Inteaz Alli

Effects of physicochemical factors on the secondary structure of β-lactoglobulin

Fourier transform infrared spectroscopy and differential scanning calorimetry were used as complementary techniques to study changes in the secondary structure of beta-lactoglobulin under various physicochemical conditions. The effects of pH (3-9), NaCl (0-2 M), and lactose, glucose and sucrose (100-500 g/l) in the temperature range 25-100 degrees C on the conformation sensitive amide I band in the i.r. spectrum of beta-lactoglobulin in D2O solution were examined. The 1692 cm-1 band in the amide I band profile had not been definitively assigned in previous studies of the i.r. spectrum of beta-lactoglobulin. The decrease in this band at ambient temperature with time or upon mild heating was attributed to slow H-D exchange, indicating that it was due to a structure buried deep within the protein. The disappearance of the 1692 cm-1 band on heating was accompanied by the appearance of two bands at 1684 and 1629 cm-1, assigned to beta-sheets. The 1692 cm-1 band was therefore attributed to a beta-type structure. beta-Lactoglobulin showed maximum thermal stability at pH 3 and was easily denatured at pH 9. On denaturation, the protein unfolded into more extensive random coil structures at pH 9 than at pH 3. After 10 h at pH 9 (25 degrees C), beta-lactoglobulin was partly denatured. Heating to 60-80 degrees C generally resulted in the loss of secondary structure. At all pH values studied, two new bands at 1618 and 1684 cm-1, characteristic of intermolecular beta-sheet structure and associated with aggregation, were observed after the initial denaturation. Differential scanning calorimetry studies indicated that the thermal stability of beta-lactoglobulin was enhanced in the presence of sugars. The Fourier transform i.r. results obtained provide evidence that sugars promoted the unfolding of beta-lactoglobulin via multiple transition pathways leading to a transition state resisting aggregation.

Thermal denaturation of mixtures of α-lactalbumin and β-lactoglobulin: a differential scanning calorimetric study

Abstract The thermal denaturation of α-lactalbumin (α-lac), β-lactoglobulin (β-lg) and a mixture of the two proteins in the presence of several sugars, sodium salts and at various pH values was studied by differential scanning calorimetry. The effects of N -ethylmaleimide (NEM), cysteine, urea and sodium dodecyl sulfate (SDS) were also investigated. The temperature of denaturation ( T d ) of β-lg decreased from 71.9°C in the absence of α-lac to 69.1°C in its presence. In contrast, an increase of 2.5°C was observed in the T d of apo-α-lac when heated in the presence of β-lg suggesting that α-lac was made more thermally stable in the presence of β-lg. Glucose and galactose had the greatest effect in stabilizing the proteins against thermal denaturation with the effect being greater for β-lg than for α-lac. A decrease in thermal stability of both proteins was observed in the presence of sodium bicarbonate; sodium ascorbate, however, had a stabilizing effect. Renaturation of α-lac was prevented in the presence of cysteine and NEM, but not in urea or SDS. Translucent gels were formed when the α-lac/β-lg mixtures were heated in the presence of all five sugars and in the presence of cysteine, urea and SDS but not in NEM. This suggests that disulfide–sulfhydryl interchange reactions may be primarily responsible for the gelation of α-lac/β-lg mixtures.

Factors affecting molecular characteristics of whey protein gelation

Abstract The effects of pH, protein concentration, NaCl, heating temperature and time on the gelation of a whey protein concentrate (WPC) and the associated changes in the molecular conformation of the individual whey proteins were studied using polyacrylamide gel electrophoresis, high performance liquid chromatography and Fourier transform infrared spectroscopy. Heat denaturation was studied using differential scanning calorimetry. The results obtained showed that varying WPC concentration affected textural properties of gels without any observed differences in the molecular behavior of α-lactalbumin and β-lactoglobulin, while heating temperature, pH and NaCl affected both molecular and textural characteristics. WPC formed firm gels at temperatures above 70 °C, at alkaline pH range and in the absence of NaCl. When the whey proteins were heated, α-lactalbumin did not aggregate above pH 7 but denatured readily to form aggregates at acid pH while β-lactoglobulin aggregated at both acid and alkaline pH regions. Denaturation of α-lactalbumin at neutral pH resulted in an increase in viscosity while denaturation of β-lactoglobulin resulted in gel formation. The aggregation of WPC, particularly at alkaline pH, was attributed mainly to β-lactoglobulin and only minimally to α-lactalbumin.

Applications of mass spectrometry to food proteins and peptides

The application of mass spectrometry (MS) to large biomolecules has been revolutionized in the past decade with the development of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) techniques. ESI and MALDI permit solvent evaporation and sublimation of large biomolecules into the gaseous phase, respectively. The coupling of ESI or MALDI to an appropriate mass spectrometer has allowed the determination of accurate molecular mass and the detection of chemical modification at high sensitivity (picomole to femtomole). The interface of mass spectrometry hardware with computers and new extended mass spectrometric methods has resulted in the use of MS for protein sequencing, post-translational modifications, protein conformations (native, denatured, folding intermediates), protein folding/unfolding, and protein-protein or protein-ligand interactions. In this review, applications of MS, particularly ESI-MS and MALDI time-of-flight MS, to food proteins and peptides are described.

Sweet and taste-modifying proteins : A review

The search for non-carbohydrate sweeteners from natural sources has led to the discovery of many intensely sweet-tasting substances. The occurrence of sweet-tasting proteins such as thaumatin, monellin, mabinlin and pentadin in the pulp of fruits of various rain forest species has provided a new approach to the potential treatment of diabetes, obesity and other metabolic disorders. These proteins can also be used as low calorie sweeteners to enhance and modify the taste of existing foods. Thaumatin is currently commercially available as a sweetener, flavor enhancer, additive to pharmaceuticals, chewing gum and animal feeds. It can also be secreted extracellularly by genetic modification of the yeast Saccharomyces cerevisiae. The taste-modifying proteins, miraculin and curculin, can be utilized in controlling the palatability of foods or in the development of new food products. This review discusses some properties of sweet and taste-modifying proteins discovered to date.

Studies on the fermentation of chopped sugarcane

Abstract The fermentation of freshly harvested and chopped sugarcane (whole plant) was studied using laboratory silos (600 g capacity). The fermentation was accompanied by a rapid drop in pH during the first 24 h. After a storage period of ten days the fermented sugarcane (canelage) contained less than 10% of the original water-soluble carbohydrates (WSC), approximately 9% ethanol (dry weight basis) and 44% more acid detergent fibre (ADF) than did the original sugarcane. Sugarcane (whole plant) that had been allowed to lie in the field for 48 h before chopping and ensilage, contained 37% less WSC than did the fresh cane; the composition of the canelage (ten-day fermentation) prepared from this “wilted” sugarcane was similar to that prepared from freshly harvested sugarcane. Canelage (pH 3.49) prepared (53 days storage) in a commercial, horizontal silo contained (dry weight basis) 0.80% ethanol, 7.29% WSC, 2.68% crude protein, 1.15% acetic acid, 1.60% lactic acid and 42.4% ADF.

The effects of ammonia on the fermentation of chopped sugarcane

Abstract Chopped sugarcane was ensiled with and without the addition of aqueous ammonia (28% NH3) in laboratory silos (4.5 kg NH3/t sugarcane). The effect of the NH3 on the pattern of fermentation was studied over a period of 42 days. Addition of NH3 resulted in: (a) an initial decrease in the population of yeasts and moulds, and Lactobacillus organisms; (b) a reduction (47.9%) in weight loss; (c) a reduction (46.4%) in the loss of water-soluble carbohydrates (WSC); (d) increased levels of lactic acid; and (e) lower levels of acid detergent fibre (ADF) when compared with untreated canelage (fermented, whole-plant sugarcane). In another experiment in which 205-litre drums were used as silos, addition of NH3 (1.25 kg NH3/t sugarcane) to chopped sugarcane also produced canelage with higher levels of lactic acid, more WSC, lower levels of ADF and a reduction in weight loss when compared with the control canelage.

Distribution, antioxidant and characterisation of phenolic compounds in soybeans, flaxseed and olives.

The distribution of free and bound phenolic compounds present in soybean, flaxseed and olive were investigated. The phenolic compounds were fractionated on the basis on their solubility characteristics in water, alcohol, dilute base and dilute acid. Reversed phase high pressure liquid chromatography (RP-HPLC) and mass spectrometry (MS) were used for identification of individual components of phenolic compounds. Antioxidant activity (AA%) of free and bound phenolic compounds was measured using the linoleic acid/β-carotene assay. The water-soluble phenolic compound fractions represented 68-81%, 50-72% and 46-56% of the total phenolic compounds measured in full-fat soybean, olive and flaxseed, respectively. Methanolic extraction of free phenolic compounds without heat, solubilised 21-56%, 42-62% and 34-51% of the total phenolic compounds measured in soybean, olive and flaxseed, respectively; methanol extraction of free phenolic compounds with heat solubilised a further 24-34%, 31-37% and 36-37% of phenolic compounds from soybean, olive and flaxseed, respectively. Further dilute alkali and dilute acid solubilised the remaining 10-40%, 1-21% and 12-29% of the total phenolic compounds from soybean, olive and flaxseed, respectively. Results indicated that the full-fat meals of soybean, flaxseed and olive showed higher antioxidant activity compared to defatted meals. RP-HPLC and LC-MS/MS profil1 for soybean, flaxseed and olive indicate two classes of phenolic compounds designated as free and bound phenolic compounds.

Potato protein isolates: Recovery and characterization of their properties

An imitation of industrial potato fruit juice (PFJ) was prepared, using Canadian variety of potatoes, and was characterized of being composed of 22.9% patatin, 53.3% protease inhibitors, and 23.7% high MW proteins. To isolate potato proteins from PFJ, several extraction techniques were explored including thermal/acidic combination, acidic, FeCl3, MnCl2, ethanol and (NH4)2SO4 precipitations, and carboxymethyl cellulose complexation. (NH4)2SO4 precipitation led to the highest yield (98.6%) and to the recovery of protein isolates enriched in patatin with high resolubility. FeCl3 precipitation resulted in the highest purification factor (6.2) and isolates with the lowest relative proportion of high MW proteins (<4.6%); however, its optimal isolate showed a wide minimum solubility pH range of 3.0-6.0. FeCl3 and MnCl2 were identified as the best precipitating agents for the enrichment of isolates with >15kDa protease inhibitors. Trypsin inhibiting activities of protease inhibitors were highly preserved upon protein isolation than the chymotrypsin ones. Acidic-based protein isolate showed the highest specific lipid acyl hydrolase activity of patatin towards o-nitrophenyl butyrate, whereas FeCl3-based one exhibited the highest activity towards 4-nitrophenyl laurate.

Protein- lipid interactions in food systems: a review

Proteins and lipids, both individually or as complexes, play important functional roles in foods. Since the 1970s food scientists have devoted attention to the nature of these interactions and particularly to their effects on functional characteristics of protein-based foods. Previously, most of the published work was devoted to the biochemical aspects of protein-lipid interactions in biological systems. This article reviews the protein-lipid interactions of both naturally occurring protein-lipid complexes and protein-lipid complexes formed by induced interactions in foods and food products. The physicochemical characteristics of known protein-lipid complexes, the nature of binding which results in formation of these complexes and the effect of the interactions on food functionality are reviewed.