Comparative assessment of drying methods, curing agents and meat-cut size on the quality and preservation of meat under rural setting perspectives

R N Ryoba, V R Muhikambele, L A Mtenga and D M Mgheni

Department of Animal Science and Production, Sokoine University of Agriculture, P.O. Box 3004, Morogoro, Tanzania.
rryoba@suanet.ac.tz   or   ruth.ryoba@gmail.com

Abstract

Effects of meat drying methods, meat cut sizes and curing agents on meat preservation characteristics in rural areas of Morogoro were assessed. Meat steak (24 kg) was divided into 3 parts of 8 kg each and was cut into sizes of  2x2x16 cm  (size 1), 4x2x16 cm (size 2) and 4x4x16cm (size3) with the respective size combination being   width, thickness and length respectively. Half of each size was “ cured” with brine solution for 16 hours and the other half was “uncured”. The brine solution used to “cure” the meat contained 5% table salt (naCl), 0.01% sodium nitrate and 0.01% sodium nitrite. Cured and uncured meat samples were dried in a kiln dryer, a modified charcoal stove based on locally produced charcoal and a solar dryer. The samples were dried while monitoring the weight at 3 hours interval up to attaining constant weights. Dried meat samples were sub-sampled for the assessment of moisture content and some were left under room temperature for monitoring shelf life and mould development. The rest of the samples were wrapped in clean aluminium foil and kept under cool (5 ⁰ C) environment while physical, chemical, microbiological and sensory evaluations were done immediately.

Results showed that curing had no effect on meat drying time using the three drying methods. Average drying times were 31, 28 and 36 h for kiln, modified charcoal stove and solar dryer respectively. Pooled moisture content  in the died meat regardless of curing status were 27-40%, 20 -24% and 12-20% for kiln, modified charcoal drier and solar dryers respectively. Both cured and uncured dried meat samples had no total bacterial and coliform growth regardless of the drying method and sizes of meat cuts. However, there were significant differences in sensory attributes for  colour, flavour, texture and general acceptability between cured and uncured samples,  methods of drying and meat sizes. Cured samples of size 2, dried in the kiln, ranked highest in sensory attributes. Meat dried in the solar dryer had the longest shelf life (>6 months) while kiln dried meat had the shortest shelf life (3 months). The decrease in percentage crude protein in cured samples implied that curing has a negative effect on protein content in the dried meat. It is concluded that meat can be preserved with sustained quality by drying in a kiln, modified charcoal stove or solar dryer with or without curing it using meat cuts size of 4x2x16cm.  

Key words: drying, kiln, meat, smoking, solar

Introduction

Fresh meat constitutes the greatest bulk of meat consumed in Tanzania (MALD 1996). However, fresh meat deteriorates quickly due to high moisture content, which is a suitable environment for microbial growth. Modern methods used in meat preservation are refrigeration, freezing, dehydration and irradiation with gamma X-ray (Forrest et al 1975) but these methods need availability of electricity. Some chemicals like curing agents, compounds of smoke and sausage ingredients are added to meat to provide some softening, colour, flavour and preservative effect on meat. Preservative action of each method is accomplished by restricting or in some completely inhibiting microbial activity, enzymatic, chemical and physical reactions that cause deteriorative changes and spoilage of meat. Preservative action of drying reduces the water activity (a_w) to such low level that microbial growth is inhibited and the meat products are stable without refrigeration.

Modern storage facilities are non existent in most rural areas and drying in the sun or over the fire dates back to prehistoric times leading to poor quality products with low shelf life. Libby (1975) indicated that farmers in villages continuously encounter both financial and nutritional losses through meat spoilage. In the rural areas of Tanzania, meat is always sold as hot carcass and cooked the same day to avoid spoilage. In addition, livestock keepers in Tanzania seldom slaughter their animals for home consumption or local markets. MALD (1996) reported that there are very few slaughter sites and meat shops in rural areas. This makes meat consumption in traditional livestock areas very low (Mtenga et al 1998). One way to promote meat consumption in rural areas is adding value to the product by proper preservation methods, making meat to be available for home use for a long time. In addition, livestock keepers close to the main roads can create a meat market for passengers in buses and lorry transporters through selling the preserved meat. There is therefore a need to study locally modified preservation technologies, which are sustainable and practically suitable for conditions in the rural areas. 

Kiln, modified charcoal stove and solar dryers are some of the thermal preservation techniques that can be used in drying and preservation of meat in rural areas. They are cheap and affordable techniques. Technically, these methods reduce fresh meat bulkiness as the meat looses its water content. Reducing water in the meat reduces the rate of its spoilage since excess water, which is often accompanied by high microbial activity promotes the spoilage of meat (putrefaction) in unpreserved meat (Libby 1975). It is apparent that effectiveness of drying of meat depends on the size (width x thickness x length) of the meat cut, whereas acceptability of the meat may be influenced by the drying method and whether the meat has been cured or not (Forrest et al 1975). In Tanzania, however, there is limited documented information on effectiveness of these meat drying techniques (JICA 2001). This study was therefore conducted to assess the effectiveness of three different drying methods as meat preservation methods on cured or uncured meat pieces cut into different sizes. Evaluation of meat quality was done by determining the shelf life, drying time, moisture content, microbial count, sensory assessment and % protein content of dried meat.

Materials and methods

Location of the study

The study was conducted at Sokoine University of Agriculture (SUA) in the Department of Animal Science and Production. 

Materials

A modified charcoal stove was designed and made at SUA, Department of Agricultural Engineering (Figure 1); kiln dryer was made at SUA, Department of Animal Science and Production (Figure 2) and solar dryer was obtained from SUA, Department of Food Science (Figure 3). Hard wood shelving was obtained from SUA carpentry workshop whereas charcoal was purchased from Morogoro town market. Fresh beef steak (24 kg) was purchased from the Morogoro Municipal abattoir. 

Sample preparation

The beef steak was from hindquarter legs and consisted mainly of the following muscles: M. biceps femoris, M. vastus lateralis and M. semitendinosus. All separable fat, visible connective tissues and meat pieces whose length was less than 16 cm were trimmed off using a clean sharp knife. Trimmed beef steak (18 kg) was then cut into flat strips of width x thickness x length of 2x2x16 cm (S1), 4x2x16 cm (S 2) and 4x4x16 cm (S 3) respectively, according to FAO (1990) standard procedures on sizes suitable for domestic use. Each lot size (6 kg) of meat was divided into two lots of 3 kg each. The 3 kg from each of the three meat sizes was cured (making a total of 9 kg) using brine solution containing 5% table salt (naCl), 0.01% sodium nitrate and 0.01% sodium nitrite as suggested by Wilson (1981), whereas  the other 3 kg from each meat size (total of 9kg)  was left uncured and used as a control. Curing was done for 16 h (Wilsion 1981, Lawrie 1981).  

Meat drying methods

 The cured meat strips were left to drip off excess curing solution by placing them on a clean perforated sterile surface for 10 minutes before drying. The uncured meat was wrapped in the aluminium foil and stored in the refrigerator (at 5 ⁰ C). The cured and uncured meat lots were further weighed to obtain their initial (fresh) weights of 1 kg each. The meat was then dried either using modified charcoal stove, a kiln dryer, or solar dryer. The drying process was carried out while monitoring weight every 3 h to constant weight. 

i) Modified charcoal stove 

Charcoal was left to burn for one hour to ensure that no smoke was produced. The hot charcoal was placed at the bottom part of the modified charcoal stove (Figure 1). The meat samples of known weight (cured and uncured of different sizes) were then aseptically hooked randomly under a wire mesh covering on top of the modified charcoal stove using clip pins while allowing the meat samples to hang at about 20 cm above hot  charcoal (65-70⁰ C). Weight of meat was monitored every 3 h to constant weight.

ii)  Kiln 

The kiln dryer was filled with charcoal and left for 1 h to allow the heat to spread evenly within the chamber (Figure 2). This was done to allow the hot charcoal to create a hot environment of not more than 70 ⁰ C. Procedure of placing meat samples in the kiln dryer and 3 hours intervals for measuring weights was similar to that of modified charcoal stove. To keep the temperature below 70 ⁰ C while avoiding heavy smoking on the meat, a handful of hard wood shavings were gradually spread over the fire. This was done for 5-10 minutes.  

iii) Solar dryer 

Both the cured and uncured meat samples of different sizes were arranged over solar trays and placed inside the solar dryer whose temperature was maintained at 55 - 60 ⁰ C in the chamber. Samples were left to dry under the sun while monitoring their weights at 3 hour intervals and the pieces of meat were turned to ensure effective drying up to constant weights. 

Experimental design 

 The experiment was carried out in a complete randomized design (CRD) in a 3 x 3 x 2 factorial arrangement with two replicates and comprising of 18 treatment combinations

Storage of samples 

Dried meat samples cured and uncured of different sizes dried by different methods were sub-sampled by randomly picking three meat strips for the assessment of moisture content. Some meat samples were left under room temperature (25⁰C) for monitoring microbial growth with focus on total count and coliform and mould growth. The rest of the samples in their respective categories were wrapped in clean aluminium foil, labelled and kept under cool environment (5⁰C). Physical, microbiological, chemical, and sensory evaluations were done immediately.  

Meat assessment for various parameters 

Physical status of dried meat

Shelf-life determination of dried meat samples stored in dry polythene bags and left at room temperature (25⁰C) was monitored weekly for 6 months to look for indication of mould growth. 

 Microbial status assessment

Total bacterial and coliform bacterial counts (cfu/ml) were determined according to IDF Standard procedures namely 100 B (IDF, 1991) and IDF Standard 73 A (IDF, 1985) respectively. 

Chemical analysis

Moisture content and crude protein (CP) contents were determined according to AOAC (1990) standard procedures.  

Sensory evaluation

Dried meat samples (of 1-cm size) from each of the 18 experimental treatment combinations (Table 1) and the corresponding replicates were randomly drawn and served to 20 panellists from the Departments of Food Science and Technology and Animal Science and Production at the Sokoine University of Agriculture.  Samples were evaluated for colour, flavour, meat texture, and general meat acceptability. For each test parameter, a four point hedonic scale according to Watts et al (1989) for  four descriptive assessments: Like very much = 4, Like moderately = 3, neither like nor dislike = 2, and Dislike = 1 was used. 

Statistical analysis 

The microbial assessment data was analyzed using General Linear Models (GLM) procedures of   Statistical Analysis System (SAS, 2001). Sensory evaluation data were analyzed by descriptive statistics and subjected to Chi-square test. In addition the MANOVA option of SAS was used to compute partial correlation coefficients among the sensory attributes. 

Results

Mould growth as index of shelf life stability

Table 2 shows that cured meat samples from all drying methods had longer shelf life than the uncured ones which developed some fungal and mould growth after three months. Meat size 3 developed fungus growth earlier than others. Solar dried meat had the longest shelf life compared to the other drying methods. Uncured meat sample size 3 was first to develop mould growth in the third month followed by size 2 in the fourth month. Size 3 charcoal modified stove cured meat, grew mould in the sixth month while kiln cured meat had mould growth from the fifth month.  

Effect of size, curing and meat drying method on meat drying time and weight loss 

Table 3 shows that curing had no significant effect on drying time and weight of dried meat samples. Similarly size of meat pieces had no significant effect on the drying time. As expected the differences in meat size were reflected in the final weight where size 1, 2 and 3 weighed 42.37, 78.17 and 155.82 g respectively. Nevertheless, the three drying methods had a significant effect on the drying time for which solar drying was relatively inferior for it had the longest drying time. Although modified charcoal stove method took the shortest time to dry the meat, it was statistically similar to the kiln dryer.

Trend of moisture loss and time taken to reach constant weights for meat samples of different sizes  S 1,  S 2 and  S 3 for cured and uncured samples  

Figure 4  shows that meat samples started attaining their constant weights at 36, 48 and 66 h of drying.  Cured meat attained their constant weights earlier than uncured meat samples of the same size.

Figure 4: Trend of moisture loss with time taken to reach constant weights for meat samples (size   S 1),  cured and uncured, in stove, kiln and solar dryer

Fig. 5 shows that solar dried samples had the lowest moisture content followed by the modified charcoal stove dried samples for both cured and uncured and for all piece sizes. As far as meat size is concerned, size 3 tended to retain the highest proportion of moisture content for both cured and uncured samples.  

Figure 5:  Effect of curing, size (S) and method of drying meat on moisture content at the end of drying

Microbiological assessment 

Results on the microbiological quality of fresh meat samples had 29 cfu/ml and 5 cfu/ml of total bacterial counts and coliform counts respectively. After drying the meat, bacteria and coliform were not detected in meat of all sizes, curing status and drying methods.  

Sensory evaluation 

Table 4 shows that cured and uncured meat samples were different   regarding sensory attributes for which cured meat samples had relatively higher preference scores than those of uncured meat.  With the exception of texture, size of the meat cuts had no effect on the rest of the sensory attributes. Nevertheless, these were influenced by the drying method.  For all the attributes, samples dried in the kiln had the highest sensory scores followed by modified charcoal stove and least scores were associated with solar drying.

Table 5 shows that for the colour attribute, there were interactions between curing and size, curing and method of drying and between curing and size and method of drying, implying that ranking the scores for either of these treatments would depend on the level of the other treatment(s).  Interactions were also observed for texture attribute where ranking the different sizes would depend on whether the meat was cured or not. 

There were significant correlations among the different sensory attributes (Table 6). 

Treatment effects on the proportion of crude protein in the dried meat samples 

Results showed that uncured meat samples had higher % CP than the cured meat samples. Size of meat also had an effect on % CP where size 1 ranked highest followed by size 3 which was not different from size 2. Meat samples dried in the modified charcoal stove had highest % CP, followed by the solar dried and least % CP in the kiln dried meat (Figures 6a, 6b).

Figure 6a. Method of drying on % CP in meat samples for cured meat of different sizes (S)	Figure 6b. Method of drying on % CP in meat samples for uncured meat of different sizes (S)

Discussion

Shelf life 

Longer shelf life obtained from cured compared to uncured meat, could be due to the preservative effect of the curing process. In addition, the raised shelf life in cured meat could also be due to the combined effect of both salt and smoke constituents especially for meat that was dried in the kiln and modified charcoal stove using charcoal. Smoke is known to have an antimicrobial and antioxidant activity on meat (Issenberg et al 1971). Smoke has been reported to cause partial surface dehydration of food (Chen La-Bo and Issenberg 1972) indicating that proper smoking of meat can be another effective preservative method. The fungal development on the uncured dried meat was probably due to the hygroscopic character of the meat that absorbed moisture from the air during storage which favoured fungal growth. Meat dried in the kiln and meat size 3 had higher moisture content compared to other drying methods and sizes that favoured mould growth. Similar findings have been reported by Forrest et al (1975) for which residual moisture in dried meat favours fungal and mould growth. The authors also reported that mould cause considerable problems in meat packing particularly if the meat is improperly handled and failure to adhere to sanitary practices. It is suggested that addition of antioxidants during packing of dried meat in the manner that eliminates oxygen and moisture prevent mould and fungus growth (Forrest et al 1975). Moisture absorption into the meat could have happened in this study because every week the samples were opened to check mould growth. In this study, the type of mould that grew on dried meat during the 3 months of storage was not investigated. Harry et al (1971) showed that the microflora of cured meats are different from those of uncured meat.  

Effect of drying methods, meat size (slice thickness) and curing on drying time and weight loss 

The differences observed in drying time between drying methods were probably due to sizes and differences in heat intensities in the drying systems. For example, the solar dryer had the longest drying time (up to 78 h) for attaining constant weight since it only depended on the intensity of sunshine. The modified charcoal stove dried samples had the shortest time of drying (up to 54 h) while the kiln dried meat for 66 h. Both kiln and modified charcoal stove used charcoal to generate heat which is obviously more intensive than that with a solar dryer and made it necessary to add some hard wood shelving to reduce the heat intensity in the kiln. The modified charcoal stove had a bigger vent allowing more ventilation than the kiln dryer, hence the shorter drying time of the former compared to the latter.

Low moisture content of dried meat from solar dryer for every size slice thickness was probably due to taking relatively longer time to dry prior to attaining constant weight. Due to unpredictable and fluctuating weather conditions during solar dehydration the time taken to complete the drying process is relatively long. Highest moisture content in samples from the kiln dryer than all other methods was probably attributed to slow ventilation since the kiln dryer had a small vent. Walter et al (1970) reported that during hot drying, meat shrivels considerably and acids from smoke cause meat protein denaturation to form the skin that prevents moisture removal from the meat surface. Similar trends were noted in the current study the light smoking was affected in the kiln dryer so as to reduce the heat build up.  Forrest et al (1975) reported that in meat drying, factors like particle size, temperature and rate of air movement must be carefully controlled. In this study only the kiln dryer had its temperature regulated to lie within the range 65-68⁰ C and not to exceed 70⁰ C while the solar dryer was regulated to 55 and 60⁰ C was maximum. 

Microbial analysis 

The major difference in total bacterial counts between fresh and dried meat samples confirmed fresh meat is not hygienic compared with the treated one (Libby 1975; Maeda et al 1997). The low bacterial counts in cured kiln and modified charcoal stove dried meat, is convincing evidence that there is combined effect of microbial destruction, curing and heat treatment. Smoke which has been reported to have germicidal effect on bacteria, could have prohibited bacterial growth regardless of its relatively high moisture levels in a kiln dryer (Libby 1975, Burt 1988). The absence of bacteria and coliform counts in the solar dried meat was to a greater extent attributable to very low moisture content.  

Sensory evaluation 

The sensory acceptability of cured meat dried using kiln and modified charcoal stove agrees with  findings by Chen La-Bo and Issenberg (1972) who showed that light smoke is primarily applied for colour and flavour improvement  and these being viewed as primary attributes that render meat acceptable. While the sun in the solar dryer just dries the meat, the kiln dryer had in addition to drying, a cooking effect due to the slow ventilation through a small vent. The meat from the kiln dryer became tenderer compared to solar dried meat. Moreover, meat dried by a solar dryer was least accepted probably due to the evidence by Lawrie (1981) that unsteady temperature ranges of solar drying cause significant meat shrinkage and surface hardening, thus reducing its consistency, tenderness as well as acceptability to consumers. Lavie (1970) and  Forrest et al (1975) reported that the combined effect of curing and smoke improves the meat colour due to the added nitrates and nitrites in the curing solution which fixes the red colour of the meat. The acceptability of cured meat visa a vis uncured was probably attributed to improved flavour brought about by the curing solution. Further more, curing process had an apparent improvement on meat texture which was portrayed by high scores from sensory panel. Higher scores in sensory attributes in the meat sizes of width x height of meat, size 2 provided more surface area for curing agents uniform incubation and heat penetration compared with slices with sizes 1 and 3.   

Effect of treatments on CP content in the dried meat 

The remarkable decrease in protein content when meat was cured indicated that curing has a negative effect on total protein. Chen La-Bo and Issenberg (1972) reported losses in available lysine in cured lean meat smoked for 10 h and uncured meat were 44.5% and 15.2%, respectively. The protein could have been digested and released in a form of ammonia resulting in losses of protein content in the cured meat. They theorized that there is a probable reaction between protein amino groups and acids in the smoke whose impact was the observed decrease in protein. This implies that curing was associated with protein denaturation. The higher protein denaturation in size 2 could have been due to more intense curing on the sample which is brought by it having the highest surface area of absorbing curing solution which leads to more CP denaturation than in the others thickness sizes. The kiln dried meat had lower CP than by modified charcoal stove dried meat and again this could be attributed to the dehydration effect arising from smoke acid interaction within the modifies charcoal stove (Forrest et al (1975).  

Conclusions and recommendations

From this study, it can be concluded that:

• All the studied drying methods are suitable for preservation of meat under Morogoro and other regions in Tanzania that have similar conditions. However, the modified charcoal stove took (28 h) the shortest time to dry the meat followed by the kiln dryer (31h) and finally the solar dryer (36 h).

  
  

• Curing of meat results in raising its shelf life and sensory attributes but reduces its crude protein content. This may suggest that where maintenance of high protein content is desired, curing should be minimized. A compromise should be drawn between preservation of the product and its quality at the time of its consumption.

  
  

• Cut size of 2 (4x2x16 cm) was found to be the best meat cut for all the drying methods.

  
  

• The authors recommend further research on the economics of the preservation methods adopted in the current investigation, measure values of external factors like ambient temperature, solar radiation and indicative biomass energy on the preservation of meat.

Acknowledgement

The authors would like to acknowledge the financial support from the TARP II SUA project funded by NORAD.

References

AOAC 1990  Association of Official Analysis Chemists. Moisture and Nitrogen content determination in meat. Official Methods of Analysis 15 ^th ed. Virginia. Pp 931- 937. 

Burt  J B 1988 Fish Smoking and Drying Effect of Smoking and Drying on the Nutritional Properties of Fish. Elsevier Applied Science, London. Pp 166. 

Chen La-Bo and Issenberg  P 1972  Interactions of some wood smoke components with amine groups in protein. Journal of Agriculture Food Chemistry 20 (6):1113- 1115. 

FAO 1990  Manual on Simple Methods of Meat Preservation, FAO, User’s, guide Rome http://www.fao.org 

Forrest  J C,  Aberle  E D, Hedrick  H B, Judge  M D  and Merkel  R A  1975  Principles of meat science. Freeman and Company USA. Pp 257-288. 

Harry  H W, George  J M, and Wilbur  A G 1971 Practical food microbiology and technology. The Avi Publishing Company, INC. Pp 176-288. 

JICA (Japan International Cooperation Agency) 2001 Preservation techniques of meat and meat products. Pp 52. 

IDF 1985  Enumeration of coliform. Colony counts technique a most probable no. At 30^o  C International Dairy Federation User’s guide, Brussels. 

IDF 1991 Enumeration of standard colony counts. Colony counts technique a most probable no. At 30^o C International Dairy Federation User’s guide Brussels. 

Issenberg  P,  Kornreich  M R and Luster  A O 1971 Recovery of phenolic wood smoke components from smoked foods and model systems. Journal of Food Science 36:107-109. 

Lavie  A 1970  The Meat Hand Book (3^rd Edition). AVI Publishing Co, West Port, Connecticut. Pp 332. 

Lawrie  R A 1981 Meat Science (4^th Edition. Percamon Press, New York. 

Libby J A 1975 Meat Hygiene (4^th Edition).Lea and Fibiger. Washington Square, Philadelphia. Pp 658. 

Maeda  G E, Peter J, Mtenga  L A and Nnko  S M 1997  Significance of condemnation of edible offal from sheep and goat slaughtered at Vingunguti abattoir, Dar Es salaam, Tanzania. Tanzania Veterinary Journal.  Pp17:98-109. 

MALD  1996  Ministry of Agriculture and Livestock Development. Preliminary Reviews of the Livestock Industry: Marketing Development Bureau Dar es salaam. 

Mtenga  L A, Makungu  R J, Muhikambele  V R, Maeda  G and Nnko  S A 1998  Rural Appraisal Study of Morogoro Urban and Dar es salaam City: Evaluation of Slaughter Systems and their Implications 0n Meat Loss, Hygiene and Marketing. SUA-NORAD project TAN 091 Publication No1 pp 47. 

SAS 2001  Statistical Analytical Systems. User’s guide, Ver. 8.2.NC USA: SAS Institute, INC. Pp 3917-2200. 

Watts  B M, Ylimaki  G L, Jeffery  L E and Elias  L G 1989  Basic sensory methods for food evaluation. International Development Research Centre, Ottawa. Canada. Pp 47- 104.

Walter  F, Robert  C D  and Aaron  E W  1970  Composition of ether-soluble fraction of a liquid smoke solution. Journal of Food Chemistry (2) pp 18.

Wilson  N R P 1981. Meat and Meat Products. Applied Science publishers, London. pp 660.

Received 8 July 2012; Accepted 16 December 2012; Published 4 January 2013

Go to top