Characterization of Extruded and Toasted Milk Protein Concentrates

Important functional properties of milk protein concentrate with 80% protein (MPC80), modified with low‐ and high‐shear extrusion, or low‐temperature toasting were compared. The effect of high‐ and low‐shear profile screws in a corotating twin‐screw extruder, and 4 different ramped temperature profiles with die temperatures of 65, 75, 90, and 120 °C were compared. Extrudates were pelletized, dried, and ground to a fine powder. Toasting was done at 75 and 110 °C for 4 h for milk protein modification. Extruded and toasted MPC80 had reduced protein solubility and surface hydrophobicity. Extrusion decreased water‐holding capacity (WHC). Toasted MPC80 had increased WHC when treated at 75 °C, but WHC decreased when heated at 110 °C. The treatments had no strong influence on gel strength. Reduced and nonreduced sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed peptide structural changes that occurred due to processing, especially for whey proteins. Results are discussed in terms of potential for application of extruded or toasted MPC80 in high‐protein nutrition bar applications.     

Effect of extrusion cooking on structure and functional properties of pea and kidney bean proteins

Pea (Pisum sativum L cv Ballet) and kidney bean (Phaseolus vulgaris L cv Pinto) seeds were extruded at 148 and 156 °C respectively. Protein solubility at various pH values and in various solvents was determined and analysis of protein fractions was carried out by SDS‐PAGE. Also, sulphhydryl and disulphide groups, water‐holding capacity (WHC), water solubility index (WSI) and oil absorption capacity (OAC) were determined. No changes in total nitrogen content of pea and kidney bean seeds occurred as a result of thermal treatment. Protein solubility from raw and extruded legumes was significantly higher in saline solutions than in water in the pH range 2–10. The solubility of proteins from extruded pea and kidney bean flours was greatly decreased with respect to native flours when extraction was in buffer (pH 7.0) alone. Extraction with buffer containing 2‐mercaptoethanol (2‐ME) or sodium dodecyl sulphate (SDS), alone or in combination, greatly increased protein extractability. As a result, the relative solubility was nearly 100% in buffer with SDS and 2‐ME for both raw and extruded samples. Total and free sulphhydryl group and disulphide contents decreased significantly (P < 0.05) after extrusion cooking. Moreover, extrusion treatment caused major changes in the band patterns of the albumin and globulin fractions obtained by SDS‐PAGE. WHC and WSI of extrudates increased significantly in both peas and kidney beans. A significant reduction in OAC was observed in extruded kidney bean flour. © 2000 Society of Chemical Industry     

Antimicrobial Activity of Nisin and Natamycin Incorporated Sodium Caseinate Extrusion‐Blown Films: A Comparative Study with Heat‐Pressed/Solution Cast Films

Antimicrobial edible films based on sodium caseinate, glycerol, and 2 food preservatives (nisin or natamycin) were prepared by classical thermomechanical processes. Food preservatives were compounded (at 65 °C for 2.5 min) with sodium caseinate in a twin‐screw extruder. Anti‐Listeria activity assays revealed a partial inactivation of nisin following compounding. Thermoplastic pellets containing food preservatives were then used to manufacture films either by blown‐film extrusion process or by heat‐press. After 24 h of incubation on agar plates, the diameters of K. rhizophila growth inhibition zones around nisin‐incorporated films prepared by solution casting (control), extrusion blowing or heat pressing at 80 °C for 7 min of nisin‐containing pellets were 15.5 ± 0.9, 9.8 ± 0.2, and 8.6 ± 1.0 mm, respectively. Since heat‐pressing for 7 min at 80 °C of nisin‐incorporated pellets did not further inactivate nisin, this indicates that nisin inactivation during extrusion‐blowing was limited. Moreover, the lower diameter of the K. rhizophila growth inhibition zone around films prepared with nisin‐containing pellets compared to that observed around films directly prepared by solution casting confirms that nisin inactivation mainly occurred during the compounding step. Natamycin‐containing thermoplastic films inhibited Aspergillus niger growth; however, by contrast with nisin‐containing films, heat‐pressed films had higher inhibition zone diameters than blown films, therefore suggesting a partial inactivation of natamycin during extrusion‐blowing.     

High Temperature Extrusion Effects on Protein Solubility and Distribution in Navy and Pinto Beans

Osborne protein solubility fractionation and polyacrylamide gel electrophoresis (PAGE) showed changes in protein distribution during high temperature extrusion of pinto and navy bean high starch fraction (HSF). A high degree of protein insolubility was found after extrusion, which resulted in a decrease in solubility of albumin and globulin fractions and an increase in the residue. An extrusion temperature of 110°C had a greater effect on solubility of albumin and globulin fractions of pinto than navy bean. Sodium dodecyl sulfate (SDS)-PAGE showed more changes in subunit patterns of albumin and globulin for pinto than for navy beans at 110°C. Higher temperatures of 135 and 150°C caused greater changes in gel electrophoretic patterns of albumin and globulin fractions of navy beans.     

Extrusion studies of mixtures containing certain meat offals: Part 2-textural properties

Thermoplastically extruded products prepared from soy grits alone, and from those containing 20% or 35% of bovine or porcine offal, at various temperatures of extrusion, were subjected to instrumental texture profile analysis (hardness, elasticity, cohesiveness, gumminess, chewiness) and shear force measurements. The offal sources were untreated, or alkali-extracted, protein from bovine and porcine lung and bovine tripe (rumen and reticulum) and protein in extracted by sodium dodecyl sulphate (SDS) from bovine tripe. For products prepared from soy grits alone, hardness, gumminess, chewiness and shear force showed maxima at 170°C, whereas elasticity and cohesiveness increased progressively in the temperature range studied (140-190°C). Products containing SDS-extracted tripe protein required more force to shear, and were more hard, gummy and chewy, than products containing untreated offal, and values for all parameters were greater in the latter than in products containing alkali-extracted offal protein. The effect of extrusion temperature on the textural parameters of the products was less than that due to the mode of offal preparation used (untreated, alkali- or SDS-extracted) or the level of incorporation. Offal source was relatively unimportant. Products containing alkali-extracted offal protein had poor internal structure and were brittle; those containing SDS-extracted protein were tough. The different textural properties of the extracted products might determine their relative suitability for use either alone, as analogues or as extenders.     

CTAB electrophoresis and immunoblotting: a new method for the determination of soy protein in meat products

A CTAB electrophoresis method that is able to separate soy protein on the basis of molecular mass is introduced. The cationic detergent CTAB (N-cetyl-N,N,N-trimethylammoniumbromide) is not as denaturing as sodium dodecyl sulphate (SDS), and thereby allows separation of proteins that are virtually unchanged from their naturally occurring state. After blotting on a nitrocellulose (NC) membrane, the protein can be detected by a soy-specific antibody. Soy protein isolates, concentrates and hydrolysates as well as meat products containing these soy protein products were separated by CTAB electrophoresis and blotted on NC membranes. As little as 0.5% soy protein in meat products could be detected, even if the meat product had been heated to 120°C for 30 min during the manufacturing process.     

Formation of acid‐precipitated soy protein–dextran conjugates by Maillard reaction in liquid systems

The conjugation reaction between soybean acid‐precipitated protein (SAPP) and dextran in liquid systems via the initial stage of the Maillard reaction was studied. Functional SAPP–dextran conjugates were prepared in 80% ethanol‐reacting system at 50 °C for 6 h, along with 95% ethanol‐reacting system at 60 °C for 24 h. The covalent attachment of dextran to SAPP was confirmed by sodium dodecyl sulphate–polyacrylamide gel electrophoresis and gel filtration chromatography.Compared to the classical dry‐heating, the reaction time of glycosylation in the two ethanol systems was largely shortened. Emulsifying activity of SAPP–dextran conjugates obtained by dry‐heating incubation and in ethanol was similar at pH 7.0 and10.0, significantly higher than that of SAPP–dextran mixture or SAPP alone. In addition, SAPP–dextran conjugates obtained in 80% ethanol‐reacting system for 6 h were completely soluble after heating at 90 °C for 20 min. The impact of various processing conditions on the formation of SAPP–dextran conjugates was investigated. This study provides important guidance to create protein–polysaccharide conjugates at mild temperatures in liquid systems.     

Rheological Behavior and Potential Cross-Linking of Pacific Whiting (Merluccius productus) Surimi Gel

Gelation behavior and potential cross-linking of Pacific whiting (Merluccius productus) surimi were affected by setting temperatures and an enzyme inhibitor. Gels of Pacific whiting surimi with salt and beef plasma protein were compared with those containing guanidine hydrochloride, sodium dodecyl sulfate, and β-mercaptoethanol. The strongest gels were formed at 25°C setting followed by 90°C heating. Hydrogen and hydrophobic bonds appeared to strongly influence gel formation, while the influence of disulfide bonds was moderate. Viscosity scanning during setting at different temperatures was also useful to estimate effects of enzymes and inhibitors.     

Purification and Characterization of β-Glucosidase from Aspergillus niger

Aspergillus niger, an isolate of soil contaminated with effluents from cotton ginning mill was grown in Czapek-Dox medium containing sawdust, Triton-X 100 and urea for production of an extracellular β-glucosidase. β-Glucosidase enzyme was purified (86-fold) from culture filtrate of A. niger by employing ammonium sulphate precipitation and gel filtration on sephadex G-75. The molecular mass of the purified enzyme was estimated to be 95 kDa by sodium dodecyl sulphate polyacrylamide gel electrophoresis. The enzyme had an optimal activity on p-nitrophenyl β-D-glucopyranoside at 50°C and pH 5.0. The K_m and V_max of the enzyme on p-nitrophenyl β-D-glucopyranoside at 50°C and pH 5 were 8.0 mM and 166 µmol/min/mg of protein, respectively. The enzyme could hydrolyze cellobiose and lactose but not sucrose. Heavy metals like Hg²⁺, Al³⁺, and Ag⁺ inhibited the activity, whereas Zn²⁺ and detergents such as Triton-X 100 and Tween-80 increased the activity at 0.01%. The enzyme activity increased in the presence of methanol and ethanol.     

Sodium Dodecyl Sulfate and Sulfite Improve Some Functional Properties of Soy Protein Concentrates

The effects of sodium dodecyl sulfate (SDS) and sodium sulfite were studied on dispersibility, water and oil absorption of soy protein concentrates from toasted soy flours. Dispersibility was improved from 6.2% to 65% by 0.5% SDS, 0.25% sodium sulfite at pH6, and 60°C. Water absorption was improved from 5.5 to 7 mL water/g concentrate by 0.5% SDS at pH 6 at 20°C. Oil absorption was improved from 4.1 to 5.9 mL oil/g concentrate by 0.5% SDS, 0.25% sodium sulfite at pH 6 and 20°C.     

Improvement in emulsifying properties of soy protein isolate by conjugation with maltodextrin using high‐temperature,short‐time dry‐heating Maillard reaction

Soy protein isolate (SPI)–maltodextrin (MD) conjugates were synthesised using Maillard reaction under high‐temperature (90, 115 and 140 °C), short‐time (2 h) dry‐heating conditions. The loss of free amino groups in proteins and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS‐PAGE) profile confirmed that SPI‐MD conjugates were formed and higher dry‐heated temperatures could increase the glycosylation degree. The emulsifying properties of SPI and SPI‐MD conjugates were evaluated in oil‐in‐water emulsions. The emulsions stabilised with SPI‐MD conjugates synthesised at 140 °C exhibited higher emulsifying stability and excellent storage stability against pH, ionic strength and thermal treatment compared with those synthesised at 90 °C, 115 °C and SPI stabilised emulsions. This might be due to a greater proportion of conjugated MD in SPI‐MD conjugates synthesised at 140 °C because of the higher glycosylation degree, and more conjugated MD on the droplet surface could provide steric effect and enhance the stability of the droplets in the emulsions.     

Extractability and thermal stability of frozen hake (Merluccius merluccius) fillets stored at −10 and −30 °C

The relationship between thermal stability changes and functionality loss was monitored in hake muscle fillets stored for 40 weeks at ?10 and ?30 °C. The evolution of changes in apparent viscosity, dimethylamine formation and extractability of muscle proteins in NaCl, sodium dodecyl sulphate (SDS) or SDS plus mercaptoethanol showed drastic differences as a function of temperature. At the higher storage temperature, both myosin heavy chain and collagen were the most severely unextracted in salt and SDS solutions, with actin becoming unextractable at the end of storage. Differential scanning calorimetry showed differences with storage time and temperature in both onset temperature and thermal denaturation enthalpy, mostly affecting the myosin transitions. Some protein denaturation occurred with little or no functionality loss. A considerably high fraction of hake muscle proteins remained in the native‐like condition even at the higher frozen storage temperature. In these conditions both apparent viscosity and myosin and actin extractability in NaCl were very low. © 2002 Society of Chemical Industry     

Biochemical and nutritional changes in fish proteins during drying

Biochemical and nutritional changes in the muscle proteins of a lean marine fish Nempiterus japonicus during drying at 50, 60 and 70°C were investigated. Solubility of proteins in water, 0.6 M NaCl, 1.5 M urea, 8 M urea and 10 g litre^?1 sodium dodecyl sulphate (SDS) decreased as drying progressed at all three temperatures; most of the decrease occurring in the initial 4 h of drying SDS-polyacrylamide gel electrophoresis of 1.5 M urea, 8 M urea and SDS extracts showed that higher molecular weight (MW) protein fractions were more sensitive to drying and disappeared much earlier from electropherograms than the lower MW protein fractions. Residual solubility of proteins near the pH range of 4–6 was found to increase during drying, but solubility at acid and alkaline pH was adversely affected. Decrease of solubility by drying was more affected at acid pH, especially at higher temperatures than at alkaline pH. Sulphydryl groups registered a regular and sharp decrease with drying except at 50°C, where initially an increase was observed. Apart from disulphide and hydrophobic bonds, free amino groups also appear to be involved in denaturation reactions during drying. Pepsin digestibility of fish muscle decreased slightly during drying but a clear relationship with drying temperature was not evident. Highly significant differences in proteins between protein efficiency ratio (PER), net protein utilisation (NPU) and biological value were observed between the drying temperatures. The PER and NPU of fish dried at 60°C were significantly higher than those dried at 50 or 70°C.     

The effect of cooking on proteins from acha (Digitaria exilis) and durum wheat

The effect of cooking on proteins from acha and durum wheat was assessed from an analysis of protein extractability, gel electrophoretic profiles, in-vitro protein digestibility (IVPD) and the amino acid compositions of wholemeal flour and residue proteins. Heating wholemeal flour samples at 100–140°C (t = 10–40 min) resulted in 0–30% and 45–55% decreases in acha and durum protein solubility, respectively. In general, high molecular weight (30–70 k Da) protein subunits were more susceptible to heat damage. For both cereals, sodium dodecyl sulphate (SDS; 10 g litre^?1) and/or dithiothrcitol (DTT; 10 mM ) increased protein solubility in unheated and heated samples. The IVPD index was 90–91% and was not significantly altered by cooking (100–120°C, t = 40 min). Cooking at extreme temperatures (140°C, t = 40 min) reduced the IVPD by 8% (P = 0.05). Osborne fractionation resulted in a durum or acha residue level of 7.8% or 55.2%. Treatment with solvent containing propanol, SDS and/or DTT at room temperature followed by SDS-polyacrylamide gel electrophoresis of non-solubilised proteins showed that the glutelin fraction of acha, with the exception of a 65 kDa subunit, was insoluble owing to strong inter-subunit hydrophobic and disulphide interactions. Wholemeal acha flour and residue protein showed a significantly greater level of hydrophobic and sulphur amino acids as well as glutamine which is associated with H-bonding. The possibility that cereal protein solubility is also dependent on protein-carbohydrate links is discussed.     

Effect of SDS on Acid Milk Coagulability

ABSTRACT To study the importance of hydrophobic interaction on the mechanism of acid milk gel formation, milk coagulation process was monitored at 30 °C in presence of various levels of sodium dodecyl sulphate (SDS). As a function of the SDS concentration, acid milk coagulability was either enhanced or reduced. Main pH‐induced biochemical changes were preserved despite the presence of SDS (such as pH‐induced demineralization and pH‐induced protein solubilization). It could be assumed that SDS‐modified casein micelles ability to coagulate by lowering of pH might seem to be governed essentially by the level of SDS‐induced k‐casein micellar dissociation, at natural milk pH.     

Freezing/defrosting/frying of sardine fillets. Influence of slow and quick defrosting on protein quality

The effect of sequential freezing/defrosting/frying on protein quality is not well known. With this in mind, fillets of fresh sardine were stored frozen, then thawed, either conventionally at 4 °C in a refrigerator or with the use of a microwave oven, and subsequently deep‐fried. Proximate and amino acid compositions, protein solubility in sodium dodecyl sulphate/β‐mercaptoethanol (SDS/β‐ME), total ? SH group content and amino acid chemical score were determined. The lowest protein concentration was observed in frozen/4 °C‐thawed sardines (CR), whilst the lowest fat content was found in both fresh/fried sardines (F) and 4 °C‐thawed/fried sardines (CF). Every step of each process studied caused a decrease in cyst(e)ine; the most important loss was recorded in CF samples and in frozen sardines fried without defrosting (Fro‐F). The lowest solubility in SDS/β‐ME and the lowest total ? SH group content were observed for Fro‐F samples and microwave‐thawed/fried sardines (MF). On the other hand, the lowest chemical score was found for Fro‐F, CF and MF samples. Although weight loss and proximate composition seemed to change less when defrosting sardine fillets using a microwave oven rather than at 4 °C, the results for SDS/β‐ME solubility and total ? SH group content suggest that a slow defrosting process (refrigerator at 4 °C) is preferable to a much quicker process (microwave oven) for thawing frozen sardine fillets before frying. Copyright © 2003 Society of Chemical Industry     

Flocculation of Kluyveromyces marxianus is induced by a temperature upshift

An upshift of the growth temperature from 26 to 40°C in the presence of calcium leads to the aggregation of Kluyveromyces marxianus cells and to the formation of flocs. Analysis of cell wall proteins, either by sodium dodecyl sulphate–polyacrylamide gel electrophoresis of extractable mannoproteins or by immunolocalization, revealed an accumulation of a protein with Mr 37 kDa (p37), upon flocculation. Immunological studies confirmed the homology of this protein with the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). When mRNA isolated from cells growing at 40°C was translated in vitro, a 35 kDa newly labelled protein was synthesized and immunoprecipitation assays showed that this protein is recognized by p37-antiserum, suggesting that the 35 kDa polypeptide might be an unglycosylated precursor form of p37. The results indicated that the presence of this cell wall mannoprotein closely related to GAPDH is dependent on the growth temperature, suggesting its role as adhesin.     

Further studies on SS bonds in cereal glutelins

Flours of wheat, rye, barley, oats, maize and sorghum have been extracted to remove albumins, globulins and prolamins. The SS bonds of the starch-glutelin residues (4 to 8% protein) have been reduced with sulphite and titrated with phenyl mercury acetate (PMA) in the presence of 3 M -guanidine hydrochloride at 37 °C and, in its absence, at 2 and 37 °C. Attention has been paid to several possible sources of error including oxidation and the rate of diffusion of PMA into protein particles. Approximate values for the diffusion coefficients of PMA through cellulose and protein films were obtained. Titrations at 37 °C in the absence of guanidine hydrochloride are unsatisfactory due to reaction of intrachain SS links. Evidence in the literature suggests that the SS bonds titratable at 2 °C are inter-chain, possibly includingsomestrained intra-chain bonds. The results imply that most of the major polypeptide chains in the cereal glutelins examined, apart from barley and sorghum, contain two such bonds. In the case of barley glutelin probably less than half the chains have two labile SS bonds. Most of the chains in sorghum glutelin appear to have a single labile bond and the polymers may contain only a few chains. The molecular weights (in thousands) of the principal polypeptide chains of the glutelins, deduced from gel electrophoresis in sodium dodecyl sulphate (SDS), are: Cappelle-Desprez wheat 44, 41; Manitoba wheat, rye and barley, 44; oats, 33, 23; maize, 23, 19; sorghum, 22, 18.     

Effect of Pulsed Ultraviolet Light and High Hydrostatic Pressure on the Antigenicity of Almond Protein Extracts

The efficacy of pulsed ultraviolet light (PUV) and high hydrostatic pressure (HHP) on the IgE binding to the almond extracts was studied using sodium dodecyl sulfate polyacrylamide-gel electrophoresis, Western blot, and enzyme-linked immunosorbent assay (ELISA) probed with human plasma containing IgE antibodies to almond allergens and a polyclonal antibody against almond major protein. Crude almond protein extracts were treated with PUV (3 pulses/s, 10 cm from lamp) for 0.5, 1, 2, 3, 4, 6, 7, and 10 min. In comparison, boiling treatments were also carried out. The HHP treatments were conducted at 600 MPa for 5, 15, and 30 min at three temperatures of 4 °C, 21 °C, and 70 °C. Western blots and indirect ELISA demonstrated a reduction in the levels of allergens and IgE binding in PUV-treated extracts at 7 min, which was found to be the optimal time for PUV exposure. Boiling was not as effective as PUV in reducing the overall IgE-binding of the almond extracts. Unlike PUV, HHP did not affect the allergen levels and IgE binding under the conditions tested.     

Effect of heat treatment on the properties of soy protein‐stabilised emulsions

The effect of heat treatment on the properties of soy protein‐stabilised emulsions was investigated. Emulsions were prepared with unheated and heat‐treated soy protein (NSP and HSP) dispersions. Heating on soy protein dispersions at 95 °C for 30 min resulted in smaller average oil droplet size, lower tendency for oil droplet flocculation, higher protein adsorption and lower viscosity. The properties of emulsions were significantly influenced by the protein concentration. The sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) profiles showed that the heat treatment on soy protein dispersions increased the protein adsorption at O/W interface. The viscosity of all samples at low shear rate was inversely proportional to the d₃₂, suggesting a positive relation to the total interfacial area per unit volume. Emulsions showed shear‐thinning behaviour. The relaxation time was found to increase with aqueous phase viscosity determined by the Cross viscosity model.