Textural and sensory characteristics of retort-processed freshwater prawn (Macrobrachium rosenbergii) in curry medium

ABSTRACT

Freshwater prawn in curry was thermally processed to three F₀ values of 6, 8, and 9 at 116°C. Total process times for F₀ values of 6, 8, and 9 were 53, 57, and 63 min, respectively. The cook value (CV) obtained at F₀ values of 6, 8, and 9 was 87.53, 107.93, and 117.55 min, respectively. Instrumental texture profile analysis revealed that except springiness, the values of hardness, gumminess, and chewiness decreased as the F₀ values increased. CIELAB values of L*, b*, and a* increased as the F₀ values were increased. The organoleptic evaluation scored the highest for the product processed to F₀ 7 min.

KEYWORDS:

• Retort pouch
• Thermal processing
• F₀ value
• Texture profile analysis
• CIELAB value
• Sensory analysis

Introduction

Thermal processing is considered as one of the most effective means of preserving food,^[1] and canned prawn curry has very good market demand. The technical and commercial feasibilities of using retortable pouches for thermal processing have been proven by various researchers.^[2–5] Retort pouches have several advantages, including shelf stability, lower weight and storage space, ease of opening and preparation, and reduced heat exposure, resulting in improved quality. Furthermore, some of the advantages of retort pouch over cans are the retort pouch-packed product needs significantly less heat than cans to achieve commercial sterility; with cooking time and energy costs reduced by half, heat penetrates the food much more quickly when it only has to reach the inside of a half-inch-thick mass rather than the much thicker mass in a round can. Unlike canned foods, the pouched foods will not be overcooked and softened into mush, ensuring better texture and taste. Most retort pouches are constructed with a 4-ply laminate consisting of polyester outside layer, a nylon second layer, an aluminium foil third layer, and a polypropylene inner layer. The melting point of polypropylene polymers is around 138°C (280°F), which is higher than the commercialized sterilization temperature of 121°C (250°F).

In India, retort pouch processing is gaining popularity over metal containers due to its unique advantages of being cheaper, having a thinner profile than cans and jars, having a shorter processing time, has lesser nutrient loss, has easier opening, the product can be eaten directly from the pouch or served on dishes, takes smaller storage space, and has easier disposal.^[6] Thermal processing is one of the most effective means of preserving food^[1] and has very good market demand. It generally involves heating of foods for a predetermined time at a preselected temperature to eliminate pathogenic microorganisms that endanger the public health as well as those microorganisms and enzymes that deteriorate the food during storage. Today, the consumer demands more than the production of safe and shelf-stable foods and insists on high-quality foods. Various researchers^[2–5] have proven the technical and commercial feasibilities of using retortable pouches for thermal processing.

Reduction of heating time using pouches has been reported by a number of workers.^[7–9] It was reported 34g/100g, 32g/100g, and 37g/100g less processing time for pouched rainbow trout, pollock, and shrimp, respectively, compared to canned products.^[8] In case of chum salmon 48g/100g reduction time was reported for in-pouches compared with cans.^[9] Ali et al.^[10] reported a reduction in processing time with increased rotation for thermally processed tuna in oil in retort pouches. Green beans processed in pouches had better flavour, texture, and overall acceptability than when processed in a can, but the colour of the canned beans was preferred.^[11] Durance and Collins^[9] reported significantly less off-flavour and greater acceptance of chum salmon in retort pouches than cans. Chia et al.^[8] reported that the pouched products were firmer in texture and lighter in colour. Gopal et al.^[12] reported that mackerel fish curry processed in retortable pouches had a shelf life of not less than 12 months at ambient temperature. Canning of Rohu (Labeo rohita) in curry medium using tin free steel (TFS) can has been reported and an F₀ value of 8.79 was found satisfactory.^[13]

One of the main features in appreciating seafood is texture.^[14] Appearance and odour are also very important for consumers. The sensory characteristics and instrumental texture attributes of papain-treated abalones have been reported.^[15] Heating of meat is accompanied by changes in appearance, smell, taste, texture, and nutritive value. Thermal sterilization may affect the sensory and nutritional aspects of fish products. During the heating of meat, numerous volatile compounds are formed that contribute to the meaty flavour. Jarvis^[16] reported that excessive heating of little tuna produced a toughening of the texture. Measurement of sensory properties of different types of fishery products has been reviewed by different authors.^[17,18] Texture profile analysis (TPA), which measures the compression force of a probe and the related textural parameters of a test food during two cycles of deformation of various foodstuffs including fruits, vegetables, bakery, meat products, and seafood, specifically retorted products, has been reported.^[9,15,19,20] Ali et al.^[21] showed an improved texture of canned sardine in oil in pouches compared to cans. Texture has been defined as “the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the sense of vision, hearing, touch and kinesthetic”.^[22] The texture of Indian mackerel deteriorated after thermal sterilization,^[23] whereas Kong et al.^[24] reported heating significantly changed the quality attributes of Salmon muscle, including colour, shear force, cooking loss, and shrinkage. Different attempts were made to improve heat penetration to cans such as agitated retorting, small disk-shaped products, thin profile containers, etc. to reduce the negative impacts of heat treatment on the qualities of foods.^[24,25,26] The first quality impact by which consumers can take a decision to acquire a product is its visual appearance. Colour is a very important quality factor in thermally processed seafood products, since it influences consumer acceptability. Tiger prawn (Macrobrachium rosenbergii) is considered a very valuable fishery product, especially in leading industrialized nations due to its taste in various forms. Texture of the meat products is considered an important sensory attribute as far as consumers’ acceptability is concerned. The present work was carried out to know the effect of thermal processing on the textural and sensory properties of freshwater prawn in curry medium at different F₀ values.

Materials and methods

Processing of prawn

Medium-sized fresh freshwater prawn (Macrobrachium rosenbergii) of size about 14–18 cm (25–30 count/kg) were procured from the local market and transported to laboratory in chilled condition (ice to prawn ratio 1:1). They were washed with chilled potable water, peeled, beheaded, and dressed into small pieces of thickness around 2 cm. The prawn pieces were hot blanched (80 ± 2°C) by dipping in 3g/100g salt solution for 10 min. The blanched prawn pieces were marinated with turmeric powder (50g/100g of the total requirement) for 30 min. The marinated prawn pieces were dried under a fan for 30 min followed by deep frying for about 12–15 s in boiling refined sunflower oil.

Preparation of curry

Ingredients used for the curry preparation are presented in Table 1 and the curry was prepared by the following method. Two pastes, namely ‘green paste’ comprising garlic, green chilli, and ginger, and ‘red paste’ comprising turmeric powder, coriander powder, cumin powder, chilli powder, and salt was prepared. Onion paste was fried in vegetable oil until a brownish colour appeared, followed by the addition of ‘green paste’, and frying continued until the colour turned reddish. The ‘red paste’ was added to the fried ‘green paste’ and heating was continued gently under low flame till the emergence of characteristic aroma. This step was followed by the addition of a required quantity of water to the fried spices and heating at low flame continued till boiling of the gravy.

Table 1. Recipe of the gravy for ‘Prawn Curry’.

Ingredients	Percentage, %	Ingredients	Percentage, %
Bay leaf	1 no.	Turmeric powder	2.0
Onion	15.0	Red chili powder	2.0
Garlic	5.0	Cumin powder	2.0
Green chilli	10.0	Coriander powder	1.0
Ginger	5.0	Oil (veg. refined)	30.0
Salt	3.0	Water	25.0

Retort pouch

Laminated flexible pouch (4-ply), consisting of 12 μm polyester (outer layer), 9 μm aluminium foil and 15 μm nylon (middle layer), and 70 μm polypropylene (inner cast) was used for packing the fish curry. Pouches (150 × 200 mm) with filling capacity of around 300 g used in the present study were purchased from M/s. Pradeep Laminators Pvt. Ltd. Pune, Maharashtra, India.

Overpressure retort

The pilot-scale overpressure retorting unit (Sakara Equipments, Bangalore, India) consisting of retort, boiler, air compressor, centrifugal pump, and the control system (PLC) was used for thermal processing. The unit used in the study is similar to the commercial-scale equipment, which produces a high degree of process reproducibility and accuracy. After processing the pouches to a required F₀ value, they were cooled rapidly to 50 ± 10°C by spraying water under pressure and further cooled in chilled water immediately after taking out from the retort. Then the cooled pouches were dried, labelled, and stored.

Filling and sealing of retort pouch

About 100 ± 5 g peeled, deveined, and flash-fried prawn pieces was packed in retort pouches. Each of the pouch was filled with hot gravy (100 ± 10 g), maintaining a pack weight of about 200 ± 10 g. Care was taken to avoid the contamination of seal area in the pouches. Adequate numbers of retort pouches were fixed with glands and thermocouples and the tip of the thermocouple was inserted into the fish pieces.

The pouches were immediately injected with steam (for about 10 s) to remove the air present in the pouches, followed by sealing with a pneumatic retort pouch sealing machine (Sunray Industries, Mysore, India). The sealed pouches were subjected to thermal processing for optimizing the F₀ value at process temperature. The detailed step for the thermal processing of ‘prawn in curry’ in retort pouch is given in Fig. 1.

Figure 1. Thermal processing of Macrobrachium rosenbergii in curry medium.

Thermal process evaluation

Filled and sealed pouches were heat processed to the required F₀ values. Ellab Eval Flex Four Channel Thermal Validation and Sterilization Monitoring System, Cat. 21401004 (Ellab A/S, Trollesmindealle 25, DK-3400 Hilleroed, Denmark), with Ellab CTF 9004 Precision Thermometer and F₀ value integrator was used to record core temperature, retort temperature, F₀ value, and cook value (CV) at a specific time interval of 60 s. The probes used for the experiments were copper/cupronickel thermocouples (Ellab SSA-12050-G700- TS) of stainless steel electrode with a length of 50 mm and diameter of 1.2 mm. The packing glands (GKJ13009C052) made from 50 mm stainless steel tubes were used for thermal processing experiments. The F₀ constants were programmed at T = 121.1°C, Z = 10°C, and the CV constants at T = 100°C, and the Z value for calculating the C value was 33°C.

Thermocouple outputs (time–temperature data) were analysed using a computer. The heat penetration data were plotted on a semi-log paper with temperature deficit (retort temperature–cold spot temperature) on log scale against time. Lag factor for heating (Jh), slope of the heating curve (Fh), time in minutes for sterilization at retort temperature (U), and lag factor for cooling (Jc) were determined. The process time was calculated by a mathematical method.^[27] Total process time was determined by adding Ball’s process time (B) and the 58g/100g of the come-up time. CV, a measure of heat treatment with respect to nutrient degradation and textural changes that occur during processing, was also determined by measuring the extent of cooking and nutritional loss during processing in a manner similar to the D value, except that the reference temperature is 100°C instead of 121°C, and the Z value is 33°C, which is required for the denaturation of thiamine.^[28]

Instrumental analysis of texture

The TPA method, based on the compression of samples with the muscle texture analyser (TA-XT Plus, Stable Micro Systems, UK), was used to objectively evaluate texture.^[29] The load cell used was a cylindrical probe of 75 mm diameter equipped with a sensor of 50 N. Cooked prawn pieces from the pouch were cut into equal-sized blocks for studying the TPA. Six samples from three pouches were used for the analysis. The texture measurement was composed of two consecutive 40% compressions of the sample at a crosshead speed of 12 mm/min.^[9,21,29] Force by time data from each test was used to calculate the mean values for the TPA parameters as described by Bourne.^[29] The values for hardness 1 and 2 are the resistance at maximum compression during the 1st and 2nd compressions. The cohesiveness (i.e. the extent to which the sample could be deformed before rupture) is the ratio of the positive force area during the 2nd compression to that during the 1st compression (Area 2/Area 1). Springiness measures the ability of the sample to recover its original form after the deforming force is removed, and is the ratio of the time duration of force input during the 2nd compression to that during the 1st compression (length 2/length 1). Chewiness indicates the work needed to chew a solid sample to a steady state of swallowing (hardness 1 × cohesiveness × springiness in kg), whereas gumminess is the product of hardness and cohesiveness.

Instrumental analysis of shear force

The maximum force required to cut the sample was determined by shear force. The piece of cooked prawn was placed on its side in the muscle texture analyser (TA-XT Plus, Stable Micro Systems, UK), and cut at the centre with a Warner–Bratzler shear blade. The load cell used for shear force study was of 50 N capacities. The crosshead speed of the machine was maintained at 5 mm/sec. The maximum shear force, determined from the force–deformation curves, was indicative of toughness and expressed as kgf.

Instrumental analysis of colour

The colour measurement of the homogenized sample was measured using a spectrocolourimeter (Colourflex EZ, Hunter Associates Laboratory, Inc, Reston, VA) with geometry of diffuse 8° and an illuminant of D65/10°.^[30] This instrument was calibrated with black and white reference tiles before analysis. A thin (approx. 1–1.5 mm) strip of prawn piece was placed over the light source and covered by an inverted black cup supplied with the equipment and post processing L*, a*, b* values were recorded. Six readings were taken for each steak and the average values were calculated. The CIELAB (L*, a*, b*) colour scale was used for the study. In this coordinate system, L* is a measure of the lightness of a sample and ranges from 0 (black) to 100 (white). The chromaticity dimension (a and b) gives understandable designation of colour as follows: a* measures redness when positive, grey when zero, and greenness when negative; b* measures yellowness when positive, grey when zero, and blueness when negative. Higher values of a* and b* indicate more saturation in colour.

Chemical and microbial analyses

The moisture content of raw and cooked meat of prawn was determined by heating the sample initially at 100°C and later at a temperature of 170°C using an automatic moisture analyser (Sartorius, Model MA 35, Bohemia, NY, USA). The crude protein content (total nitrogen×6.25) was determined by the micro-Kjeldhal method.^[31] Fat was determined using the Soxhlet extraction system.^[31] Ash content was determined by ashing at 550°C ± 10°C for 6 h in a muffle furnace. Total volatile basic nitrogen (TVBN) was determined by the distillation method.^[31] The thiobarbituric acid (TBA) value was determined using the method of Tarladgis et al.^[32] The absorbance was determined by a spectrophotometer at 532 nm against a blank containing distilled water and TBA solution. The TBA value was expressed as mg malonaldehyde per kg meat. The total plate count (TPC) was estimated by the spread plate technique.^[33] The average number of colonies was calculated and expressed as CFU/g of the sample.

Commercial sterility test

The samples processed at different F₀ values and at different temperatures were incubated at 37°C for 15 d and at 55°C for a minimum of 5 d. The incubated pouches were aseptically opened and 1–2 g of the samples were taken by sterilized forceps and inoculated into the sterilized fluid thioglycolate broth in test tubes. Sterilized liquid paraffin was put onto the top of the broth to create an anaerobic condition and incubated at 37°C for 48 h and at 55°C for 4 d, respectively.^[34]

Sensory test

Thermally processed prawn curry pouches were randomly selected (three retort pouches for each treatment) and heated in boiling water for 5 min. The contents of the sample including the curry were placed in coded white enamel plates and served warm to panellists in separate booths equipped with proper illumination. Water was provided to the panellists for use before and after the evaluation of each sample, to restore taste sensitivity. The changes in the sensory characteristics of the prawn curry samples were evaluated by a panel of 10 researchers from the Institute, who have previously participated in the evaluation of similar products, on a 10-point scale.^[35,36] A sensory score of 6 was taken as the limit of acceptability. The panellists were asked to assign a score of 1–10 (1 = dislike extremely; 2 = dislike very much; 3 = dislike moderately; 4 = dislike slightly; 5 = neither like nor dislike; 6 = light slightly; 7 = like moderately; 8 = like very much; 9 = like extremely; 10 = excellent) for colour, flavour, chewiness, succulence, toughness, fibrosity, and overall acceptability.^[36]

Statistical analysis

The SPSS 10.00^[37] statistical package was used for analysis of the experimental results. The results were expressed as mean ± standard deviation.

Results and discussion

Biochemical and microbiological quality of fresh and cooked prawn meat

Proximate analysis of fresh Macrobrachium rosenbergii revealed that the moisture, protein, fat, and ash contents were 75.26 ± 0.78g/100g, 17.66 ± 0.53 g/100g, 4.03 ±0.09 g/100g, and 2.13 ± 0.44g/100g, respectively. Studies on the biochemical composition of the fresh prawn meat indicated medium fat content. A proximate composition of freshwater prawn with 77.5 ± 0.7g/100g moisture, 18.7 ± 2.3 g/100g protein, 1.7 ± 0.3 g/100g fat, and 1.1 ± 0.2 g/100g ash was reported.^[38] The higher fat content of prawn used in this study may be because the harvesting time coincides with its pre-spawning feeding season (March). Fresh prawn meat had a pH of 7.06 ± 0.02, a TBA value of 0.38 ± 0.01 mg malonaldehyde/kg meat, total volatile base nitrogen (TVBN) content of 6.51 ± 0.23 mg/100g, and TPC of 4.16 × 10⁴ CFU/g. The TPC, TBA, and TVBN levels were below the approved food standards limit.

Thermal processing

Based on the culinary style preferred in the locality, prawn in curry medium was prepared and the product developed was processed at a medium temperature of 116.0°C. At this process temperature, the curry packed in the retort pouch was subjected to three different F₀ values (6, 8, and 9). Heat penetration characteristics of the thermally processed prawn in curry in the retort pouch processed to F₀ 6, 8, and 9 are shown in Fig. 2. Total process times taken to reach the F₀ values of 6, 8,and 9 for prawn in curry were 53, 57, and 63 min, respectively. These process times were sufficient to obtain commercially sterile products. It was observed from the results that as the F₀ value increased, the total process time also increased proportionally. CV represents the extent of cooking of the product. It also showed a proportionate increase with increase in F₀ values. CV for the prawn in curry was 87.53, 107.93, and 117.55 min when the samples were processed to F₀ values of 6, 8, and 9, respectively.

Figure 2. Heat penetration characteristics, cook value, and F₀ value of ‘Prawn in curry’ thermally processed at 116°C F₀ 6 (a), F₀ 8 (b), and F₀ 9 (c).

In addition to the destruction of microbes, there were also degradation of nutrients, textural changes (usually softens) as well as inactivation of enzymes.^[39] CV that evaluates the impact of thermal processing on food should be minimized at any given lethality.^[40] Several works on the retort pouching of fishery products have been reported. The F₀ value recommended for fish products is in the range of 5–20.^[41] An F₀ value of 8.43 was reported as satisfactory for fish curry products.^[12,42] According to Mallick et al.,^[13] F₀ value of 8.79 was said to be sufficient to obtain a commercially sterile product. An F₀ value of 6.94 with CV of 107.24 min and a process time of 50.24 at 116°C was found to be satisfactory for Tilapia fish curry.^[43]

Instrumental analysis of TPA

The results of TPA of prawn in curry processed to F₀ values of 6, 8, and 9 are shown in Fig. 3. The hardness (kgf) values for F₀ 6, 8, and 9 were 0.68, 0.62, and 0.56, respectively. It is observed from the result that the hardness decreased as the F₀ values increased, which might be attributed to the heat-induced muscle decomposition and fragmentation during sterilization. The springiness value for thermally processed prawn in curry to F₀ values of 6, 8, and 9 was 0.44 mm, 0.47 mm, and 0.51 mm, respectively. The springiness value, which is the rate at which a deformed material recovers to its undeformed condition after the deforming force is removed, showed the reverse trend of hardness. The cohesiveness, which is the extent to which food can be deformed before it ruptures, was estimated 0.41, 0.43, and 0.46 for thermally processed prawn in curry to F₀ 6, 8, and 9, respectively, and showed no marked changes, while F₀ increased to F₀ 8. The gumminess value was found to be 0.35, 0.29, and 0.23, respectively, for thermally processed prawn in curry to F₀ 6, 8, and 9 and showed a decreasing trend as the F₀ values increased. A similar trend was observed in case of chewiness values and was recorded as 0.28 kgf.mm, 0.19 kgf.mm, and 0.11 kgf.mm, respectively, for F₀ 6, 8, and 9. These decreasing trends were mainly due to the increase in the heat treatment. This may be due to the denaturation of protein and the destruction of muscle cells during thermal processing.^[44,45] Tang et al.^[46] reported that the higher sterilization temperature improved the textural as well as colour and sensory acceptability of the retort pouch-processed traditional Chinese dish made of carp.

Figure 3. TPA of ‘Prawn in curry’ thermally processed at 116°C to F₀ values 6, 8, and 9.

Instrumental analysis of shear force

The shear force of prawn in curry thermally processed to different F₀ values is shown in Fig. 3. The shear force, which is the maximum force required to cut the sample and is also indicative of toughness, for thermally processed prawn at F₀ values of 6, 8, and 9 was 1.22 kgf, 1.05 kgf, and 0.97 kgf, respectively. This showed that the shear forces decreased as the F₀ values increased for the products. The product processed to F₀ value of 6 had the highest shear force, whereas it was the least for F₀ value of 9. The decrease in the shear force for prawn followed the same trends as hardness, gumminess, and chewiness with increase in the F₀ values. Decrease in the toughness of thermally processed squid with increase in cooking time was reported by some authors.^[45,47] The relationship between shear force with hardness and chewiness has been reported by various authors.^[48,49,50]

Instrumental analysis of colour

The changes in the colour parameters during the thermal processing of prawn in curry to different F₀ values are shown in Fig. 4. The L* value (lightness) was 54.0, 49.36, and 46.42 for prawn thermally processed to F₀ values of 6, 8, and 9, respectively. In the case of a* value (green to red), it was 3.83, 4.86, and 5.37 for shrimps processed to F₀ values of 6, 8, and 9, respectively. The b* value (blue to yellow) followed the same trend as observed in the case of the L* value. From the figure, it was observed that there was a decreasing trend for L* and b* values, whereas increasing trends were observed for a* values with increase in the F₀ values. This may be due to prolonged heat treatment, which affects the colour of the prawn.^[51] The increase in the red colour (a* value) in the prawn may be due to the release of astaxanthin pigment by prolonged heat treatment.^[52,53] The decrease in lightness (L* value) and yellow colour (b* value) of the thermally processed prawn upon increase in the F₀ value may be due to the formation of the red pigment as a result of prolonged heat treatment, or nonenzymatic browning or Maillard reactions.^[54]

Figure 4. Colour profile of ‘Prawn in Curry’ thermally processed at 116°C to F₀ values 6, 8, and 9.

Sensory analysis

Prawn in curry processed to three different F₀ values were analysed by 10 trained sensory panellists. The results of the sensory analysis are presented in Fig. 5. The sensory score given by the panel for colour of the product was found to be 7.86, 8.85, and 8.56 for thermally processed prawn to F₀ 6, 8, and 9, respectively. In case of flavour, panellists scored 8.53, 9.23, and 8.74 for processed prawn to F₀ values 6, 8, and 9, respectively. It was observed from the above result that the retort pouch-processed prawn experienced an increase in colour and flavour with increase in F₀ value. This may be explained by the prolonged heating, which favours the development of colour and flavour in the finished product. Textural parameters like succulence, toughness, and fibrosity showed decreasing trends, whereas not much change in the chewiness parameter was observed. Cooking affects the muscle firmness and texture of the final products.^[50] The decrease in the textural parameters may be due to the higher cooking time. The highest overall acceptability score given by the panellists for the products processed to F₀ value 7 was 8.86 ± 0.21.

Figure 5. Sensory analysis of ‘Prawn in curry’ thermally processed at 116°C to F₀ values 6, 8, and 9.

Conclusion

Freshwater prawn curry was prepared and thermally processed at three different F₀ values, i.e., 6, 8, and 9. It was observed from the results that the total process time increased as the F₀ values increased. The sensory textural parameters and instrumental textural parameters such as hardness, gumminess, and chewiness followed the same trend and showed decreasing trends as the F₀ values increase. The shear force for the product decreased with increase of F₀ values. The organoleptic evaluation scored the highest for the product processed to F₀ 7 min.

Acknowledgements

The technical assistance provided by all technical staff of the Dept. of Fish Processing Technology & Engineering, College of Fisheries (CAU), Lembucherra, Tripura (India), is gratefully acknowledged.

Funding

The authors are thankful to the Ministry of Food Processing Industries, Govt. of India, New Delhi, for providing financial assistance (Sanction Order No. 66 /MFPI/R&D/2011 dt. 18/12/2012) for this research project.

Additional information

Funding

The authors are thankful to the Ministry of Food Processing Industries, Govt. of India, New Delhi, for providing financial assistance (Sanction Order No. 66 /MFPI/R&D/2011 dt. 18/12/2012) for this research project.