Surimi (and surimi-like material) processing water contains high organic loads that may represent an environmental problem, e.g., poultry processors (Velazquez et al., 2007). Therefore, it is essential to find out new efficient technologies for these by-products, with environmentally sound procedures requiring low energy consumption (Dumay et al., 2008; Stine et al., 2012).
The processing water generated in the surimi processing contains 0.5 to 2.3% of total proteins which are composed of sarcoplasmic proteins with small amounts of myofibrillar proteins (Morrisey et al., 2000; Velazquez et al., 2007). The wash water is generally discarded back into the plant’s waste stream, resulting in the loss of valuable components, especially soluble proteins with good functional and nutritional properties that could be recovered and fractionated (Sanmartín et al., 2009; Tacharatanamanee et al., 2004). This is an irresponsible waste of raw-material that is not acceptable in a world with increasing populations that needs proteins (Nølsoe and Underland, 2009). Recovering proteins would reduce the negative environmental impact and the cost of waste disposal, beyond generate potential profits (Bourtoom et al., 2009).
The recovery of proteins may potentially improve yield beyond recycle the water from surimi obtaining processes (Lin et al., 1995) and them added back at a low percentage to surimi products (Stine et al., 2012). Numerous studies have been published on recovering proteins from surimi process (Bourtoom et al., 2009; Dumay et al., 2008; Lin et al., 1995; Rawdkuen et al., 2009; Sanmartín et al., 2009; Stine et al., 2012; Velazquez et al., 2007).
Functional proteins can be efficiently recovered from low-value muscle processing by-products using isoelectric solubilization/precipitation and subsequently be used in value-added human foods (Ramirez et al., 2007). The extraction mechanism is characterized by acid- or alkaline-aided solubilisation of the muscle protein at low or high pH to separate soluble proteins of non-soluble materials, like bones, skin, connective tissue, cellular membranes, and neutral storage lipids through centrifugation (Nølsoe and Underland, 2009; Rawdkuen et al., 2009). Then, precipitated proteins are recovered by centrifugation and decanting (Sanmartín et al., 2009). The benefits of each method with reference to quality, quantity and end use of the recovered proteins need thorough investigation (Bourtoom et al., 2009).
The extracting efficiency to isolate muscle proteins depends mainly on the nature of the raw material to be treated, as well as on the final application, once that the species respond differently to acid and alkaline solubilization (Sanmartín et al., 2009). The increase in the mechanical properties may induce a higher level of protein-protein interaction and thus stronger gels (Velazquez et al., 2007). The importance of high gel strength is dependent on the intended use of the product. The acid and/or alkaline methods provide higher protein yields due to the additional recovery of sarcoplasmic proteins and increased lipid removal caused by the high-speed centrifugation step. However the temperature should be kept below 15°C during the entire isolation process to retain important functional properties like gel strength (Sanmartín et al., 2009).
The aim of this work was to evaluate the effect of adding soluble proteins, recovered by acidic pH-shifting, from mechanically separated chicken meat surimi like-material(MSM-SLM) wastewater on the composition and texture of MSM-SLM.
Material and Methods
Fresh MSCM was supplied from a local poultry processing plant and transported under refrigerated conditions to our laboratory. It was produced in 3 mm particle size using a meat-bone separator, (Model 694 Baader, Germany) operating at inlet 6°C and outlet 10°C, from broiler’s necks, frames and backs, 24 h after the slaughtering.
Mechanically separated chicken meat (MSCM) was washed in 3 cycles, utilizing in each cycle a washing solution: MSCM ratio of, 4:1 (w/v) for 10 min at 7°C. The stirring was kept constant at 220 rpm. It was utilized 0.5% NaHCO3 solution for the first and second washings and 0.3% NaCl solution for the last one. The first and second centrifugations were carried out at 3,000 g for 15 min, while the third one at 7,000 g for 25 min. (Cortez-Vega et al., 2015).
The fractions of supernatant containing fat and water-soluble proteins obtained from each washing cycle were mixture. Then the soluble protein was isolated by the process of pH variation at the isoelectric point in pH 5, according to methodology adapted from elsewhere (Nølsoe and Underland, 2009). The homogenate was ice-cooled and the pH adjusted to 10.8 for alkaline protein isolation by drop-wise addition of 2 M NaOH. The pH-adjusted homogenates were centrifuged at 10,000 g for 20 min at 4°C. The precipitation was performed by adjusting the pH of the supernatant to pH 5.0 by drop-wise addition of 2 M NaOH. After that, the material was again centrifuged at 10,000 g for 20 min at 4°C. Then, the supernatant was poured off and the recovered protein was collected and maintained at 4°C until all batches were processed.
To prepare the gel, surimi-like material (SLM) was added of recovered protein (RP) in order to obtain 0, 10, 20 and 30% (w/w) of RP in the mixture. The RP-SLM mixture was added of 2.5% NaCl and chopped for 5 min at 4°C to obtain the homogenous sol. The sol was then stuffed into stainless steel cylinders (30 mm i.d., 30 mm ht) and both ends of casing were sealed tightly. Two-step heated gels were prepared by setting the sol at 40°C for 30 min, followed by heating at 90°C for 20 min. The gels were then cooled in iced water and stored for 24 h at 4°C prior to analysis.
Moisture, crude protein, crude fat and crude ash contents were determined in triplicate according to the methods described by AOAC (1995). Moisture was determined by the oven drying method at 105°C until constant weight (method 950.46), protein by the Kjeldhal method (method 928.08), fat by the Soxhlet method (method 960.39) and ash by using the muffle oven technique (method 920.153).
pH was measured using a digital pH meter (Model PA 200, Marconi Instruments, Inc., Brazil). About 10 g of sample (MSCM, MSCM-RP, MSCM-SLM, MSCM-RP-SLM) was cut into small pieces to which 50 mL of distilled water was added and slurry was made using a blender and the pH was recorded.
The myofibrillar proteins was determined according to the method described by Jin et al. (2007) and Kuo and Chu (2003). Myofibrilla proteins were isolated from samples by homogenizing 4 g of minced samples in a Metvisa homogenizer (Model TA-2, SC, Brazil) for 10 s in 10 vol. (v/w) of a 2°C isolating medium containing 100 mM KCl, 20 mM potassium phosphate (pH 7.0), 1 mM EDTA and 1 mM sodium azide. The homogenate was sedimented at 1,000 g for 15 min and the supernatant decanted. The sediment was resuspended at 1,000 g for 15 min and the supernatant decanted. The sediment was again resuspended in 5 vol. (v/w) of the original isolating medium and passed through a polyethylene strainer to remove connective tissue and debris. Five more volumes, resulting in 10 vol. (v/w) total of the original isolating medium, were used to further facilitate passage of myofibrilla protein through the strainer. Again, the supernatant was sedimented at 1,000 g for 15 min and the supernatants decanted. The sediments were washed three more times by suspending in 5 vol. (v/w) of the original isolating medium, and were sedimented at 1,000 g for 15 min. Finally, the sedimented myofibrilla proteins were resuspended in 5 vol. (v/w) of the original isolating medium. The protein concentration was determined by the biuret procedure described by Clark and Switzer (1977).
Texture analysis of surimi gel was carried out using a texture analyzer (Model 3 TA-XT2 plus; Stable Micro Systems, Surrey, England). Gels kept at 4°C were equilibrated at room temperature (28-30°C) before analysis. Cylindrical samples, 2.5×3.0 cm, were prepared and placed in the texture analyzer equipped with a spherical plunger (5 mm diameter; 60 mm/min depression speed). The results were expressed as breaking force (g) and deformation (mm) representing the hardness and cohesiveness of the surimi gels, respectively. Gel strength (g.cm) was expressed as the product of breaking strength and deformation. Analogously, samples were submitted to a cutting/shearing test using a knife blade. The cutting strength (N) is correlated to the firmness of the sample and the work of shear (N.s) indicated the total energy (work) required to shear (Cortez-Vega et al., 2015).
Results and Discussion
Proximate composition and pH of mechanically separated chicken meat (MSCM), mechanically separated chicken meat surimi like-material (MSCM-SLM), mechanically separated chicken meat recovered protein (MSCM-RP) and mechanically separated chicken meat surimi like-material added with recovered protein (MSCM-RP-SLM) at different levels are given in Table 1.
The composition of the unwashed MSCM is in accordance with results previously reported (Cortez-Vega et al., 2015; De Oliveira et al., 2014; Perlo et al., 2006; Rivera et al., 2000; Smyth and O’Neill, 1997). As the MSCM composition varies in function of the raw material utilized (Cortez-Vega et al., 2015), a variable number of washings may be necessary to obtain surimi-like material with reduced fat contents. Here, the fat content of the MSCM-SLM was significantly reduced to 11.98% and protein increased to 83.64% (dry basis) (p<0.05) after three washing cycles (Table 1).
*Dry basis: UW MSCM, unwashed mechanically separated chicken meat; SLM, surimi-like material; SLM W3, SLM obtained after three washing cycles; RP, Recovered protein; nd, not determined.
Average and standard deviation calculated from triplicate analyses of five samples.
Different letters in the same column indicate significant differences at p<0.05.
It is important to highlight that the protein recovery was very high for a conventional method for SLM obtaining. Rawdkuen et al. (2009), e.g., recovered protein from minced tilapia muscle using three methods. The highest protein recovery was obtained in the acid-aided process (85.4%), followed by the alkaline-aided process (71.5%) and conventional method (67.9%).
The MSCM-SLM was then added of sarcoplasmic soluble proteins left in the supernatant after the centrifugations by the acid-aided process. Thus, the most obvious finding observed was that the increase of the RP concentration in the MSCM-SLM significantly augmented the protein content from 83.64% to 93.45% and reduced fat content from 11.98% to 2.78%. This was expected since the majority of the water-soluble components, including sarcoplasmic proteins and inorganic salts, were removed during the leaching for MSCM-SLM obtaining. Despite the concentration of the myofibrillar proteins is increased during this process, the addition of recovered proteins from other nature has decreased the final myofibrillar protein content of the MSCM-SLM, as much as RP was added. In agreement, Rawdkuen et al. (2009) found decreases in myoglobin and lipid contents during protein recovery.
Ash and moisture contents were also reduced, but the difference was in most of the cases not statistically significant (Table 1). Washing increased the pH (p>0.05) of the MSCM, but the addition of recovered protein on the MSCM-SLM reduced it (p>0.05) (Table 1).
Texture is shown in Table 2. SLM prepared from MDCM has the texture parameters increased with the washings (Nowsad et al., 2000). However, the addition of recovered to the SLM-MDCM showed a decrease in texture as the RP increased from 10 to 30% (Table 2).
Previous studies report breaking forces of 946 g (Cortez-Vega et al., 2012) and 1003.4 g (Cortez-Vega et al., 2015) and gel strengths of 838.2 g.cm (Cortez-Vega et al., 2012) and 646.85 g.cm (Cortez-Vega et al., 2015) for SLM-MSCM obtained after three washing cycles. The addition of recovered protein was responsible for a drastic decrease in both the parameters, reaching 252.36 g and 185.23 g.cm for breaking force and gel strength, respectively, when 30% of RP was included in the MSCM-SLM (Table 2). The same behavior was observed for the cutting strength. Cortez-Vega et al. (2015) reported a cutting strength of 9 N for MSCM-SLM obtained after three washing cycles while here the MSCM-SLM (from three washing cycles) added with 30% RP presented a cutting strength of 6.97 N.
SLM, surimi like material; RP, Recovered protein. Gel strength = breaking force x distance to rupture.
*Average and standard deviation calculated from triplicate analyses of five samples.
Different letters in the same column indicate significant differences (p<0.05) between treatments.
It was reported that gels prepared from conventional surimi presents greater breaking force than that from acidaided processes (Rawdkuen et al., 2009). The reduction in the texture can be associated with the settling effect on the gel structure (Nowsad et al., 2000) that decreased with the addition of recovered protein, once that the gel-forming ability of surimi varies with the function of the myofibrillar proteins (Rawdkuen et al., 2009). This is underlined by the pH decrease observed with the RP addition (Table 1). The obtained results suggest that the protein was extremely denatured due to the acidic pH-shift process (Rawdkuen et al., 2009) and that the neutralization of acidified protein did not recover the level of gel strength found before acidification (Shikha et al., 2006). Moreover, the sarcoplasmic proteins may have had adversely effected the strength of myofibril protein gels due bounding to the myofibrils during the heat treatment (Rawdkuen et al., 2009).
Ramirez et al. (2007) related that the recovered proteins from surimi wash water retained the gel-forming ability of surimi, but failed to gel unless beef plasma protein was added, which was obtained with the addition of potato starch, transglutaminase, and phosphate to the recovered proteins.
The reduced characteristics of RP-MSCM-SLM gels are due the presence of tropomyosin, troponin and myosin light chain, which were removed with washings, but re-added in the formulations. These compounds interfere in protein-protein interactions involved in gel formation. However, it was also found in the literature that surimi with a substitution 10% of recovered proteins did not have diminished the texture properties (Lin et al., 1995). Other authors observed the decrease in the mechanical properties only if high contents of recovered protein were added to the surimi (Velazquez et al., 2007).
Several successful applications have been reported for fish protein recovered from the wastewater derived from the surimi obtaining processing. However the recovered protein from the surimi-like material obtaining processing from mechanically separated chicken meat was responsible to reduce the texture properties of the surimi-like material when added back even at the low percentage of 10%. On the other hand, the final protein content of the surimi-like material added of recovered protein has significantly increased and the crude fat significantly reduced, which turns the obtained intermediary product an option to applications in processed meat products where low gel strength is as suitable as high gel strength products.