ABSTRACT

Calcium hypochlorite commercially known as bleaching powder is used as a bleaching agent in shrimp processing industries in many countries and known to effect biochemical alteration in shrimp muscle. Studies were, therefore, undertaken to determine their effect in different concentrations viz., 10, 20, 30, 40 and 50 ppm with different time intervals on the quality of head-on, headless shell-on and peeled giant freshwater prawn (Macrobrachium rosenbergii) by determining biochemical and organoleptic aspects. Myofibrillar protein solubility of fresh head-on, headless shell-on and peeled samples were 90.5%, 90% and 88%, respectively indicating a gradual decrease in protein solubility with increasing concentration of bleaching powder. Decrease in protein solubility was also higher in samples kept at longer duration in different concentration of bleaching powder. At a given concentration of 50 ppm for 30 min treatment, the loss of myofibrillar protein was higher (26.14%) in peeled samples than those of head-on and headless shell-on samples (20.44% and 21.11%). Shelf life of bleaching powder treated prawn samples was found to be reduced to 4 - 5 days in iced condition compared to 6 - 7 days for control samples. Peeled samples were also found to be more susceptible to bleaching powder than that of head-on and headless shell-on samples.

1. Introduction

A wide variety of chemicals are used in meat, fish and shrimp processing industries from treatment of water, cleaning and sanitizing equipment and fish working premises to those permissible to be added in food. While calcium hypochlorite [Ca(OCl)₂], sodium hypochlorite, quaternary ammonium chloride are used as disinfectants; ammonium sulfate, sodium tri polyphosphate, sodium carbonate, potassium bicarbonate, sodium acid pyrophosphate and some other chemicals are used for better processing and reducing processing losses during food processing. The USFDA of the United States, EU of the European Union, Food and Drug Regulations (FDR) of Canada are some important regulatory authorities playing key roles in selecting or banning certain chemicals or food additives. Since food safety issue has become a major part in import and export of international food trade, processing industries are compelled to strictly follow guidelines set by the authorities.

Bangladesh has become one of the largest producers of giant freshwater prawn (Macrobrachium rosenbergii de Man) and the product has got higher acceptability and greater demand among the consumer in the international market. However, due to inadequate facilities during harvesting, post-harvest handling and processing, the finished products of Bangladesh sometimes do not satisfy the criteria of standard quality and thus suffer serious losses. Post-harvest losses of prawn have been reported at different stages of handling and transportation [1]. Furthermore, prawn and shrimp muscle are known to contain higher percentage of sarcoplasmic and myofibrillar protein and lower amount of stroma protein, higher percentage of unsaturated fatty acids that are degraded quickly through autolytic, microbial and oxidative spoilage. As Bangladesh is a country with tropical environment and meteorological condition, it’s suitable for quick post-mortem changes in any organism, changes in its protein takes place at a faster rate compared to other temperate countries. Myofibrillar protein that is the form of contraction apparatus and soluble in high ionic strength salt solution and constitute of 70% - 80% of total fish protein. Fish myofibrillar protein contains myosin, actin, paramyosin, tropomyosin and some other proteins. Their properties largely affect the quality of fish flesh and processed meal. The changes of these biochemical parameters in post-mortem fish muscle are closely related to organoleptic changes. On the other hand, the changes of these biochemical parameters in fish muscle are decreased quickly during post harvest chemical treated conditions.

In shrimp processing industries of Bangladesh, prawn and shrimp are treated with various chemicals including sodium tri polyphosphate [2], sodium carbonate, sorbitol, aluminum sulphate, sodium acid pyrophosphate, sodium hexameta-phosphate. All these additives are approved by USFDA and Canadian FDR, and shrimp processors incorporate these additives according to buyer’s demand. Besides these additives, some other chemicals are used in these processing plants which have been banned in foods. One such chemical is calcium hypochlorite which is popularly known as bleaching powder. Calcium hypochlorite [Ca(OCl)₂] is a white or grayish-white powder. The commercial name of calcium hypochlorite is bleaching powder which is mainly used in raw shrimp and prawn mainly to bleach and reduce the number of bacterial load. It is suspected that various biochemical changes also take place in shrimp and prawn muscle including reduction of protein solubility. Denaturation of myofibrillar protein may occur due to use of bleaching powder which might affect the quality of fish/prawn.

Assessing and selecting the quality of fish and fishery products is of great importance to produce a level of quality which will satisfy both the customer and statutory food legislation. Over the years, many different methods of quality assessment have been developed and investigated in an attempt to determine the most suitable index for use in quality control testing. There are a number of chemical methods widely used to assess the degree of freshness of fish. Some important well known methods are determination of NPN value and myofibrillar protein solubility. In post mortem fishes, the muscle undergoes a series of biochemical changes that markedly alter the internal environment of cell and its protein constituents. The most important reaction is the turn over of ATP and denaturation of muscle protein which ultimately effect on gel forming ability of meat paste. Post-mortem changes influence the quality of fish depending on the storage conditions. The changes include physical, biochemical and bacterial. The physical changes are those which are perceived with the senses, i.e. appearance, odour, texture and taste. It has been reported that post-mortem pH fall influences the quality of muscle as meat, particularly in the important characteristics of texture, water holding capacity and myofibrillar protein solubility [3].

The present study reports the changes in quality and shelf life of giant freshwater prawn and bleaching powder treated prawn under three different conditions viz., head-on, headless shell-on and peeled during ice storage condition under different concentration of solution with different time intervals.

2. Materials and Methods

2.1. Raw Material

Live giant freshwater prawn samples with an average body weight 63.6 ± 0.5 gm and size 15 - 17 cm were caught from a private farm of Fulpur Upazila in Mymensingh district and transported to Department of Fisheries Technology, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh in an insulated box with ice (1:1). The samples were processed into three categories which were called head-on, headless shell-on and peeled. One hundred and eight samples were prepared for bleaching powder treatment.

2.2. Bleaching Powder Treatment

Samples were separated into two groups. One group was treated with 5 different concentrations (10, 20, 30, 40, and 50 ppm) of bleaching powder at a 30 min interval and another group was treated with two fixed concentration (20 and 50 ppm) of bleaching powder at different time (10, 20, 30, 40, 50 and 60 min) intervals. Here, 3 samples (head-on, headless shell-on and peeled) were used as control and 54 samples were used for determination of myofibrillar protein solubility. Rest 54 samples were used to evaluate the organoleptic quality of prawn samples.

2.3. Determination of Myofibrillar Protein Solubility

2.3.1. Preparation of Myofibrils

Myofibrils were prepared from prawn muscles immediately after excision according to Perry and Grey [4] with slight modification. The muscle was chopped by a meat grinder and chilled minced muscle (50 g) was homogenized for 1 min in 5 volumes of 39 mM borate buffer (pH 7.1) containing 25 mM KCl and 0.1 mM DTT. The homogenate was centrifuged for 15 min at 600 × g. The residue obtained was again homogenized and centrifuged for 15 min. The light-coloured upper layer of the residue consisting mainly of myofibril was recovered with small volume of 39 mM borate buffer (pH 7.1) containing 0.1 M KCl and 0.1 mM DTT. The suspension was centrifuged for 15 min to remove the supernatant. Myofibrils were diluted with 4 volumes of 39 mm borate buffer (pH 7.1) containing 0.1 M KCl and 0.1 mM DTT, and coarse materials removed by centrifuging at 400 × g. The suspension was centrifuged again for 15 min at 600 × g to sediment myofibril. After the pellet was washed three times in the same way, myofibrils were suspended with a desired volume of 39 mm borate buffer (pH 7.1) containing 0.1 M KCl to make a concentration of 10 - 15 mg/ml.

2.3.2. Myofibrillar Protein Solubility

Two ml of myofibrillar suspensions (5 mg/ml) were homogenized with 2 ml of 1 M KCl plus 100 mM phosphate buffer (pH 7.0) using a homogenizer. The homogenate was allowed to stand at refrigerated temperature (4˚C) overnight. The suspension was centrifuged for 30 min at 400 × g in cool condition. The protein in supernatant was determined by the biuret method [5].

2.4. Organoleptic Quality of Prawn

Organoleptic quality of prawn was also determined by dipping in different concentration of bleaching powder with different time interval. The organoleptic changes of prawn after dipping in 10 ppm, 20 ppm, 30 ppm, 40 ppm and 50 ppm bleaching powder solutions for 30 minutes were not significantly different among different concentration for each category sample (head-on, headless shellon and peeled). Again, the organoleptic changes of prawn samples after dipping in 20 ppm and 50 ppm bleaching powder solutions for 10, 20, 30, 40, 50 and 60 minutes were not significantly different among different time interval. So, from the 1st group of samples soaked in two concentration (20 and 50 ppm) for 30 minutes and from the 2nd group of samples soaked in 20 ppm and 50 ppm bleaching powder solutions for maximum time (60 minutes) were isolated. Then those samples were kept in ice storage for 10 days to evaluate the organoleptic changes such as odor, texture, color (with shell), color of flesh and general appearance according to ECC freshness grade for fishery products with slight modifications [6].

2.5. Statistical Analysis

For the statistical analysis of data, one-way analysis of variance (ANOVA) and Tukey’s Test was performed using the SPSS (Statistical Package for Social Science, version-12.0). Significant results (p < 0.05) were further tested by using Duncan’s multiple range test (DMRT) at 5% level of significance to identify significant differences among means.

3. Results and Discussion

Studies were conducted on the changes in myofibrillar protein solubility in samples treated with calcium hypochlorite and in control samples. In case of fresh headon, headless shell-on and peeled prawn, initial myofibrillar protein solubility was 90.5%, 90% and 88% which decreased gradually with the increasing of bleaching powder concentration and time of treatment (Figure 1). There were mainly two groups of samples which were treated with different concentrations of bleaching powder at different time interval and were obtained different amount of myofibrillar protein.

3.1. Effect of Bleaching Powder on the Myofibrillar Protein Solubility of Prawn

3.1.1. Changes in Myofibrillar Protein Solubility of Prawn by Dipping in 5 Concentrations of Bleaching Powder Solutions for a Constant Time (30 min) Interval

The results of biochemical changes specially myofibrillar protein solubility of prawn by dipping in 10 ppm, 20 ppm, 30 ppm, 40 ppm and 50 ppm bleaching powder solutions for 30 minutes were significantly different for each category sample (head-on, headless shell-on and peeled). Again, these changes of head-on and headless shell-on were nearly similar but significantly different from peeled.

Figure 2 shows that, in 10 ppm calcium hypochlorite solution, the samples soaked either in head-on, headless shell-on or peeled conditions for 30 minutes the protein solubility was decreased into 81%, 80% and 78% which is about 10% less than the original concentration. The solubility was decreasing with increasing concentration