried directly in the sun or in a solar drier with no other treatment and after treatment with vinegar or 1 N HCl.

TABLE 1. Characteristics of untreated meat and meat treated with vinegar or hydrochloric acid, dried directly in the sun or in a solar drier and stored for up to one week

Treatment

Gas

Odour

Drip

Surface colour

ERVa (%)

pH

Frozen normal 96 5.8

Direct sun drying

Untreated + H2S + green/black 40 7.0
Vinegar dark brown 55 5.2
Hydrochloric acid dark brown 52 3.8

Solar drier

Untreated + H2S + green/black 10 7.1
vinegar dark brown 57 5 4
Hydrochloric acid dark brown 5 3.0

Minus sign (—) = no change in quality; plus sign ( + ) = very positive formation. H2S = hydrogen sulphide odour.
a. Extract release volume.

The untreated samples dried in a solar drier showed signs of spoilage on the second day; those dried directly in the sun spoiled after three days. It appears that the solar drier accelerated spoilage because it produced higher air temperatures (50-70 C) than sun drying. The spoilage in both cases was characterized by the development of gas, with a hydrogen sulphide odour, and excessive dripping and maceration, a sign of impending putrefaction. Both samples developed greenish and black spots and a foul smell in three to six days. In addition, the meat attracted a lot of flies.

The low ERV of the untreated samples—40% for those dried directly in the sun and 10% for those dried in the solar drier, compared with 96% for the deep-frozen reference sample—showed extreme spoilage. The ERV indicates the degree of proteolytic degradation of the meat, which increases the water-soluble proteins, thus restricting the ERV [6]. The higher temperatures in the solar drier increased meat proteolysis; hence the lower ERV [1]. The alkaline pH of 7.0-7.1 also indicates excessive proteolysis and breakdown of amino acids.

The samples treated with vinegar or HCl were successfully preserved by drying. After being stored for a week, they showed no gas formation and no hydrogen sulphide odour, and had ERV values ranging from 52% to 57%. The surface colour became dark brown, as would be expected due to the formation of metamyoglobin from the oxidation of myoglobin. On cooking after elusion of residual acidity and rehydration, however, the samples treated with HCl disintegrated and turned into a broth. Vinegar, on the other hand, did not turn the meat into liquid on cooking, but made it tender. Further studies with HCl were therefore discontinued, but vinegar was included in other studies along with the other bacteriostatic reagents.

Chang'aa was ineffective in achieving chemical stabilization, as the meat started developing a foul odour within 24 hours of soaking. Further experimentation with ethanol was therefore also discontinued.

 

Treatment with brine, honey, and glycerol

Treatment with brine, honey, or glycerol as a bacteriostatic agent protected the meat from spoilage while drying (table 2).

TABLE 2. Quality of dried meat treated with brine, honey, or glycerol, after four weeks' storage (characteristics assessed by sight and smell)

Treatment

Gas

Odour

Drip

Surface colour

Frozen normal

Direct sun drying

Brine cooked meat
Honey + fermented dark brown (white spots)
Glycerol dark brown

Solar drier

Brine cooked meat
Honey dark brown (white spots)
Glycerol dark brown

Minus (—) and plus( + ) signs as in table 1.

No changes were noted in terms of gas formation, odour, or drip formation in the samples treated with brine or glycerol, which effectively arrested spoilage, whether the meat was dried directly in the sun or in a solar drier.

The solar drier appeared to be more effective than direct sun drying for meat treated with honey. Gas was formed and a fermented odour developed in the sun-dried honey-treated samples. The spoilage was due to the growth of yeast, which did not occur with the solar drier. This can be explained by the high temperatures achieved by the solar drier (> 50 C) and perhaps a higher rate of drying, which are not conducive to yeast growth.

All the samples changed surface colour. Those treated with brine turned to the colour of cooked meat, while those treated with honey or glycerol became dark brown.

Weight loss, moisture, and water activity

Samples treated with brine, honey, or glycerol and dried in the solar drier lost 43%-54% of their weight, compared with 42%-51% for samples treated similarly and dried in the sun (table 3). The differences in weight loss might not appear significant considering that their measured moisture contents ranged from 12% to 43% and 18% to 44% respectively for the two drying methods. However, given the differences in the drying conditions, especially with respect to temperature and air flow, the weight loss of the samples dried in the drier may not all be attributable to moisture loss. As shown in figure 3 (see Figure. 3. Adsorption isotherm for air-dried beef at 20C), the water activity in air-dried beef corresponding to the lowest 12% and 18% moisture content of samples at 20C would be approximately 0.63 and 0.75 respectively [8]. However, the measured lowest water-activity values corresponding to 12% and 18% moisture level were 0.87 and 0.83 respectively. This inverse situation confirms that the weight loss in the drier-dried samples was due to the loss of other materials, most likely volatile substances, in addition to moisture. This is likely, considering the high temperatures of 50-70 C in the solar drier, where the air flow is more restricted than in open sun-drying conditions.

TABLE 3. Quality of dried meat treated with brine, honey, or glycerol, after four weeks' storage (measured values)

Treatment

Weight loss (%)

Moisture content (%)a

Water activity

ERV (%)

Rehydration (%)

pH

Frozen

0

79

0.96

82

 

5.5

Direct sun drying

Brine

42.5

44

0.85

24

91.3

5.8

Honey

43.7

28

0.91

48

63.5

5.2

Glycerol

50.7

18

0.83

60

70.0

6.0

Solar drier

Brine

43.5

43

0.89

68

70.0

5.7

Honey

53.0

19

0.92

36

57.0

5.2

Glycerol

54.0

12

0.87

48

52.0

5.5

a. Wet-weight basis.

Water activity is a more accurate measure for predicting the ability of microbes to grow in food than moisture content [3]. Glycerol treatment produced the lowest water-activity values, followed by brine and honey; nevertheless, brine is known to be a better preservative than glycerol at any given water-activity level [8].

Many bacteria, including those that cause spoilage and pathogens, grow most rapidly at water-activity levels in the range of 0.995-0.980, but only at near-optimum temperatures [3]. Below this range, and especially at nonoptimum temperatures, microbial growth is severely hindered. Yeasts, especially the xerophilic types, can grow at a water-activity range as low as 0.85-0.60 [3]. Pathogens can remain viable for long periods under these conditions but cannot grow. The yeasts that grew in the honey-treated, sun-dried samples where water activity was 0.91 may have been of the xerophilic types naturally present in honey. That the yeasts failed to grow in the honey-treated samples dried in the solar drier, with 0.92 water activity, however, was probably due to the high temperatures in the drier, which killed them.

Microbial counts

Table 4 shows very low microbial counts and levels of growth during three weeks of storage. The counts were well below the 10(6)/cm considered critical for meat spoilage [2]. The slight rise in the total viable microbe count in the second week could be attributed to sampling error and aerobic contamination during the analysis, since different sets of samples were packaged and analysed separately after different periods of storage.

TABLE 4. Microbiological status of dried meat after one, two, and three weeks of storage

Treatment

Colony-forming units (cfu) per gram

Total viable

Yeasts and moulds

Lipolytic

1

2

3

1

2

3

1

2

3

Brine(20%)

12

15

ND

12

7

ND

4

3

ND

Honey
100%

300

230

ND

300

300

ND

ND

ND

ND

75%

300

29

30.177

300

29

30.177

ND

10

ND

Glycerol

16

78

ND

ND

ND

ND

12

20

ND

Vinegar

14

ND

ND

ND

ND

2

2

ND

ND

1, 2, and 3 in the column headings indicate weeks of storage.
ND= not detected.

However, when honey diluted with 25% water was used, a step aimed at reducing the cost of the honey, the total viable count, mainly yeasts, increased to a high level of about 10(7)/g toward the third week of storage. As already noted, these yeasts are likely to be the natural xerophilic types in honey. Their uneven inoculation from honey onto the meat cut for experimentation could account for the difference between the counts in weeks 1 and 2.

Very low lipolytic counts were detected, and they showed no evidence of growth during storage.

Other measurements

The ERV values of the samples could not assist in comparing the effectiveness of direct sun drying and the solar drier, as the sun-dried samples treated with brine had much lower ERV values than those dried in the drier, while the reverse was true for the samples treated with honey or glycerol.

Samples dried in the solar drier had relatively lower rehydration values and poorer rehydration properties than those dried directly in the sun.

Neither pH nor the performance of the solar drier over direct sun drying could assist in predicting any change due to spoilage.

 

Organoleptic evaluation

Samples treated with 10% brine, 100% honey, glycerol, and vinegar—all dried in the solar drier—and frozen meat were evaluated by nine panellists (table 5). The data were subjected to a two-way analysis of variance to test for the consistency of the panellists and the treatments. Both variables were significant at p <.01 using the F test. Separation of the panellists into consistent groups by Duncan's multiplerange test showed that one panellist consistently scored the meat higher than the others. Accordingly, that person's data were omitted. On further analysis of the remaining data, the treatments still had significantly different scores.

TABLE 5. Organoleptic scores of cooked samples of preserved meat

Treatment

Average

score

Acceptance

rank

Brine (10%)

2.50c

4

Honey (100%)

2.04c

5

Glycerol

2.73b

3

Vinegar

3.00b

2

Frozen

4.63a

1

Data based on eight panellists.
Scores with different superscripts are significantly different at p < .01.

According to Duncan's multiple-range test, the frozen meat was the best, followed by the vinegar and glycerol-treated samples, which were about equally well accepted. The brine- and honey-treated samples were also accepted equally but ranked last. As noted, vinegar had the effect of tenderizing meat. Honey, on the other hand, imparted an unusual fermented off-flavour, which could explain its low score. The major problem with all the treated samples compared with frozen meat was a loss of meaty flavour.


Conclusion


Beef cuts can be preserved equally well using a solar drier or by direct sun drying, provided they are protected from microbial and biochemical deterioration with bacteriostatic chemicals, particularly those that lower water activity. Vinegar, brine, and glycerol were equally effective in protecting the meats, but brine and vinegar are more cost effective than glycerol. The preserved meats tend to lose the meaty flavour that is preserved in frozen meat, a subject for further investigation.


Acknowledgements


The authors thank the Food and Agriculture Organization of the United Nations, which provided funds for the research. Appreciation is also extended to Jane Njenga of the Department of Food Technology and Nutrition of the University of Nairobi, who carried out most of the analyses and experiments.


References


  1. Kaundia TJ. Extension of shelf life of fresh meat and selected meat products without refrigeration. Master's thesis, Department of Food Technology and Nutrition, University of Nairobi, 1987.
  2. Brown MH. Meat microbiology. London: Applied Science Publishers, 1982:13-67.
  3. International Commission on Microbiological Specifications for Foods. Microbial ecology of foods: Vol 1. Factors affecting life and death of microorganisms. New York: Academic Press, 1980:92-111.
  4. Karmas R. Fresh meat technology. Park Ridge, NJ, USA; London: Moyes Data Corporation, 1975.
  5. Nout MJR. Aspects of the manufacture and consumption of Kenya traditional fermented beverages. Doctoral thesis, University of Wageningen, Netherlands, 1 976.
  6. Wilson NRP, Dyett EJ, Hughes RB, Jones CRV. Meat and meat products: factors affecting quality control. London: Applied Science Publishers, 1981.
  7. Speck ML. Compendium of methods for the microbiological examination of food. Washington, DC: American Public Health Association, 1976.
  8. Iglesias HA, Chirife J. Handbook of isotherms: water sorption parameters for food and food components. New York: Academic Press, 1982.
  9. Karel M, Fennema OR, Lund DB. Principles of food science: Part 11. Physical principles of food preservation. New York; Basel: Marcel Dekker, 1975.

Contents - Previous - Next