| Summary: | Increased sales of surimi seafood, with the majority as crabstick in the United States indicates that surimi based products are becoming more popular. With growing popularity, there is increased competition for market share. Under these circumstances, some companies may be willing to sacrifice product quality in order to facilitate manufacture and reduce price as a means of gaining market share. During the last 10 years, poor quality imports, which exceeded nearly 2 million pounds a year, have negatively impacted the domestic market. The most important factor in surimi production is the textural properties imparted by the fish protein. Consequently, crabstick quality is directly related to the amount of fish protein from raw surimi that is used. A high quality crabstick product typically contains 40% or higher fish protein. The protein content and quality of raw surimi are varied by fish species and surimi grade. As a way to increase profits, lower quality products can be sold while claiming they are made primarily of high quality fish protein surimi. Indirect enzyme-linked immunosorbent assays (ELISA) technique was used to identify and quantify the use of dried egg white (DEW) and whey protein concentrate (WPC) in crabsticks. The use of SDS-PAGE for the quantification of protein additives has had limited success due to the high shear and high temperature processes of surimi crabstick. Monoclonal (anti-heat denatured ovalbumin) and polyclonal (anti-β-lactoglobulin) antibodies were used. Antibodies showed no significant cross-reactivity with non-target crabstick proteins. An optimized extraction solution of 10% SDS and 2.5% βME yielded high extractability with improved consistency. Quantification of DEW and WPC was achieved using the optimized extraction solution and indirect ELISA. Estimated DEW values were within 7% of actual values, WPC samples were within 17%. Inter-assay coefficients of variance for DEW ranged from 0.9% to 3.1% and those of the WPC were 1.0% to 8.0%. A competitive enzyme-linked immunosorbent assay (ELISA) was developed for quantification of Alaska pollock (AP) surimi in crabsticks. Identification of fish species is complicated by processing, cooking, and additional ingredients. ELISA is a powerful tool for identification and quantification of fish species. Polyclonal antibodies were raised in rabbits against a 15-amino-acid peptide (Ala-Pro-Lys-Lys-Asp-Val-Lys-Ala-Pro- Ala-Ala-Ala-Ala-Lys-Lys) determined from the myosin light chain 1 (MLC 1) of AP. Immunoblotting showed the anti-pep-AP antibody had no significant cross-reactivity with protein additives. However, cross-reactivity of the MLC 1 from Pacific whiting, and threadfin bream surimi was observed. MLC 1 was purified from AP surimi and used as the coating protein in the competitive ELISA. MLC 1 was serially diluted and had a R² of 0.9845 following a logarithmic curve. All estimations of AP surimi were within 9% of the actual value. Inter-assay coefficients of variance ranged from 4.2 to 4.9%. Antibodies were produced by injection of a synthesized 15-amino acid peptide or by whole myosin light chain 1 isolated from Alaska pollock (AP). A direct sandwich ELISA was tested using extracts prepared from AP, Pacific whiting (PW), true cod (TC), tilapia (T), and catfish (C). Fish extracts were studied using SDS-PAGE and Western blotting. A standard curve was created for each fish and used to estimate 3 different verification samples. All estimations were within 10, 37.5, 30, 43, and 34% of the actual value for AP, PW, TC, T, and C, respectively. When one or more fish species was mixed together with AP the estimation of the Alaska pollock content became much less accurate. This study confirms a direct sandwich ELISA accurately detects the quantity of AP. Testing found the sandwich ELISA developed exhibited cross-reactivity with other protein sources such as beef, chicken, pork, shrimp, and clam. Purified Chinook salmon myosin was studied using SDS-PAGE and densitometric analysis to determine its purity, which was 94%. Myosin subjected to linear heating began to form aggregates at > 24 °C as measured by turbidity at 320 nm. Conformational changes, as measured by surface hydrophobicity (S[subscript o]), began at 18.5 °C and continued to increase up to 75 °C after which it decreased slightly. Total sulfhydryl content (TSH) showed similar trends from 18.5 to 50 °C after which point the TSH began to drop. Surface reactive sulfhydryl groups (SRSH) gradually increased as the temperature increased from 18.5 to 50 °C and then followed a similar trend as the TSH decreased from 55 to 80 °C. Differential scanning calorimetry showed four peaks, three endothermic (27.9, 36.0, 45.5 °C) and one exothermic (49.0 °C). Dynamic rheological measurements provided information concerning the gelation point of salmon myosin which was 31.1 °C as samples were heated at 2 °C/min. Purified tilapia myosin was digested with α-chymotrypsin and purified to obtain heavy meromyosin (HMM) and light meromyosin (LMM). Tilapia myosin, HMM, and LMM were studied using SDS-PAGE. Myosin, HMM, and LMM were linearly heated from 10 to 90 °C and showed protein denaturation/aggregation during heating as measured by turbidity at 320 nm. Conformational changes as measured by surface hydrophobicity (S[subscript o]) showed a marked increase for myosin and HMM between 30 and 40 °C and reached a stable plateau at 70 °C. LMM, in an extremely small magnitude, also showed a continuous increase to 70 °C. Total sulfhydryl content (TSH) showed that the –SH residue content of HMM was nearly double that of LMM. Surface reactive sulfhydryl groups (SRSH) for myosin and HMM were relatively unchanged from 10 to 30 °C but increased significantly from 30 to 50 °C. SRSH content of LMM was lower than that of the TSH content of LMM but both showed a slightly decreasing trend as the sample was heated. Differential scanning calorimetry showed 3 (17.5, 41.9, and 49.9 °C), 2 (43.0 and 67.1 °C), and 3 (40.4, 51.7, and 69.0 °C) major peaks for myosin, HMM, and LMM, respectively. Dynamic rheological measurements demonstrated crossover points, which are generally recognized as gelation point, 40.3 °C for myosin and 27.0 °C for HMM. Obtaining antibodies that would bind the target protein in the highly processed crabstick proved to be a key for the success for assay development. The anti-pep-AP antibody proved to be very accurate in the qualification and quantification of Alaska pollock surimi used in crabsticks. Using the appropriate ELISA format, competitive vs. indirect, proved to be pivotal in obtaining accuracy and repeatability of the assay. Using a direct sandwich ELISA for the qualification and quantification Alaska raw fish fillets provided good estimation of Alaska pollock used in the verification sample. However, the Alaska pollock MLC 1 antibodies exhibited high levels of cross reactivity with other fish species. It has been shown that salmon myosin can be affected by heating. As myosin was heated in solution myosin aggregates began to form as evidenced by the increased turbidity. Myosin denaturation and gel formation was also supported by the thermal transition points determined from differential scanning calorimetry and dynamic rheology. The exposure of buried hydrophobic and sulfhydryl groups of myosin was increased as the myosin was linearly heated. The study of tilapia myosin, HMM, and LMM carried out from chymotryptic digestion of myosin allowed for important insights into the thermo stability and gelation properties of tilapia.
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