Sulfite-radical anions in isolated soy proteins

ABSTRACT:  Aqueous mixtures of manganese and sulfite, at levels found in isolated soy proteins (ISP) and defatted soy flakes, spontaneously react in the presence of oxygen to produce methanethiol from the 1-electron oxidation of methionine. The carbon and sulfur of methanethiol originate from the methyl-carbon and sulfur of methionine. Similar aqueous mixtures of sulfite, manganese, and oxygen also produce sufficient levels of free radicals to degrade fluorescein. The degradation of methionine by free radicals generated in the sulfite, manganese, and oxygen reaction mixture is inhibited by the free radical spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide. Processing ISP with either L-cystine or potassium iodate reduces the free sulfite content of ISP and reduces the headspace methanethiol from aqueous ISP slurries to nondetectable levels. ISP processed without additives contained sufficient levels of free radicals to generate methanethiol from the oxidation of added methionine. There were no detectable levels of methanethiol produced when methionine was added to ISP processed with iodate.     

Residual Sulfur Metabolites in Isolated Soy Proteins: Sulfite to Cysteine

ABSTRACT: Mean methanethiol headspace concentrations above aqueous slurries of isolated soy proteins (ISP) increased 17-to 36-fold over the controls with the addition of L-cysteine. Corresponding hydrogen sulfide levels were also greatly increased. Dithiothreitol, sodium sulfite, and glutathione increased headspace methanethiol from aqueous ISPs 23-to 44-fold, 8-to 9-fold, and 5-fold, respectively, but did not elevate hydrogen sulfide. These observations, along with the effects from the addition of dithiothreitol/O-acetyl-serine, the addition of a pyridoxial phosphate inhibitor and the intrinsic sulfite content of ISP samples (22 to 31 ppm), indicate that methanethiol from soy proteins is formed by way of components of a sulfite-to-cysteine pathway.     

A ≥ 100000-MW Soybean Protein Fraction that Inhibits the Formation of Methanethiol and Hydrogen Sulfide in Aqueous Slurries of Isolated Soy Proteins with Added L-Cysteine

ABSTRACT: The addition of L-cysteine to aqueous slurries of commercial isolated soy proteins (ISP) increased methanethiol headspace levels by 17- to 36-fold over the control. Corresponding levels of hydrogen sulfide were about 10 to 19 times greater than methanethiol. Neither methanethiol nor hydrogen sulfide were detected when L-cysteine was added to aqueous slurries of hexane-defatted soy flour. The production of hydrogen sulfide and methanethiol in aqueous slurries of commercial ISP was inhibited by the addition of a component(s) recovered from the pH 4.6 supernatant obtained during laboratory preparation of ISP by isoelectric precipitation. The inhibitory component had a molecular weight (MW) of ≥ 100000 and an isoelectric point of about 5.9. This component was not serine acetyl transferase. Its inhibitory properties were inactivated at 70°C and diminished with elevated levels of methionine. Adding the ≥ 100000-MW soluble-proteins (from the isoelectric precipitation step) back to a nonheat-treated laboratory ISP during processing reduced the methanethiol level by 88%.     

Lipoxygenase Independent Hexanal Formation in Isolated Soy Proteins Induced by Reducing Agents

ABSTRACT:  Compared to corresponding controls, 6.5 mM dithiothreitol (DTT) elevated headspace hexanal level over aqueous slurries of both commercial isolated soy proteins (ISP) and laboratory ISP prepared with 80 °C treatment. Further analysis revealed that lipoxygenase (LOX) activity was not detected from these ISP, indicating that LOX is not involved in the observed hexanal increase. Levels of the induced headspace hexanal over the ISP aqueous slurries were proportional to the amount of DTT added in the range of 0 to 65 mM. Subsequent systematic investigations with model systems revealed that iron was required for the reducing agent-induced hexanal formation from linoleic acid. Erythorbate, another reducing agent, can also induce hexanal formation in both ISP and model systems. As a comparison, the LOX activity and hexanal synthesis in defatted soy flour were examined. The corresponding results showed that defatted soy flour maintained high LOX activities and that hexanal synthesis in such sample was significantly inhibited by high concentration DTT (above 130 mM). Data from the current investigation demonstrate the existence of LOX independent hexanal formation induced by reducing agents in ISP and the potential requirement of iron as a catalyst.     

Methionine is the Methyl Group Donor for Sulfite-Associated Methanethiol Formation in Isolated Soy Proteins

ABSTRACT:  Addition of 3.2 mM sodium sulfite resulted in a 5.1-time higher amount of methanethiol in aqueous slurries of isolated soy proteins (ISP) over the corresponding controls after being stirred for 60 min. Introduction of 4.0 mM methionine to the slurries induced a 2.7-time increase. However, when both sodium sulfite (3.2 mM) and methionine (4.0 mM) were added, a 25.7-time higher amount in methanethiol concentration was observed. Similar results were obtained when sodium sulfite and methionine were added to a partially purified protein fraction prepared from defatted soy flakes. Mass spectra of the methanethiol formed with addition of L-methionine-methyl-¹³C₁ and unlabeled sulfite showed that the carbon-13 labeled methyl group was actually integrated into methanethiol. But no incorporation of isotopic sulfur was observed when ³⁴S-labeled sodium sulfite was applied. The free methionine contents in commercial and lab-prepared ISP and in-process protein extracts ranged from 10.34 μg/g to 52.74 μg/g, which were not proportional to the indigenous methanethiol contents. Results from the current study showed that methionine is an important methyl group donor for methanethiol in ISP associated with sulfite as a reducing agent.     

Carbon-centered radicals in isolated soy proteins

ABSTRACT:  Solid-state electron paramagnetic resonance (EPR) spectroscopy of commercial samples of isolated soy proteins (ISP) revealed a symmetrical free-radical signal typical of carbon-centered radicals ( g = 2.005) ranging from 2.96 × 10¹⁴ to 6.42 × 10¹⁴ spins/g. The level of free radicals in ISP was 14 times greater than similar radicals in sodium caseinate, 29 times greater than egg albumin, and about 100 times greater levels than casein. Nine soy protein powdered drink mixes contained similar types of free radicals up to 4.10 × 10¹⁵ spins/g of drink mix, or up to 6.4 times greater than the highest free-radical content found in commercial ISP. ISP samples prepared in the laboratory contained trapped radicals similar to the levels in commercial ISP samples. When ISP was hydrated in 2.3 mM sodium erythorbate or 8.3 mM L-cysteine, frozen and dried, the level of trapped free radicals increased by about 17- and 19-fold, respectively. The ESR spectrum of defatted soybean flakes contained overlapping signals from the primary free-radical peak ( g = 2.005) and a sextet pattern typical of manganese-II. The manganese signal was reduced in the laboratory ISP and very weak in the commercial ISP.     

Comparison of soybean oils, gum, and defatted soy flour extract in stabilizing menhaden oil during heating

ABSTRACT:  Capabilities of crude soy oil, degummed oil, gum, and defatted soy flour extract in preventing the oxidation of menhaden oil and its omega-3 fatty acids, DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), during heating were evaluated. The menhaden oil mixed with defatted soy flour extract demonstrated the greatest stability by producing the lowest TBA reactive oxidation products and retaining the highest concentrations of DHA and EPA after heating at 150 °C for 30 min. A range of 62.8% to 71.5% of DHA and 67.7% to 75.9% of EPA remained in the fish oil with defatted soy flour extract, while only 29.9% of DHA and 37.2% of EPA were retained in the fish oil with no addition. Stabilizing capability from highest to lowest was defatted flour extract > gum > degummed oil = crude oil. The defatted flour extract had the highest level of total phenolic content (11.3 μg catechin equivalent/g), while crude oil, degummed oil, and gum contained 7.1, 6.1, and 6.0 μg catechin equivalent/g, respectively. The level of isoflavones in the defatted soy flour extract was 55 mg/g, which was over 100 times higher than in the crude oil or gum. Although isoflavones were not detected in the degummed oil, it contained the highest level of tocopherols (414 μg/g), whereas the lowest level (215 μg/g) was found in the defatted flour extract. The order of free radical scavenging capability measured from high to low was the defatted soy flour extract, crude oil, degummed oil, and gum.     

CORN MEAL/SOY FLOUR BLENDS: CHARACTERISTICS AND FOOD APPLICATIONS

SUMMARY –Protein quantity and quality of degermed corn meal were improved by supplementation with toasted defatted soy flour. Taste panel results indicated no significant change in preference, acceptability and similarity to corn meal when as much as 20% toasted defatted soy flour was contained in the blend. Flavor stability was adequate in blends containing toasted defatted, commercial processed full-fat or extrusion-cooked full-fat soy flours at the 15% level. Product color was changed more by the inclusion of toasted defatted than of full-fat soy flour in the blends. Satisfactory performance was found in chapatti and northern corn bread containing degermed corn meal with and without 15% toasted defatted, 20% commercial processed full-fat or 35% extrusion-cooked full-fat soy flours.     

Analysis of Lunasin in Commercial and Pilot Plant Produced Soybean Products and an Improved Method of Lunasin Purification

Abstract: Lunasin is a bioactive peptide present in soybean. It is important to quantify lunasin concentration in soy products to assess its potential impact as functional food. The objectives of this study were to analyze lunasin in commercial soymilk products and implement an efficient method to isolate and purify it from defatted soybean flour. Defatted soybean flour was suspended in water, and the extract was loaded in a pre‐equilibrated diethylaminoethyl column and bound proteins eluted using a step gradient of salt. Most lunasin was eluted from the column at 0.2 to 0.4M NaCl as quantified by immunoassays and purified using ultracentrifugation and ultrafiltration techniques. Lunasin purity was ≥90% and a standard curve was used to quantify its concentration in soymilk products. Concentration of lunasin in soy products, including organic soymilk, soy protein shakes, and soy infant formulas, ranged from 1.78 to 9.26 mg lunasin/100 g product. The concentration per serving ranged from 1.59 ± 0.01 to 22.23 ± 0.74 mg lunasin with variability depending on brand and size per serving. Steam‐ground‐cooked soy had the highest concentration of lunasin (22.23 ± 0.74 mg/serving), similar to some commercial products. Ground‐cooked soymilk contained roughly half the concentration of lunasin (14.39 ± 1.4 mg/serving). Soy infant formulas that used soy protein isolate revealed lower concentrations of lunasin (P < 0.05). It was concluded that all soymilk products analyzed contained lunasin, and a more efficient method to isolate lunasin with higher purity was developed. Practical Application: Soy foods have shown to play a role in cardiovascular health prevention. The quantification of lunasin in commercially available soy products can add to the already existing health claim for soy foods and encourage consumers to include soy products in their diets.     

Dairy and Vegetable Protein Blends by Co-Extraction and Co-Ultrafiltration

A process was developed for the manufacture of blends of dairy and vegetable proteins by co-extraction and co-ultrafiltration. Defatted soy flour proteins were extracted using either acid or sweet whey as the solvent. The whey/soy slurry was separated in a two stage extraction process using a centrifugal desludger. The proportion of whey protein in the blend was determined by the number of extractions and the total amount of whey used. The combined extracts were processed by ultrafiltration to increase the protein content of the blends. Solubility characteristics of the blends indicate potentially good functional properties.     

Effect of Sequestering Intrinsic Iron on the Electron Paramagnetic Resonance Signals in Powdered Soy Proteins

This investigation examined iron in powdered soy protein products using electron paramagnetic resonance (EPR) spectroscopy, and the effect that selectively binding free iron in isolated soy protein (ISP) had on the occurrence of metastable radicals in powdered soy proteins. EPR analyses of soybean defatted flour, commercial ISP and laboratory ISP samples revealed a peak at g = 4.3 characteristic of high‐spin ferric iron in a rhombic‐coordinated environment. Commercial ISP samples examined contained higher levels of the rhombic ferric iron than laboratory‐prepared ISP samples. During the first 6 wk of storage the primary singlet EPR signal at g = 2.0049 in the commercial ISP samples approximately doubled, and the laboratory prepared samples increased by about 9‐fold. The EPR signal was initially about 4‐times higher in the freshly prepared commercial samples compared to the corresponding laboratory ISP. Laboratory ISP samples prepared with added deferoxamine to sequester endogenous iron exhibited a large increase in the high‐spin ferric iron EPR signal at g = 4.3. ISP treated with deferoxamine also exhibited a multiple‐line EPR signal at about g = 2.007, instead of the typical singlet signal at g = 2.0049. The power at which the signal amplitude was half‐saturated also changed from about 1 mW in the control ISP to about 20 mW in the deferoxamine treated ISP. The multiple‐line EPR spectrum from the ISP treated with deferoxamine increased during storage over a 6‐wk period by about 6‐fold. The observed changes in EPR line‐shape, g‐value, and power saturation with the deferoxamine treatment indicate that the primary free‐radical signal in powdered ISP samples may be from stabilized tyrosine radicals with spin densities distributed over the aromatic ring.     

Effect of moisture, lipids, and select amino acid blocking agents on the formation and stability of metastable radicals in powdered soy proteins

Incremental increases in the moisture content of powdered soy protein products from 4.4% to 13.4% produced an inverse effect on the ability of soy proteins to maintain metastable free radicals. The corresponding reduction in electron paramagnetic resonance signal was not due to dielectric loss in the range of moisture contents examined. Subsequent evaluations of various treatments were conducted after drying soy proteins with molecular sieve to a water activity below 0.085 in order to minimize the influence from variations in moisture. Isolated soy protein (ISP) samples, prepared with "defatted flour" that had been further extracted with chloroform/methanol (2 : 1), had a 96% reduction in total lipids compared to the control ISP samples. The initial rate of radical accumulation in the "reduced-lipid" ISP for the first 3 wk was not significantly different from the initial rate of radical increases in the control ISP. After 3 wk, radical accumulation in the "reduced-lipid" ISP continued to increase, but at a rate that was less than the control. These findings indicate that the initial reactions contributing to the formation of metastable radicals in the powdered ISP are not strongly dependent on associated lipids. Blocking sulfhydryl groups during ISP preparation with N-ethylmaleimide did not significantly slow the rate of radical accumulation compared to the control ISP. Blocking arginine residues in ISP samples with phenylglyoxal caused an increased rate of radical accumulation for the first 4 wk. PRACTICAL APPLICATION: Levels of metastable radicals in powdered soy protein products typically range from 10 to 100 times greater than the free radicals in other food protein sources. This current research examines various compositional and treatment parameters that might be used to minimize the content of free radicals in foods containing soy proteins.     

利用脱脂花生饼粕生产豆腐的研究

本文研究了大豆中掺合花生及其脱脂饼粕所制备的豆腐的品质和出率,发现掺合脱脂花生粕的效果最好,其次是半脱脂花生饼,再次是未脱脂花生。花生饼粕的掺合量可达20％,豆腐制品质地均匀,外表光滑,结构稳定,风味比100％大豆制品好。     

Abundant proteins associated with lecithin in soy protein isolate

A new method for fractionating acid-precipitated soy proteins, so called soy protein isolate, involving three step acidification of a water extract of defatted soy flour was developed. Soy protein isolate was shown to be mostly composed of not only β-conglycinin and glycinin, but also a group of lipophilic proteins associated with lecithin (phospholipids). The proportions of three major proteins were 23%, 46%, and 31%, respectively. The proportions changed as the nitrogen solubility index (NSI) of defatted soy flour changed. The yield of lipophilic proteins depended on the NSI of defatted soy flour, different from those of β-conglycinin and glycinin. The variation in the proportions of the three major proteins may be due to the yield of lipophilic proteins.     

Nutritional Improvement of Soy Flour Through Inactivation of Trypsin Inhibitors by Sodium Sulfite

Treatment of raw soy flour at 75°C with 0.03M sodium sulfite for 1 hr completely inactivated trypsin inhibitors leaving no sulfite residues in the soy proteins. Rat feeding studies showed that the protein efficiency ratio (PER) increased from 1.55 for raw flour to 2.11 for heated flour and 2.49 for flour heated in the presence of sulfite. This nutritional improvement was accompanied by enhancement in in vivo nitrogen digestibilities. Pancreas weights were elevated in rats fed raw soy flour, but not in those consuming heated soy flour, with or without sodium sulfite. Knowledge of the factors that decrease the stability of potentially toxic trypsin inhibitors may be useful for improving the quality of certain foods.     

Purification, Thermal Stability, and Antigenicity of the Immunodominant Soybean Allergen P34 in Soy Cultivars, Ingredients, and Products

ABSTRACT:  Protein P34 ( Gly m Bd 30K) is the immunodominant allergen in soybean ( Glycine max L.). The objectives of this study were (1) to study the effect of thermal treatment on P34 antigenicity and secondary structure after isolation and purification of P34 from soybean by chromatographic techniques; (2) to identify the variability of P34 allergen within 138 accessions from a diverse USDA soybean germplasm collection by ELISA; and (3) to quantify P34 immunoreactivity in various commercial soy ingredients and products. Thermal processing decreased P34 antigenicity. Soybean accessions with the highest P34 content were ancestral (12 mg/g defatted flour) followed by modern (10 mg/g defatted flour) and exotic (8 mg/g defatted flour). The cultivar that emerged as the lowest-expressing P34 accession was PI548657 (2.3 mg/g defatted flour). Among commercial soy ingredients, soy flour yielded the highest P34 antigenicity (32 mg/g extracted protein) followed by soy protein isolate (29 mg/g extracted protein) and soy protein concentrate (24 mg/g extracted protein). Among soy consumer products, soymilk presented the highest P34 antigenicity, ranging from 7 to 23 mg/g extracted protein, followed by tempeh (8 mg/g extracted protein), soy infant formula (3.4 mg/g extracted protein), soy powder (2 mg/g extracted protein), and soy cheese products (0.50 mg/g extracted protein). Korean miso, soy sauce, soy chili mix, soy nuts, soy cream cheese, soy meat patty, texturized soy protein, and soy cereal exhibited undetectable P34 antigenicity (detection limit = 0.45 ng). Selecting soybean varieties with low levels of this allergen, or via processing, could potentially make soybean products less antigenic.     

脱脂对燕麦淀粉理化性质影响研究

研究了燕麦粉的脱脂条件和脱脂对燕麦淀粉性质的影响.结果表明:燕麦粉脱脂条件为料液比为1∶4,脱脂时间4h;燕麦淀粉经脱脂后粗脂肪和损伤淀粉含量分别降低81.3％、26.9％;燕麦破损淀粉含量由4.64％下降至3.63％;燕麦淀粉经脱脂透光率增加、沉降体积变小、冻融吸水率下降;经脱脂后燕麦淀粉溶解度下降,膨润力变高;经脱脂燕麦淀粉峰值黏度升高,谷值黏度基本不变,衰减值、最终黏度、回生值、糊化温度均有所下降.     

In situ examination of starch granule-soy protein and wheat protein interactions

The objective of this study was to evaluate the association between heat and pressure-treated soy proteins and wheat prime starch relative to the level of granule surface proteins present. Soy isolates were manufactured from defatted soy flour and from two types of textured soy flour. Conditions for binding between wheat starch and soy fractions were established by altering pH, protein and sucrose concentrations. Conformations of exogenous proteins bound to the wheat starch granule surface were evaluated using an amyloglucosidase assay. Proteins present on the granule surface were removed and soy protein binding was reevaluated. Thermal and pressure processing of soy protein significantly influenced binding kinetics. Textured soy proteins exhibited increased wheat starch granule adsorption characteristics compared to untreated soy protein. Removal of native wheat starch granule surface proteins decreased the binding of added proteins. This suggests that native granule proteins may mediate the binding of exogenous protein.     

Soybean Flour Lipoxygenase lsozyme Mutant Effects on Bread Dough Volatiles

Lipoxygenases are implicated in improvement of bread quality after addition of enzyme active soy flour. However, differences among the three major isozymes in soybeans in terms of impact on bread characteristics are not known. Differences exist among soybean lipoxygenase isozymes in terms of stability and generation of volatile flavor and aroma. Soybean flour with different lipoxygenase isozymes present might affect bread dough volatiles or defatting may affect activity of the isozymes. Full fat or defatted soy flour could be added to bread dough. Defatting selectively reduced lipoxygenase 2 activity with little effect on lipoxygenases 1 and 3. Addition of full fat soy flour from mutant isolines indicated that as with soybean homogenates, lipoxygenase 2 is mostly responsible for undesirable aroma compounds. Much higher levels of volatile alcohols were found in soy flour amended bread dough than in soy flour homogenates alone.     

不同大豆蛋白对小麦面条加工品质和血糖生成指数的影响

为改善小麦面条品质,在小麦粉中添加大豆蛋白(全脂豆粉、脱脂豆粉、大豆分离蛋白、大豆浓缩蛋白、大豆组织蛋白),以拉伸特性、蒸煮特性、微观结构以及血糖生成指数为评价指标,研究大豆蛋白添加量(0%、5%、10%、15%、20%)对面条加工品质及血糖生成指数的影响。结果表明:随蛋白添加量的增加,脱脂豆粉、全脂豆粉的增加使面条的拉断力呈明显上升趋势,拉伸距离呈先上升后下降趋势,大豆分离蛋白、大豆浓缩蛋白添加量的增加使拉断力呈先上升后下降趋势,拉伸距离呈下降趋势,而大豆组织蛋白的增加使面条拉伸性能均逐渐降低;当大豆蛋白添加量为10%时,混合粉面条拉伸特性较优;五种大豆蛋白的添加均使面条的烹煮损失率及断条率上升,全脂豆粉的增加使面条的吸水率、膨胀率逐渐下降,其余四种均使其上升;微观结构表明,脱脂豆粉和全脂豆粉的添加使混合粉面条的面筋网络结构更加连续均匀,而其余三种大豆蛋白添加量的增加造成混合粉面条微观结构的劣变;利用体外复合酶方法测定GI值,血糖生成指数依次为:小麦粉 > 脱脂豆粉 > 全脂豆粉 > 大豆组织蛋白 > 大豆浓缩蛋白 > 大豆分离蛋白。