The production of volatile compounds in model casein systems with varying fat levels as affected by low-frequency ultrasound

The effects of low-frequency ultrasound on the production of volatile compounds in model casein protein systems containing various fat concentrations of 2%, 4% and 6% (w/w) were investigated. Ultrasound application was performed at 20 kHz for up to 10 min which corresponded to energy densities ranging from 9.54 to 190.8 J mL⁻¹. Similar volatile compounds were detected both in pure fat and mixtures of casein and fat (CF) systems. These volatiles belonged to the groups of aldehydes, ketones, esters, alcohols and hydrocarbons, which were the products of oxidation of lipids or protein degradation due to acoustic cavitation. The amount of fat in the casein systems had minor effects on the production of volatiles, whereas the production of volatile compounds was significantly affected by the ultrasound treatment. Short sonication times <5 min generated similar volatile profiles to the untreated samples. In contrast, prolonged sonication for 5 and 10 min considerably increased the production of volatile compounds and the amounts of fatty acids. Thus, the application of low–frequency ultrasound for short periods should be considered to minimise the production of volatile compounds which can ultimately affect the taste.     

Preconcentration of yoghurt base by ultrafiltration for reduction in acid whey generation during Greek yoghurt manufacturing

The feasibility of applying ultrafiltration (UF) to concentrate a yoghurt base prior to fermentation during Greek yoghurt manufacturing was studied as a way of minimising acid whey (AW) generation. Two Greek yoghurts were prepared by concentrating a milk base through UF to 13.8% and 17.5% (w/w) (GY‐2, GY‐3). Production of GY‐3 resulted in zero AW discharge, while the GY‐2 production discharged 78% lower AW compared to the control (GY‐1). GY‐2 exhibited a hard gel structure, low syneresis, maximum viscosity properties and a high protein and fat content. Applying both, UF and straining, resulted yoghurts with required structural attributes while substantially reducing the generation of AW.     

Stability of oil–water primary emulsions stabilised with varying levels of casein and whey proteins affected by high-intensity ultrasound

This study evaluates physical and chemical stability of ultrasound-assisted grape seed oil primary emulsions stabilised by varying compositions of caseins to whey proteins (80:20, 60:40, 50:50 and 40:60) at different sono-operating conditions (81.9 and 117.0 J mL⁻¹). Physical and chemical stabilities were influenced by both sonication energy densities and milk protein compositions. Emulsions prepared at 81.9 J mL⁻¹ energy density with ≥40% whey protein fraction (60:40, 50:50, 40:60 and WPI) showed greater physical stability than the emulsions sonicated at 117.0 J mL⁻¹ which exhibited physical instability due to the depletion flocculation mechanism at the critical casein concentration (≥40%). The emulsion oxidative stability was found to be affected by sonication conditions as 117.0 J mL⁻¹ induced the oxidation reactions once the whey concentration exceeds 40%. Therefore, ultrasound prepared emulsions with casein to whey ratios of 60:40, 50:50, 40:60 and WPI at 81.9 J mL⁻¹ energy density was found to be stable for 10 days at 4 °C.     

In vitro immunogenicity of various native and thermally processed bovine milk proteins and their mixtures

In vitro immunogenicity of various native and thermally processed (72°C/15 s and 100°C/30 s) bovine milk protein fractions, their mixtures, whey, and skim milk, was studied by analyzing the immune response of T helper (Th) cells in human peripheral blood mononuclear cells. The secretion of Th type cytokines induced by the protein stimulants was quantified while determining the heat-induced protein denaturation. Purified whey proteins, caseins and whey fraction, and skim milk provoked substantial immune responses at various degrees, indicating their potent immunogenicity. The protein mixtures prepared using the fractionated whey proteins with or without caseins appeared less immunogenic in both native and heat-treated forms, implying their potential of producing less immunogenic dairy products. The 100°C/30 s treatment significantly altered the immunogenicity of most of the potent protein stimulants, which mostly coincided with their levels of protein denaturation. The 72°C/15 s treatment caused the least protein denaturation but altered the immunogenicity of several protein stimulants notably, including heat-stable caseins and α-lactalbumin.     

Effects of pH,calcium-complexing agents and milk solids concentration on formation of soluble protein aggregates in heated reconstituted skim milk

The proportion of protein present in the supernatant after centrifugation and the formation of soluble protein aggregates in heated (90 °C for 10 min) reconstituted skim milk were investigated as a function of (a) pH, (b) milk concentration 9–21%, w/w (milk solids non-fat, MSNF) and (c) addition of calcium-complexing agents (orthophosphate or EDTA). Compositional changes in milk, resulting from pH adjustment or salt addition, altered the distribution of caseins between the serum and micellar phases of milk prior to heating, and influenced the amount and composition of soluble aggregates formed during heat treatment. Although milk pH was a good predictor of the aggregation behaviour of the proteins during heating of milk of different solids concentrations, neither the proportion of protein in the supernatant prior to heating nor the pH alone could predict the aggregation behaviour when the composition was adjusted with mineral salts.     

Feasibility of Spray Drying Concentrated Acid Whey After Nanofiltration

Concentration of acid whey and crystallisation of lactose followed by spray drying in the production of acid whey powder are severely restricted by the presence of lactic acid (LA) and calcium (Ca). These compounds cause stickiness during spray drying. The present study examined the effects of removing LA and Ca by membrane processing to varying extents in order to improve the feasibility of the spray drying of an acid whey stream. Spray drying of unaltered acid whey achieved a powder recovery of ~?18% in the collection vessel and ~?31% in the cyclone. Removal of LA and Ca by 30 and 40%, respectively, by nanofiltration significantly increased the powder recovery to ~?58% in the collection vessel. Powder particles decreased in size giving a D[4,3] of ~?18 μm, with SEM images confirming the presence of well separated spherical powder particles. A solubility of >?75% was also achieved. However, removing Ca >?60% compromised the spray drying process. FTIR analysis suggested that water molecules in the hydration layer of lactose and the structural changes of both lactose and protein molecules at the molecular level appeared to play an important role in governing the extent of the drying feasibility. In addition, the formation of calcium lactate may restrict the diffusion of lactose molecules, once the appropriate stoichiometry was reached. Thus, manipulating LA and Ca concentrations in a particular acid whey stream can improve the spray drying process and production of a non-sticky acid whey powder.     

Electrophoretic characterization of protein interactions suggesting limited feasibility of accelerated shelf-life testing of ultra-high temperature milk

Accelerated shelf-life testing is applied to a variety of products to estimate keeping quality over a short period of time. The industry has not been successful in applying this approach to ultra-high temperature (UHT) milk because of chemical and physical changes in the milk proteins that take place during processing and storage. We investigated these protein changes, applying accelerated shelf-life principles to UHT milk samples with different fat levels and using native- and sodium dodecyl sulfate-PAGE. Samples of UHT skim and whole milk were stored at 20, 30, 40, and 50°C for 28 d. Irrespective of fat content, UHT treatment had a similar effect on the electrophoretic patterns of milk proteins. At the start of testing, proteins were bonded mainly through disulfide and noncovalent interactions. However, storage at and above 30°C enhanced protein aggregation via covalent interactions. The extent of aggregation appeared to be influenced by fat content; whole milk contained more fat than skim milk, implying aggregation via melted or oxidized fat, or both. Based on reduction in loss in absolute quantity of individual proteins, covalent crosslinking in whole milk was facilitated mainly by products of lipid oxidation and increased access to caseins for crosslinking reactions. Maillard and dehydroalanine products were the main contributors involved in protein changes in skim milk. Protein crosslinking appeared to follow a different pathway at higher temperatures (≥40°C) than at lower temperatures, making it very difficult to extrapolate these changes to protein interactions at lower temperatures.     

An improved extraction and purification method for obtaining high-quality chitin and chitosan from blue swimmer (Portunus pelagicus) crab shell waste

Food Science and Biotechnology - Portunus pelagicus shell waste is highly accumulated in seafood processing factories and has low commercial applications. The objective of this study was to modify...     

The effect of ultrasound on casein micelle integrity

Samples of fresh skim milk, reconstituted micellar casein, and casein powder were sonicated at 20 kHz to investigate the effect of ultrasonication. For fresh skim milk, the average size of the remaining fat globules was reduced by approximately 10 nm after 60 min of sonication; however, the size of the casein micelles was determined to be unchanged. A small increase in soluble whey protein and a corresponding decrease in viscosity also occurred within the first few minutes of sonication, which could be attributed to the breakup of casein-whey protein aggregates. No measurable changes in free casein content could be detected in ultracentrifuged skim milk samples sonicated for up to 60 min. A small, temporary decrease in pH resulted from sonication; however, no measurable change in soluble calcium concentration was observed. Therefore, casein micelles in fresh skim milk were stable during the exposure to ultrasonication. Similar results were obtained for reconstituted micellar casein, whereas larger viscosity changes were observed as whey protein content was increased. Controlled application of ultrasound can be usefully applied to reverse process-induced protein aggregation without affecting the native state of casein micelles.     

A Comparison of the Effectiveness of Sonication, High Shear Mixing and Homogenisation on Improving the Heat Stability of Whey Protein Solutions

Upon ultrasonic treatment at 20 kHz, protein aggregates in a dairy whey solution were broken down. In addition, when sonication was applied to a heated solution of denatured and aggregated proteins, there was a dramatic reduction in viscosity and aggregate size, which was maintained after re-heating. This observed heat stability may be due to shear forces that are induced by acoustic cavitation. To determine whether high shear mixing or homogenisation is able to cause similar effects to that of acoustic cavitation, sonication, high shear mixing and homogenisation were performed on 5 wt% whey protein concentrate solutions at identical energy density levels, which was based on the power drawn in each system. Homogenisation provided similar particle size and viscosity reductions as sonication while high shear mixing was less efficient in decreasing particle size. Cavitation was shown to be absent in both the mixing and homogenisation configurations, indicating that the shear forces generated are responsible for the observed particle size and viscosity reduction. In addition, heat stability was achieved in all systems indicating that a combination of heat treatment and any method that generates high shear forces can be used to improve the heat stability of whey proteins.