Chemical and physical lipophilization of proteins

With a view to enhance the amphipathic nature of food proteins, chemical and physical modification were carried out. Soy glycinin and α_S1-casein were lipophilized by chemically attaching naturally occurring fatty acids to them. The covalent attachment of fatty acyl residues to these proteins caused an increase in their emulsification activity. Soy proteins and the maize protein, zein, were associated with soy lecithin and phosphatidate, respectively, by sonication. The emulsification activity of phospholipid-protein complexes was greatly increased after they were treated with 50% ethanol or enzyme digestion.     

Lipid components that reduce protein solubility of soy protein isolates

A lipid fraction from a commercial soy protein isolate (SPI), previously found to be detrimental to SPI solubility, was analyzed by size-exclusion liquid chromatography, by high-performance liquid chromatography (HPLC), and for chemical composition. The molecular weight of most of this material was greater than 1,100 daltons. This lipid fraction was water-soluble yet required a strong nonpolar solvent mixture to elute it from a C₁₈ HPLC column. The lipid material was alkaline (pH 8.7) and composed of 3.0% nitrogen, 1.6% phosphorus, 17.5% nonvolatile crude fatty acids primarily hydroxylated), 10.4% long-chain bases, 9.9% hexuronic acid, 3.2% hexosamine, and 6.6% total sugar. The molecular weight, chemical composition, and physical characteristics (solubility characteristics, surfactant characteristics, and appearance) of this material were all similar to those reported for phytoglycolipid.     

Nonpolar-volatile lipids from soy protein isolates and hexane-defatted flakes

Lipid extracts from two samples of commercial soy protein isolates (SPI) and two samples of commercial hexane-defatted flakes were fractionated by silici acid-column chromatography. The material eluted with 100% chloroform was collected, further fractionated by silica solid-phase extractions, and analyzed by gas chromatography-mass spectroscopy by using mass spectra, retention times of authentic standards, and Kovats indices for identification. Thirty-eight compounds were identified and quantitated in the lipid fractions from soy protein isolates (SPI); 23 of these are reported for the first time as components of SPI. An additional 13 compounds are reported for the first time as components of hexane-defatted soybean flakes. The major classes of compounds reported for the first time associated with SPI include: butyl, methyl, and ethyl esters of fatty acids; phenols, diphenyls and phenyl esters; and abietic acid derivatives. Dehydroabietinal at 0.180 to 0.191 ppm of the protein isolates was the most abundant aldehyde in the SPI lipid extracts. The third most abundant aldehyde found in SPI after dehydroabietinal and hexanal was 2-butyl-2-octenal (0.065 to 0.086 ppm). Dehydroabietic acid methyl ester was present in SPI (0.309 to 0.459 ppm). Dehydroabietene (0.628 ppm) and abietatriene (0.396 ppm) were tentatively identified in one sample of hexane-defatted flakes.     

Changes of isoflavones during processing of soy protein isolates

Soy protein isolate (SPI) is a widely used food ingredient and is made by extracting soy flour (SF) under slightly alkaline pH, followed by precipitation, washing, and drying. Soy foods and foods containing soy protein ingredients have great potential in the prevention of cardiovascular diseases and cancers. These health benefits have been attributed to isoflavones in soy protein ingredients. However, the current processing techniques were developed many years ago without this knowledge. The objective of this study was to investigate the mass balance of different isoflavones during manufacturing of SPI and to provide basic information to assist further development efforts leading to preservation of soy isoflavones in soy protein ingredients. The study revealed that only about 26% of the total isoflavones in SF remained in SPI. The percentages of total isoflavones lost during extraction, precipitation, and washing were 19, 14, and 22%, respectively. Washing was the step where most isoflavones were lost. The isoflavone profile of the SPI was different from that of SF. The former contained much more aglucones (genistein and daidzein), while the latter had almost none. The high content of aglucones in SPI was probably due to the hydrolysis of glycosides.     

Composition,nutritional, and functional properties,and quality criteria of soy protein concentrates and soy protein isolates

The available commercial soy protein concentrates and soy protein isolates afford the food processor concentrated sources of protein with some interesting and varied functional properties. Each class of products is mild to bland in flavor and light in color. The concentrates contain at least 70% protein and the isolates 90%. The nutritional quality of the proteins is fair to good and can be excellent either by supplementation with 1.5% methionine or by appropriate blending with other sources of proteins. The concentrates provide the food manufacturer with products where a high protein content for unit of volume or wt is needed. The isolates are available for uses where the functional properties reside solely in the protein and the nonprotein components may interfere. The adaptability of the proteins to modification by controlled processing conditions has made it possible for the manufacturers to produce a diversity of products that should be of interest to practically all food formulators.     

Rheological properties of emulsions containing modified soy protein isolates

The feasibility of replacing common emulsifiers with soy protein isolates (SPI) in low-calorie salad dressings was evaluated. Structural modifications of SPI were obtained by thermal-acidic treatment with or without neutralization (TH1.6N and TH1.6, respectively). Modification of flow properties of TH1.6 and TH1.6N emulsions by thermal treatment and different protein concentrations was evaluated through shear stress vs. shear rate measurements in a rotational viscometer. TH1.6N isolates generated emulsions with higher shear stress and apparent viscosity than those prepared with TH1.6. Heated TH1.6N emulsions at 10% protein gave the highest values of shear stress and plastic flow behavior. These emulsions had high consistency, viscosity, and elasticity. TH1.6N isolates had lower emulsifying capacity than TH1.6, probably due to the higher protein aggregation produced during neutralization, which prevented protein unfolding. These isolates would be suitable for the preparation of stable emulsions with adequate consistency and elasticity.     

Identification of trans-3,5-dimethoxystilbene in commercial soy protein isolates

A previously unidentified component of the lipid extracts from commercial soy protein isolates (SPI) was analyzed by gas chromatography-mass spectrometry (GC-MS), high resolution mass spectrometry (HRMS), ultraviolet-visible spectroscopy, and GC-Fourier transform infrared spectrometry (FTIR). All these data, together with mass spectra of derivatives obtained by hydrogenation, indicated the structure of an unsymmetrical dimethoxystilbene. Subsequently, standard trans-3,5-dimethoxystilbene, synthesized according to established procedures, was found to have identical retention times and spectra by GC-MS and GC-FTIR with the compound isolated from commercial SPI. Laboratory SPI prepared from Probst, Stressland, and Burlison variety soybeans contained no detectable amounts of either trans-3,5-dimethoxystilbene or dehydroabietinal.     

Retention of isoflavones and saponins during the processing of soy protein isolates

The mass balance of saponins during processing of soy protein isolates (SPI) was established, and the effects of precipitating and washing (P/W) temperatures (0, 10, 25, 40, and 50°C) on the retention of isoflavones and saponins were investigated in this study. About 41% of total saponins in soy flour (SF) were found to remain in SPI during processing, whereas 42% remained unextracted in the solid waste. None was detected in the whey or wash water. The study also revealed that only about 27% of total isoflavones from SF remained in the final SPI when P/W was performed at 50°C. As much as 40% of the total isoflavones could be retained in SPI when P/W was conducted at 25, 10, or 0°C. When the P/W temperature was 50°C, the percentages of total isoflavones lost during extraction, precipitation, and washing were 28, 22, and 6%, respectively. When the temperature was changed to 0°C, the percentages of isoflavones lost during extraction, precipitation, and washing were 28, 11, and 5%, respectively. The P/W temperatures did not affect the distribution of saponins in different streams during the processing of SPI. Lowering the P/W temperature did not significantly lower the protein content in SPI unless the temperature was reduced to 0°C.     

Thermal and mechanical properties of plastics molded from urea-modified soy protein isolates

Soy protein has been considered as a potential alternative of some petroleum polymers in the manufacture of plastics. The purpose of this investigation was to characterize the thermal and mechanical properties of plastics made from urea-modified soy protein. Soy protein isolate was separated from the defatted soy flour, modified with various urea concentrations, and compression-molded into plastics. Differential scanning calorimetry showed that the temperatures of denaturation and the enthalpies of denaturation of the modified soy protein decreased as urea concentrations increased above 1 M. At the same urea concentration, molded plastics made from the modified soy proteins showed a similar temperature of denaturation as the modified soy protein, but a lower enthalpy of denaturation. Tensile strength and Young's modulus of the molded plastics from the modified soy proteins increased as urea concentration increased and reached their maximum values at 8 M urea modification. Both storage modulus and glass transition temperature of the plastics from the modified soy proteins increased as urea concentration increased. The plastics made from the 2 M urea-modified soy proteins showed improvements in elongation, tough fracture behavior, and water resistance. The urea may function as a denaturant, a plasticizer, and a filler.     

Food uses of wheat gluten

New separation methods for wheat gluten have increased production and made possible the use of mass wheat as a raw material. This widens the use of gluten in the fields where functional as well as baking properties are important. Solubility, swelling, viscosity and nutritional aspects of gluten are reviewed. The current applications of wheat gluten are mainly in baked products, breakfast foods, and meat analogs. Many chemical modifications of gluten have been developed. Modifications are not in use, but the increased gluten production will bring them into use.     

Textural properties of cheese analogs containing proteolytic enzyme-modified soy protein isolates

Cheese analogs were prepared from untreated or proteolytically modified soy protein isolates (SPIs), replacing 60% of casein, to explore their potential to replace higher-priced milk proteins. Quality attributes of cheese analogs were evaluated by texture profile analysis with the Instron and melting spread. Compared with commercial milk-based cheeses, ranging from hard-type (Cheddar) to soft-type products (Mozzarella), textural properties of cheese analogs were markedly different; they were harder and more fracturable with no measurable adhesiveness. The use of enzyme-modified SPI significantly (P < 0.05) lowered both hardness and fracturability of cheese analogs and also brought about adhesiveness, all of which fell within the range observed for dairy cheeses. Although melting spread of cheese analogs was improved by the use of enzyme-modified SPI, it was still inferior to those of dairy cheeses and needed further improvement. Treatments of SPI with alcalase and trypsin were more influential in modifying textural properties of the resulting cheese analogs than those with other proteases studied.     

尿素变性对大豆分离蛋白粘接强度和分子结构的影响

大豆分离蛋白基胶粘剂由于具有环境友好性、生物降解性和可再生性而受到人们的关注。研究了尿素变性对大豆分离蛋白粘接强度及对蛋白分子结构的影响。结果表明,蛋白质经过尿素变性后,随着尿素浓度的增加,蛋白质分子展开的程度过大反而对粘接强度有不利的影响。在对榉木进行粘接时,1mol/L尿素变性获得的粘接强度最大。     

Effects of preheating treatment on thermal property and adhesion performance of soy protein isolates

The objective of this research was to study the effects of preheating treatment and thermal-setting temperature on the thermal properties and adhesion performance of esterified and cross-linked soy protein isolates. Preheating treatment was achieved by heating a soy protein isolate suspension (5% solid) for 20 min at 60, 80, 110 or 130°C. Thermal-setting temperatures of 130, 160, 190 and 220°C were achieved by adjusting the temperature of the hot press. Differential scanning calorimetry and sodium dodecyl sulfate-polyacrylamide gel electrophoresis were used to determine changes in the protein structure before and after preheating treatment. Preheating treatment had a significant effect on protein structure and adhesion performance. Adhesion strength of control and esterified soy protein isolates reached maximum at 80°C preheating temperature. Severe preheating (over 110°C) caused complete denaturation of proteins and loss of their native structure and was, therefore, detrimental to adhesion performance. Thermal-setting temperature also had a significant effect on protein structure and wet strength of the soy protein isolates. Wet adhesion strength of unmodified, esterified, and cross-linked soy protein isolates increased by 170%, 128% and 80%, respectively, as the thermal-setting temperature increased from 130 to 220°C.     

Effect of gallic acid on the aroma constituents of soymilk and soy protein isolates

Volatile compounds from soymilk were analyzed by gas chromatography/olfactometry/mass spectrometry (GCO/MS) with direct injection of various volumes of static headspaces. The most powerful odorants, determined by the minimum headspace volume required to detect by olfactometry, were (i) hexanal, (ii) acetaldehyde, (iii) methanethiol, (iv) dimethyl trisulfide (DMTS), and (v) 2-pentyl furan. Analyses of soymilk prepared with the addition of 100 ppm gallic acid revealed that the only two detectable odorants were hexanal and acetaldehyde. Sensory analyses of the soymilk treated with 100 ppm gallic acid produced a significantly lower score (P=0.0006) for overall odor intensity compared with the control soymilk. Aqueous slurries of soy protein isolates (SPI) prepared with the addition of 100 ppm gallic acid also had lower odor intensities than the control SPI (P<0.0001). GCO/MS analyses of headspace volatiles revealed that the gallic acid treatment had removed all detectable levels of methanethiol and DMTS while having no significant effect on the level of hexanal (P=0.81).     

Study of wheat gluten plasticization with fatty acids

The plasticizing effect on wheat gluten of a series of saturated fatty acids with different carbon chain length (from 6 to 10 carbons) was investigated. Plasticized materials were obtained by mixing in a counter-rotating batch mixer. The wheat gluten plasticization with the saturated fatty acids was shown to be thermodependent. Differential scanning calorimetry enabled the determination of the compatibility limits between wheat gluten and the fatty acids. Plasticizing effect, as judged from the shift of the temperature of the glass to rubbery transition, was determined by dynamic mechanical thermal analysis. As results showed a series effect on several times, it was thus possible to approximate the plasticizing effect of the different saturated fatty acids with a unique Couchman-Karasz equation. The molecular size distribution of the plasticized gluten was analyzed by size-exclusion high performance liquid chromatography (SE-HPLC). Mixing of plasticized wheat gluten induced variable and complex changes in the protein size distribution and resulted in a predominant polymerization, indicating a gluten reactivity in the presence of the fatty acids. The water vapor permeability of films obtained by pressure molding from the mixed plasticized samples showed improved barrier properties in comparison with films from wheat gluten plasticized with glycerol. The use of saturated fatty acids open thus new ways for the plasticization and the improvement of properties of gluten based biodegradable plastics.     

Cereal bars with soy protein and wheat germ, physicochemical characteristics and texture during the storage

Studies analyzing cereal bars have reported on consumer characteristics and preferences in sensory analyses and on their market growth, however little has been published on their physicochemical data and texture properties. Thus the objective of this research was to provide information about the storage of a cereal bar formulation with high protein and vitamin levels based on soy protein and wheat germ, packaged in 3 different films (A: PET/PEBD; B: PETmet/PEBD; C: PET/PEBD/AL/PEBD), during 6 months under environmental conditions of temperature (25 +/- 2 degrees C) and relative humidity (56%). The moisture content, water activity, pH and total acidity of the cereal bars were determined. The textural measurements accompanied during storage were breaking strength, hardness and cohesiveness. The cereal bars presented variations in water activity (Aw), moisture content and total acidity during storage. The moisture content of the bars tended to increase, which led to a significant (p = 0.05) influence on the texture characteristics of breaking strength and hardness, in the different packaging films tested. The increase in the values for breaking strength (A: 4756,5N; B: 5093,0N; C: 5575,6N) at 45 days of storage was attributed to a possible crystallization of the agglutinating syrup used for the bars. The textured soy protein used in the formulation could also have contributed to this fact due to its hygroscopic character, also interfering in the decrease in the cohesiveness measurements (deformation) with time. The effect of the different barrier properties of the packaging films tested was significant (p < 0.05) in the stability of the cereal bars during storage.     

Water absorbance and thermal properties of sulfated wheat gluten films

Wheat gluten films of various thicknesses formed at 30–70°C were treated with cold sulfuric acid to produce sulfated gluten films. Chemical, thermal, thermal stability, and water uptake properties were characterized for neat and sulfated films. The sulfated gluten films were able to absorb up to 30 times their weight in deionized water. However, this value dropped to 3.5 when the film was soaked in a 0.9% (w/w) NaCl solution. The films were also soaked 4 times in deionized water, and each soaking resulted in a reduced water uptake capacity. The temperature of film formation had no effect on the final water uptake properties. Also, thinner films had higher concentrations of sulfate groups than thicker films; this resulted in higher water uptake values. In addition, sulfated gluten films had comparable glass‐transition temperatures but lower thermal stabilities than the neat gluten films. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010