Influence of blanching temperatures on the biochemical, microbiological, and sensory characteristics of freeze-dried Metapenaeus affinis (Jinga shrimp) during ambient storage

Introduction

Seafood is an important source of high-quality protein, essential amino acids, vitamins, and minerals, and plays a significant role in global food and nutritional security. Aquatic animal foods contributed approximately 15% of animal protein and 6% of the total protein supply worldwide in 2021(Food and Agriculture Organization [FAO], 2024) [16] . Owing to the presence of omega-3 fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), seafood consumption has increased considerably. It is widely recognized for its health benefits (Rahmaniya and Sekharan, 2018) [37] . Regular consumption of seafood has been associated with proper growth, brain development, and a reduced risk of several lifestyle-related diseases, including cardiovascular disorders (Guallar et al., 2002; Krauss et al., 2000; Liao and Chao, 2009; Singh et al., 1997; Von Schacky et al., 1999) [18,23,25,41,45] . India is one of the leading seafood-exporting countries, exporting 16, 98, 170 MT of marine products valued at ₹62, 408.45 crore (US$ 7.45 billion) during FY 2024-25. Frozen shrimp remains the most valuable export commodity, accounting for 69.46% of total export earnings. In contrast, dried marine products contribute comparatively less to export value despite their shelf-stable nature, highlighting the need for improved processing technologies and value-added seafood products (Marine Products Export Development Authority [MPEDA], 2025) [29] . Among commercially important crustaceans, Metapenaeus affinis (H. Milne Edwards, 1837), commonly known as Jinga shrimp, is widely distributed throughout the Indo-Pacific region and supports an economically important fishery along the Indian coast (ICAR-Central Marine Fisheries Research Institute [CMFRI], 2016, 2025) [9,20] . Shrimp is highly valued because of its excellent nutritional profile, containing high-quality protein, vitamin B12, selenium, omega-3 highly unsaturated fatty acids, and astaxanthin, a potent natural antioxidant (Dayal et al., 2013; Venugopal, 2008) [13,44] . However, shrimp is highly perishable due to its high moisture and protein content, which promotes microbial proliferation and biochemical deterioration soon after harvest (Akonor et al., 2016) [1] . Therefore, effective preservation methods are essential to maintain product quality, safety, and marketability. Drying is one of the oldest and most widely adopted preservation methods for seafood. By reducing water content, drying inhibits microbial growth and enzymatic activity, thereby extending shelf life (Rahman and Perera, 2007) [36] . Nevertheless, conventional drying methods often result in undesirable changes in nutritional value, texture, colour, and flavour because of prolonged exposure to heat (Al-Hilphy et al., 2022) [3] . To overcome these limitations, advanced drying technologies such as freeze-drying have gained considerable attention (Wang et al., 2011) [46] . Freeze-drying removes water from frozen materials through sublimation under low-temperature and vacuum conditions, thereby minimizing thermal damage and preserving the original characteristics of the product (Donsì et al., 2001) [15] . This technique has been widely recognized for its ability to retain nutritional quality, flavour, colour, and sensory attributes while producing lightweight products with extended shelf life (Berk, 2013) [6] . Freeze-dried seafood products have gained increasing commercial importance because of their convenience, long storage stability, and ease of transportation (Crapo et al., 2010) [12] . Furthermore, the porous structure developed during freeze-drying enhances rehydration characteristics and improves consumer acceptability (Liu et al., 2022) [26] . Pretreatments applied before drying can significantly influence the quality and stability of the final product. Among these, blanching is commonly employed to reduce microbial load, inactivate enzymes, and improve product quality (Arroqui et al., 2003a; Arroqui et al., 2003b) [4,5] . The effectiveness of blanching depends on processing conditions such as temperature, time, and salt concentration, which may influence the biochemical, microbiological, and sensory characteristics of the finished product (De Corcuera et al., 2004 ) [14] . Optimisation of blanching conditions before freeze-drying may therefore contribute to improved storage stability and product acceptability. Despite the commercial importance of M. affinis and the advantages of freeze-drying, research regarding the optimization of blanching conditions as a pre-treatment for this species remains limited. Therefore, the present study was undertaken to evaluate the influence of different blanching temperatures on the biochemical, microbiological, and sensory quality of freeze-dried M. affinis packaged in multilayer pouches and stored under ambient conditions.

Materials and Methods

Raw Material and Experimental Design

Fresh Jinga shrimp (Metapenaeus affinis) weighing approximately 39 kg were procured from the Veraval Fish Landing Centre, Gujarat, India. Immediately after procurement, the shrimp were adequately iced and transported to the Fish Processing Technology Laboratory, College of Fisheries Science, Kamdhenu University, Veraval, in insulated ice boxes. During transportation, the temperature was maintained between 1 °C-2 °C to preserve freshness and minimize biochemical and microbial deterioration. Upon arrival at the laboratory, the shrimp were thoroughly washed with chilled potable water to remove adhering slime, dirt, and other extraneous materials. The samples were manually beheaded, peeled, and deveined before being subjected to experimental treatments. The experiment was conducted in a Completely Randomized Design (CRD) comprising four treatments. The control treatment (T 0 ) consisted of shrimp samples without blanching, whereas treatments T 1 , T 2 , and T 3 involved blanching in 1.5% sodium chloride solution at 70, 80, and 90 °C, respectively, for 2 min. Each treatment was replicated three times.

Processing and Freeze-Drying

Processed shrimp were divided into four equal portions corresponding to the treatment groups. For blanching treatments, a 1.5% (w/v) sodium chloride solution was prepared and maintained at the respective temperatures. Shrimp samples were immersed in the blanching solution for 2 min, then drained. Following pretreatment, all samples were subjected to vacuum freeze-drying using a vacuum food freeze dryer (PVFD10, Precise Vacuum Systems Pvt. Ltd., India). Samples were initially frozen at-25 °C for 360 min. Primary drying was carried out under vacuum (600 mTorr; 0.8 mbar) at a shelf temperature of 40 °C, followed by secondary drying at 50 °C for 360 min. After completion of the drying cycle, the freeze-dried shrimp were packed in multilayer high-barrier pouches (PES/Al/BOPA/LDPE) and sealed using a foot-operated impulse sealer. Packaged samples were stored at ambient temperature for 180 days.

Biochemical Analysis 2.3.1 pH

The pH of shrimp samples was determined using a digital pH meter (HANNA HI-98107). Ten grams of the sample were homogenized with 20 mL of distilled water, filtered, and the pH of the filtrate was recorded at room temperature.

Trimethylamine Nitrogen (TMA-N)

Trimethylamine nitrogen was estimated using the microdiffusion method of Beatty and Gibbons (1937). Trichloroacetic acid (TCA) extracts were prepared by homogenizing shrimp samples with 20% TCA, followed by filtration and volume adjustment. The liberated trimethylamine was trapped in standard sulphuric acid and quantified by titration against standard sodium hydroxide using Tashiro's indicator. Results were expressed as mg TMA-N per 100 g sample.

Volume of sample taken × Weight of sample × 100

Total Volatile Base Nitrogen (TVB-N)

TVB-N was determined from the same TCA extracts using the micro-diffusion technique described by Beatty and Gibbons (1937). Volatile basic nitrogenous compounds liberated from the sample reacted with standard sulphuric acid in the diffusion chamber and were subsequently quantified by titration. Results were expressed as mg TVB-N per 100 g sample.

TVB-N (mg/100 g) = (V s -V b ) × 0.14 × Volume of extract Volume of sample taken × Weight of sample × 100

Peroxide Value (PV)

The extent of lipid oxidation was assessed by determining peroxide value using the iodometric method described by AOAC (2006). Lipid extracts obtained from shrimp samples were reacted with potassium iodide, and the liberated iodine was titrated against standardized sodium thiosulphate solution. Peroxide value was expressed as milliequivalents of peroxide per kilogram of lipid (meq/kg).

Peroxide value (meq/kg) = 1000 × (V -X) × Volume of extract W × Aliquot sample volume × 100

Microbiological Analysis

Microbiological quality was assessed throughout storage. Ten grams of sample were aseptically homogenized with 90 mL sterile phosphate-buffered saline to obtain an initial 10⁻¹ dilution. Further serial dilutions were prepared as required for microbial enumeration.

Total Plate Count (TPC)

Total viable bacterial count was determined by the pour plate method using Plate Count Agar (PCA). Aliquots of appropriate dilutions were plated, mixed with molten agar, and incubated at 37±1 °C for 48 h. Colonies were counted and expressed as colony-forming units per gram (CFU/g).

TPC (CFU/g) = Average No. of colonies × Dilution factor

Sensory Evaluation

The sensory quality of freeze-dried shrimp samples was evaluated using a nine-point hedonic scale developed by Peryam and Pilgrim (1957). A semi-trained panel consisting of ten members assessed colour, appearance, odour, texture, and overall acceptability. Samples were presented under identical conditions, and panelists assigned scores ranging from 1 (dislike extremely) to 9 (like extremely). A score of five was considered the minimum acceptable limit.

Statistical Analysis

All experiments and analytical determinations were carried out in triplicate. The data obtained were expressed as mean±standard deviation. Statistical analysis was performed using analysis of variance (ANOVA) as described by Snedecor and Cochran (2014) to determine significant differences among treatments and storage periods. [10,27] . Similar increases in pH during storage have been reported in dried seafood products. Hossain et al. (2019) [19] observed pH values increasing from 6.27-6.80 to 7.65-7.86 in salt-smokedried Mystus tengara during 180 days of storage. Likewise, Rabie et al. (2016) [35] reported an increase in pH from 6.12-6.62 to 6.37-6.98 in dried shrimp (Pandalus borealis) stored for six months at ambient temperature. Among the treatments, T 2 maintained the lowest final pH (7.08±0.05), indicating better biochemical stability and reduced protein degradation during storage. The results suggest that blanching at 80 °C prior to freeze-drying effectively retards the formation of alkaline spoilage compounds and improves the storage stability of freezedried M. affinis shrimp. [8,31] . Similar increases in TVB-N during storage have been reported in dried seafood products. Rabie et al. (2016) [35] observed TVB-N values increasing from 17.22-18.35 mg/100 g to 28.36-36.07 mg/100 g in dried shrimp (Pandalus borealis) during six months of storage. Likewise, Jarag et al. (2025) [21] reported an increase from 8.70-14.51 mg/100 g to 19.48 mg/100 g in dried Jawala shrimp during 90 days of storage. According to Connell (1995) [11] , spoilage in dried fish products is generally associated with TVB-N levels of 100-200 mg/100 g. In the present study, only the unblanched control (T 0 ) exceeded 100 mg/100 g after 180 days, whereas all blanched treatments remained below this limit.

Results and Discussion

[32] . The progressive increase observed during storage reflects ongoing degradation processes and the accumulation of volatile amines responsible for characteristic fishy odours. Similar trends have been reported by Jarag et al. (2025) [21] , who observed TMA-N values increasing from 2.00±0.01 mg/100 g to 6.70±0.05 mg/100 g in dried Jawala shrimp during storage.Despite the increase, all treatments remained below the maximum permissible limit of 50 mg/100 g recommended for dried seafood products (Spinelli and Dassow, 1982) [42] . [22,24] . Similar increases in peroxide value during storage have been reported in dried seafood products. Majumdar et al. (2018) [28] observed PV values increasing from 12.08 to 25.35 meq/kg lipid in sundried Mystus vittatus during storage. Likewise, Rasul et al.

(2019) [38] reported an increase from 13.84 to 27.87 meq/kg in dried Trichogaster fasciata under ambient storage conditions. Rabie et al. (2016) [35] also reported progressive lipid oxidation in dried shrimp during storage, reflected by increasing peroxide values.

According to Shahidi and Zhong (2010) [40] , acceptable peroxide values in fish products generally range between 10 and 16 meq O₂/kg, while values exceeding 20 meq/kg are considered indicative of rancidity and reduced consumer acceptability (Connell, 1995) [11] . In the present study, all blanched treatments (T 1 , T 2 , and T 3 ) remained within acceptable limits throughout storage, whereas the control sample (T 0 ) approached the upper acceptable limit by day 180. [21] . The lower microbial counts observed in blanched samples can be attributed to thermal inactivation of microorganisms and spoilage-associated enzymes prior to freeze-drying. Comparable reductions in microbial load following blanching have also been reported in dried seafood products (Omodara and Olaniyan, 2012) [33] .

According to the Food Safety and Standards Authority of India (FSSAI, 2023) [17] standards, the permissible limit for aerobic plate count in dried fishery products is 4.0 log CFU/g. All blanched treatments (T 1 -T 3 ) remained within the acceptable limit throughout storage, whereas the unblanched control (T 0 ) slightly exceeded the regulatory threshold at the end of the storage period.

Sensory Characteristics

Sensory evaluation is a widely accepted method for assessing the quality and freshness of aquatic products using human senses, and in the present study, a 9-point hedonic scale was used where 9 indicated excellent quality and scores below 5 were considered unacceptable due to spoilage-related deterioration ( [2,7,39] . All sensory attributes of freeze-dried shrimp (M. affinis), including appearance, colour, odour, texture, and overall acceptability, showed a consistent declining trend during 180 days of ambient storage across all treatments, with statistically significant interaction effects between blanching treatments and storage period (p<0.05 [30,31,43] , and the results are consistent with previous reports on dried and freeze-dried seafood where gradual sensory degradation occurs during prolonged storage (Pratheep & Solanki, 2025; Rabie et al., 2016) [34,35] . Overall, the findings indicate that while all samples experienced quality loss over time, blanching treatments, especially T 2 , were most effective in preserving sensory acceptability and extending the shelf stability of freeze-dried shrimp under ambient conditions.

Conclusion

The present study demonstrates that thermal blanching pretreatments significantly influence the storage stability and overall quality of freeze-dried Jinga shrimp (Metapenaeus affinis) stored under ambient conditions. Among the evaluated conditions, blanching in a 1.5% NaCl solution at 80 °C for 2 minutes (Treatment T 2 ) emerged as the optimal pretreatment. Over the 180-day storage period, T 2 most effectively retarded biochemical degradation, maintaining the lowest levels of pH, Total Volatile Base Nitrogen (TVB-N), Trimethylamine Nitrogen (TMA-N), and Peroxide Value (PV). Furthermore, while the unblanched control samples exceeded the FSSAI permissible limits for microbial load and fell below acceptable sensory thresholds by the end of the storage period, the T 2 blanched samples maintained a safe Total Plate Count (2.44±0.22 log CFU/g) and retained the highest overall sensory acceptability score (6.50±0.53). Ultimately, integrating an 80 °C blanching step prior to freeze-drying, combined with packaging in high-barrier multilayer pouches (PES/Al/BOPA/LDPE), is a highly effective processing strategy. This method minimizes biochemical deterioration, ensures microbiological safety, and preserves the sensory attributes of freeze-dried M. affinis, offering a viable approach for the seafood industry to produce premium, shelf-stable dried shrimp for both domestic and export markets.

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