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A comparative study of shrimp feed pellets processedthrough cooking extruder and meat mincer
Abstract：
Performance of three types of shrimp feed pellets, processed through cooking extruder, meat mincer and pellet mill (commercial diet served as the control), was compared by studying the results on proximate composition, physical properties and growth of Penaeus monodon F. juveniles in aquariums. A single screw variable length cooking extruder, designed and fabricated at the Institute, was used to extrude the feed mix dough under optimal conditions of screw speed (92 rev min 1), L:D ratio (14), barrel temperature (94.4°C) and moisture content of feed mix (35.4%). No commercial binder was used in cooking extruder and meat mincer. The commercial diet was found to be most water stable among the three. There was significant difference between the water stability of extruder pellet and commercial diet only at 30 min of immersion period at PB0.05 and PB0.01 for remainder of the period up to 4 h. Whole wheat flour, used in the formulation might have contributed to water stability of extruder pellets. Compacted sinking type pellets with minimum expansion ratio and high density was achieved in the extruder run under the optimal processing conditions. Feed conversion ratio (FCR) was essentially the same for the commercial and extruder pellets.
Keywords: Shrimp feed pellet types; Extrusion cooking; Pellet properties
1.Introduction:
By extrusion cooking optimum pellet characteristics, such as water stability and high density, would be achieved if the proper selection of process conditions and inclusion levels of raw ingredients are made (Wood, 1995). Binders such as modified or synthetic substances need not be used in extrusion cooking process to enhance the water stability of pellets, whereas inclusion of such binder is a necessity in pellet milling (Cuzon et al., 1994). Extruders are very costly equipment and need high precision manufacturing technique for fabrication. A laboratory single screw cooking extruder was, therefore, designed and fabricated at the workshop of the Institute and was used to process shrimp feed (Rout, 1997). Preliminary studies on extrusion characteristics of shrimp feed mix, containing alginate as binder, was carried out in this extruder and found that extruded pellets gave better performance than the pellets processed through meat mincer (Gokulakrishnan and Bandyopadhyay, 1995; Bandyopadhyay, 1996). The present investigation was undertaken to study the performance of feed pellets processed through the extruder without using any commercial binder and to compare it with that of pellets processed through meat mincer and of a commercial diet processed through pellet mill.
2. Physical characteristics
Water stability of pellets was obtained as the percentage of pellets retained on wire mesh screen after immersion in saline water for a period of time and drying in air oven. Baskets made of brass wire mesh, 0.5 mm in size, were used for the purpose. Triplicate baskets containing :20 g pellets spread in one layer were 74 R.K. Rout, S. Bandyopadhyay : Aquacultural Engineering 19 (1999) 71–79 immersed in saline water flowing through a tank under mild agitation by air stones. Immersion times varied between 30 and 240 min. Expansion ratio was measured as the ratio of average diameter of dry pellet to the die hole diameter. The results were reported as mean9S.D. of five measurements. The method of pycnometer using toluene was followed to determine the true density of pellets. The values were reported as mean of three replications. Settling velocity was obtained from the time taken by the pellets to fall between two marks, 87 cm apart, in a plexiglass column, 100 cm long and 24.5 cm in diameter, containing saline water. The upper mark was well below the liquid surface, thus ensuring that the terminal velocity was attained by the pellets (Backhurst and Harkar, 1988). The pellets, 3–5 mm long by 2 mm diameter, were dropped in the dry condition, which is usually followed in practical feeding. Each test was repeated ten times and the results were reported as mean.
3. Results
Proximate composition and gross energy of the three feeds are given in Table 2. Statistical analysis of the results (ANOVA) showed that there were significant differences at PB0.05 in all the components except for gross energy. However, the paired t-test showed significant difference at PB0.05 between the extruder and commercial feed but showed no significant difference between extruder and mincer feeds.Expansion ratio, true density and settling rate of the three feeds are shown in Table 3. The density and settling rate were highest for the extruder feed, but the differences were statistically significant at PB0.05. ANOVA and paired t-test showed that true densities of extruder and mincer pellets and of extruder and commercial ones were significantly different (PB0.05), whereas settling rates of extruder and mincer pellets and of extruder and commercial pellets were significantly different at PB0.05 and PB0.01, respectively. Expansion ratio was higher with the extruder feed than with the mincer feed but difference was significant at PB0.01. The average values of pellet water stability of the three feeds measured after 30–240 min of immersion are shown in Fig. 1. The lowest water stability was observed with the mincer feed, whereas the highest was with the commercial one. The ANOVA and t-test showed that there was significant difference (PB0.01) among the feeds throughout the test period. The paired t-test showed significant difference at PB0.05 between the commercial and extruder feeds only at 30 min of immersion and at PB0.01 at rest of the immersion periods.
4. Discussion
Results showed that statistical difference in proximate composition was significant between the commercial feed and feeds processed in the present work due to difference in their formulation. However, there were significant differences in the physical characteristics and growth of animals among the three feeds. The highest water stability of pellet-milled feed among the three types was observed. This was probably due to the use of a special quality binder in the formulation. Comparison of water stability between the commercial and extruded pellets indicated that extrusion cooking might produce good water stable diets with whole-wheat flour as binder, thereby saving the high cost of commercial binding agents. Most commonly used natural binders, such as high gluten wheat flour or cereal starches, require proper control of heat, moisture and time for full activation (Lim and Cuzon, 1994). Ordinarily, mincer type equipment does not provide such conditions. On the other hand, extrusion cooking can produce compacted sinking type pellets by proper feed formulation and selection of extrusion conditions. The extruded feed has higher expansion ratio than mincer feed. This gives an impression that mincer feed is more compact or dense than extruded feed. However, based on density, extruded feed is much heavier. This indicates that mincer feed is loosely compacted. The settling rate appears to correlate well in the density of pellet. Weight gain of shrimp decreased with mincer pellets, but the difference between commercial and extruder pellets was not significant. High water stability contributed to little loss of nutrients with the latter type pellets, whereas in the former type pellets the loss of nutrients was greater. Therefore FCR was the highest with the mincer pellets and was essentially the same with the commercial and extruder pellets, viz. 1.9 and 2.03, respectively. Shrimp feed pellets of desirable characteristics could be achieved by processing the feed mix in the cooking extruder developed by the authors.
References
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