Role of Sorbitol in Manufacturing Dried Seafood from Heated Squid Meat

FISHERIES SCIENCE 2001; 67: 524–529

Original Article

Role of sorbitol in manufacturing dried seafood from heated squid meat

T KUBO AND H SAEKI*

Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan

SUMMARY: Japanese common squid meat was heat-treated at 80∞C for 1 min, cured with 1.0 M sorbitol solution (pH 7.0) at 4∞C for 18 h, and dried further at 30∞C (60% relative humidity) for 16 h. Osmotic dehydration during the sorbitol curing process and slow moisture vaporization at the initial drying period were observed regardless of the heat denaturation of muscle protein. Simultaneously, lowering the amount of moisture vaporized in the falling rate of the drying period caused a shortening of the total drying time. Furthermore, the shear force of the dried product from heat-treated meat was kept at a lower value by sorbitol curing, although the suppression effect of sorbitol on the hardening of dried meat was lost by protein denaturation. These results are useful for understanding the role of sorbitol in reducing drying time and in eliminating excess hardening of dried squid products.

KEY WORDS: dehydration, drying, moisture, sorbitol, texture, squid, meat.

INTRODUCTION lighted. We believe that the effects of sorbitol contribute to a reduction in drying time and the The basic purpose of food drying is to create an elimination of excess hardening of dried fish and environment that is deleterious to the growth of squid products. spoilage mechanisms.1 To reduce the water activity In the seafood industry, dried food products are of dried products, food materials are often cured produced from various kinds of materials; that is, with sorbitol, NaCl, and other ingredients. Recent not only fresh but also frozen and boiled fish and studies have shown that when such ingredients are shellfish. In addition, squid mantles are skinned introduced to materials in the curing process, they by heating at 50–80∞C in a matter of minutes in affect the drying characteristics of the materials ordinary process lines.6 Therefore, it is necessary and the texture of the final products.2–4 In a previ- to investigate how the protein denaturation of ous paper, we studied the role of sorbitol in the materials affects the role of sorbitol when drying process of dried seafood products and found that seafood products. In the present study, heat- the drying characteristics of fish and squid meats treated squid meat was cured with sorbitol solu- were affected by sorbitol; in other words, the mois- tion and then dried. Moisture removal during the ture was vaporized slowly during the initial drying curing and drying processes was examined, as period of the sorbitol-cured meats, and the criti- well as whether there was a relationship between cal moisture content was significantly decreased the texture of dried squid products and moisture when the sorbitol content of dried products transportation. was increased.5 Furthermore, the hardening of the dried products was effectively suppressed by curing with sorbitol. The importance of osmotic MATERIALS AND METHODS dehydration during the sorbitol-curing process in shortening the drying process was also high- Materials

Quick-frozen Japanese common squid (Todarodes pacificus) (mean weight, 276 ± 68 g, n = 25) was *Corresponding author: Tel./Fax: 81-138-40-5515. Email: obtained at a local fish market. Cellulose tubing saeki@fish.hokudai.ac.jp and membrane (for dialysis) was purchased from Received 31 August 2000. Accepted 21 November 2000. Sankou Jun-yaku Co., Ltd (Tokyo, Japan). Sorbitol Drying sorbitol-cured squid FISHERIES SCIENCE 525

was provided by Towa Chemical Industry Co., Ltd for 2 min at 20 000 r.p.m. with a homogenizer (Tokyo, Japan) and all other chemicals (reagent (model AM-6; Nissei Co. Ltd, Tokyo, Japan). The grade) were obtained from Wako Pure Chemical homogenate was dialysed against 2000 mL of 0.5 M Industries, Ltd (Osaka, Japan). NaCl and 40 mM Tris-HCl (pH 7.5) for 16 h at 4∞C and centrifuged at 15 000 ¥g for 30 min. Solubility was expressed as the percentage of protein con- Preparation of sorbitol-cured and centration in the supernatant with respect to that dried squid meats of the total protein solution before centrifugation. The protein concentration was determined by the Biuret method using bovine serum albumin as a Samples of squid meat were taken from the middle 7 of the skinned mantle, which had a thickness of standard. A Ca-ATPase assay of the homogenate 4.3 ± 0.7 mm (n = 40). The test piece (1.5 cm wide) was performed at 25∞C in a mixture containing was cut perpendicularly to the body axis (6 cm in 0.5 M NaCl, 5 mM CaCl2, 1 mM adenosine tri- length) and wrapped in a cellulose membrane. phosphate (ATP), 25 mM Tris-maleate (pH 7.0), Twelve sample pieces were obtained from each and 0.2–0.3 mg/mL of protein. The reaction was individual. stopped by adding HClO4 to a ﬁnal concentration of 5%. The inorganic phosphate liberated The raw meat pieces (RM) were packed into a 8 polyethylene bag and then heated in hot water at was measured using Gomori’s method. 80∞C for 1 min. The heat-treated meat pieces (HM) were wrapped in a cellulose membrane and cured in 2000 mL of 1.0 M sorbitol containing 20 mM Measurement of moisture content and Tris-acetate (pH 7.0) at 4∞C for 18 h. After removing analysis of drying characteristics the cellulose membranes, the sorbitol-cured meat pieces (HCM) were dried at 30∞C and 60% relative The dried sample pieces were chopped and humidity for 0–16 h in a humidity cabinet (model incubated at 105∞C for 16 h, and weight loss was PR-1G; Tabai Espec Corp., Tokyo, Japan). The air- estimated as moisture. Changes in the moisture ﬂow rate was regulated to about 2.5 m/s. content of sample pieces during drying were mea- The dried sample pieces (HCDM) were adjusted sured, and the drying characteristic curves were to 40 mm lengths by cutting off both ends and drawn so as to compare moisture transportation. then used for texture evaluation. In this work, sor- In the drying characteristic curves, the moisture bitol-cured meat (RCM), dried meat (RDM), and content at which an abrupt decrease in the drying cured dried meat (RCDM) were also prepared rate occurred was regarded as the critical moisture from RM without heat-treatment under the same content. Furthermore, the amounts of moisture conditions. removed from the squid meats by osmotic dehydration and moisture vaporization were calculated to understand the effect of sorbitol curation on Measurement of sorbitol concentration moisture transportation.

Sorbitol-cured meat (5 g) was homogenized with cold distilled water (40 mL) followed by cen- Mechanical assessment of texture trifugation at 10 000 ¥g for 30 min, and 7.5% trichloroacetic acid (final concentration) was The modified shear head of a Warner–Bratzler’s added to the supernatant. The mixture was kept at apparatus was attached to a rheometer (model room temperature for 30 min and then centrifuged RUD-JS; San Kagaku Co., Ltd, Tokyo, Japan), which at 3000 ¥g for 30 min. After adjusting the pH to is eminently suitable for measuring the texture of 7.0 with 1.0 M KOH, the supernatant was used dried squid meat.9 A sample piece was attached for sorbitol analysis with an enzyme analysis between the sample stopper and the stainless steel kit (D-Sorbitol/Xylitol Analysis Kit; Boehringer cutter blade, which measured 0.45 mm thick, and Mannheim, Mannheim, Germany). was then cut longitudinally by the blade in slab form at a speed of 140 mm/min. The shear force (gw) was measured as the highest load value Measurement of solubility and Ca-ATPase during sample cutting and expressed as the mean activity of squid meats value of eight sample pieces. There were no significant differences in the Pieces of squid meat (5 g each) were homogenized shear force among meat pieces from different in 40 mL of 0.1 M NaCl and 0.5% Triton X-100 specimens of squid mantle (n = 6, P > 0.05). 526 FISHERIES SCIENCE T Kubo and H Saeki

Statistical analysis and 2.18 g H2O/g dry solid (HCM) by osmotic dehydration during the curing process and, as a result, Tukey’s multiple range test10 was performed by there was no difference between the moisture using a Minitab 10 system for personal computers content of both sorbitol-cured meats (P > 0.05). (Minitab Inc., State College, PA, USA).

Moisture transportation from squid meats RESULTS AND DISCUSSION during drying

Effect of protein denaturation on sorbitol Moisture changes of RM, RCM, and HCM during permeation and osmotic dehydration drying and their drying characteristic curves are during sorbitol curing presented in Fig. 1. When compared to the same drying time, the moisture contents of RCM and Table1 shows the effect of heat on the biochemical HCM were always lower than those of RM during properties of muscle protein. When RM was heated the drying process (Fig. 1a). Also, no differences at 80∞C for 1 min, its solubility in 0.5 M NaCl were found in a time course of moisture decrease decreased greatly and myoﬁbrillar Ca-ATPase and in the drying characteristic curves between activity was completely lost. These results indicate both cured meats. These results indicate that that the muscle protein of the squid meat was protein denaturation has no effect on moisture denatured by the heat treatment. vaporization from sorbitol-cured squid meats. Table2 shows changes in the moisture content Although a constant-rate drying period and a and sorbitol concentration of different squid meats falling-rate drying period were observed in the during the heat treatment and further curing with drying characteristic curves of all samples (Fig. 1b), 1.0 M sorbitol for 18 h at 4∞C. The moisture content the critical moisture contents of RCDM and HCDM was diminished from 3.10 g H2O/g dry solid (RM) (1.60 g H2O/g dry solid) were signiﬁcantly lower to 2.94 g H2O/g dry solid (HM) (P < 0.05) by the than that of RDM (2.60 g H2O/g dry solid). The heat treatment. However, the heat denaturation decrease in the critical moisture content in the of squid muscle protein had little effect on the drying process implies effective moisture vapor- amount of sorbitol permeated in the different ization from the surface of the dried meat, as meats. The moisture contents of RM and HM reported in a previous paper.4 Therefore, the result decreased greatly to 2.21 g H2O/g dry solid (RCM) of Fig. 1 indicates that effective moisture vaporization was achieved during the drying process by sorbitol curing, regardless of the heat denaturation Table 1 Effect of heat treatment on solubility and of muscle protein. Ca-ATPase activity of squid meats Figure 2 shows the amounts of moisture removed from sample meats by osmotic dehydra- Sample Solubility Ca-ATPase activity tion and moisture vaporization. In the preparation (%) (mmol Pi/min·mg of protein) process of RCDM, 28.1% of total moisture was RM 89.0 1.19 dehydrated in the curing process, and 20.3% of that HM 4.6 0.00 was vaporized in a constant-drying rate period. In contrast, in the case of HCDM, the amounts of RM, raw meat; HM, heat-treated meat. moisture dehydrated in the curing process and vaporized in a constant-drying rate period were 24.4% and 18.7%, respectively, although 5.2% of Table 2 Effect of heat treatment on moisture and moisture was removed by the earlier heat treat- sorbitol concentrations of squid meats ment at 80∞C for 1 min. As a result, the same Sample Moisture Sorbitol amount of moisture (40%) was vaporized in a (g H2O/g dry solid) (mol/kg) falling-rate drying period in RCDM and HCDM. RM 3.10a 0.00 These results indicate that the heat denaturation of HM 2.94b 0.00 squid muscle protein has little effect on osmotic RCM 2.21c 0.53d dehydration during the sorbitol-curing process HCM 2.18c 0.52d and on moisture transportation during the drying process. Different letters mean that results are statistically different As shown in Fig. 2, the amount of moisture (P < 0.05). RM, raw meat; HM, heat-treated meat; RCM, sorbitol-cured vaporized in a falling-rate drying period was meat without heat treatment; HCM, heat-treated, sorbitol- diminished by sorbitol curation. In a previous cured meat. paper, we reported that the reduction of a falling- Drying sorbitol-cured squid FISHERIES SCIENCE 527

(a)

(b)

Fig. 2 Comparison of moisture transportation during the heating, sorbitol curing and drying processes. The amount of moisture removed from samples at each manufacturing step was expressed as the percentage of moisture content of raw meat. Equilibrium moisture content was also shown. RDM, dried meat without heat treatment; RCDM, sorbitol-cured and dried meat without heat treatment; HCDM, heat-treated, sorbitol-cured and dried meat.

sorbitol curing, which was observed in RCM and HCM, was caused by a reduction of the falling-rate Fig. 1 (a) Time course of moisture change of squid drying period, regardless of heat denaturation of meats during drying process and (b) their drying char- the muscle protein. acteristic curves. () Raw meat (RM) and () sorbitol- cured meat without heat treatment (RCM), and () heat-treated and sorbitol-cured meat (HCM) were dried at 30∞C and 60% relative humidity. Effect of protein denaturation on the textural change of squid meat during processing

The shear force of RM, RCM, HM, and HCM was rate drying period contributes to a shortening 1299 gw, 1149 gw, 695 gw, and 699 gw, respectively. in drying time.5 This result led us to investigate There was no difference in the shear force between the effect of sorbitol curation on the drying time RM and RCM (P > 0.05), and also between HM and of squid meat in the present study. As presented HCM (P > 0.01). Thus, it is apparent that sorbitol in Table 3, the shortening of the drying period by curing has no effect on the shear forces of squid 528 FISHERIES SCIENCE T Kubo and H Saeki

Table 3 Comparison of drying time of squid meats Sample Constant rate Falling rate Total drying of drying of drying time period (h) period (h) (h) RM 0.7 5.2 5.9 RCM 1.6 2.2 3.8 HCM 1.5 2.2 3.7

Drying time was calculated the period taken for sample moisture content to reach 1.0 g H2O/g dry solid. RM, raw meat; RCM, sorbitol-cured meat without heat treatment; HCM, heat-treated, sorbitol-cured meat.

Fig. 4 Relationhip between shear force and moisture content of dried squid meats. () Dried meat without heat treatment (RDM); () sorbitol-cured and dried meat without heat treatment (RCDM); () heat-treated, sorbitol-cured and dried meat (HCDM).

The relationship between shear force and moisture content of the different dried meats was inves- tigated. As presented in Fig. 4, the shear force of HCDM was always lower than that of RDM when compared at the same moisture content. For example, the shear force of HCDM was lower than Fig. 3 Change in shear force of squid meats during that of RDM by 1.5 times at 1.0 g H2O/g dry solid of drying. () Dried meat without heat treatment (RDM); moisture content, and by 1.9 times at 2.0 g H2O/g () sorbitol-cured and dried meat without heat treat- dry solid of moisture content. These results suggest ment (RCDM); () heat-treated, sorbitol-cured and that excess hardening of dried meat was sup- dried meat (HCDM). pressed by sorbitol curing even when squid meat protein was denatured and that the suppressing effect of sorbitol on dry-hardening was impaired. meats, although the shear force of raw squid meat In a previous study, we pointed out that the role was diminished by the treatment of heat. of sorbitol in manufacturing dried seafood prod- The shear force changes of RDM, RCDM, and ucts includes the reduction of moisture content by HCDM during the drying process were examined osmotic dehydration during curing, slow moisture to investigate the effect of sorbitol on the texture vaporization at the initial drying period, and sup- of dried squid meats. As shown in Fig. 3, the shear pression of the hardening of dried products.5 As force of RDM increased 1.6 times during the drying shown in Figs 1 and 2, the former two effects were period (16 h). In contrast, no textural change was also conﬁrmed in the heat-treated squid meat. In observed in RCDM. We have already reported that addition, lowering the amount of moisture vapor- sorbitol has a strong effect on suppressing the ized in the falling-rate drying period caused a hardening of dried ﬁsh and squid meat products.5 shortening in total drying time, as presented in However, the shear force of HCDM was increased Table3. Furthermore, the shear force of dried prod- to as much as that of RDM. This result obviously ucts was kept at a lower value by sorbitol curing, as indicates that the suppressing effect of sorbitol on presented in Figs 3 and 4, although the suppression meat hardening was impaired by heat denatura- effect of sorbitol on the hardening of dried meat tion of the squid muscle protein. disappeared with protein denaturation. The results Drying sorbitol-cured squid FISHERIES SCIENCE 529

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