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.     

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.     

Effect of ultrasonication on microbiological,chemical and sensory properties of raw,thermized and pasteurized milk

The combined effect of ultrasonication and heat treatment on microbiological, chemical and sensory properties of raw, thermized and pasteurized milk was studied. Microbiological parameters monitored included total viable counts and psychrotrophs, chemical parameters included thiobarbituric acid and volatiles determination and sensory evaluation included the monitoring of odour and taste as a function of storage time. Results showed a 1–2.1 log cfu mL^?1 reduction in total viable counts and psychrotrophs for raw, thermized and pasteurized milk up to 6 days of storage. For raw and pasteurized milk ultrasonication resulted in a taste score equal to or lower than that of untreated milk. However, thermized milk, ultrasonicated for 2 min gave a higher taste score than its untreated counter part on day 4 of storage. With regard to lipid oxidation, malondialdehyde ranged between 1.20 and 1.95 mg kg^?1. Finally, volatile compounds identified in all samples were mainly products of lipid oxidation that increased in concentration with sonication and storage time.     

Effect of milk fat concentration and gel firmness on syneresis during curd stirring in cheese-making

An experiment was undertaken to investigate the effect of milk fat level (0%, 2.5% and 5.0% w/w) and gel firmness level at cutting (5, 35 and 65 Pa) on indices of syneresis, while curd was undergoing stirring. The curd moisture content, yield of whey, fat in whey and casein fines in whey were measured at fixed intervals between 5 and 75 min after cutting the gel. The casein level in milk and clotting conditions was kept constant in all trials. The trials were carried out using recombined whole milk in an 11 L cheese vat. The fat level in milk had a large negative effect on the yield of whey. A clear effect of gel firmness on casein fines was observed. The best overall prediction, in terms of coefficient of determination, was for curd moisture content using milk fat concentration, time after gel cutting and set-to-cut time (R² = 0.95).     

Effects of added whey protein aggregates on textural and microstructural properties of acidified milk model systems

Non-fat milk model systems containing 5% total protein were investigated with addition of micro- or nanoparticulated whey protein at two levels of casein (2.5% and 3.5%, w/w). The systems were subjected to homogenisation (20 MPa), heat treatment (90 °C for 5 min) and chemical (glucono-delta-lactone) acidification to pH 4.6 and characterised in terms of denaturation degree of whey protein, particle size, textural properties, rheology and microstructure. The model systems with nanoparticulated whey protein exhibited significant larger particle size after heating and provided acid gels with higher firmness and viscosity, faster gelation and lower syneresis and a denser microstructure. In contrast, microparticulated whey protein appeared to only weakly interact with other proteins present and resulted in a protein network with low connectivity in the resulting gels. Increasing the casein/whey protein ratio did not decrease the gel strength in the acidified milk model systems with added whey protein aggregates.     

Isolation and characterisation of κ-casein/whey protein particles from heated milk protein concentrate and role of κ-casein in whey protein aggregation

Milk protein concentrate (79% protein) reconstituted at 13.5% (w/v) protein was heated (90 °C, 25 min, pH 7.2) with or without added calcium chloride. After fractionation of the casein and whey protein aggregates by fast protein liquid chromatography, the heat stability (90 °C, up to 1 h) of the fractions (0.25%, w/v, protein) was assessed. The heat-induced aggregates were composed of whey protein and casein, in whey protein:casein ratios ranging from 1:0.5 to 1:9. The heat stability was positively correlated with the casein concentration in the samples. The samples containing the highest proportion of caseins were the most heat-stable, and close to 100% (w/w) of the aggregates were recovered post-heat treatment in the supernatant of such samples (centrifugation for 30 min at 10,000 × g). κ-Casein appeared to act as a chaperone controlling the aggregation of whey proteins, and this effect was stronger in the presence of α_S- and β-casein.     

Fat Reduces Volatiles Production in Oil Emulsion System Analyzed by Purge-and-Trap Dynamic Headspace/Gas Chromatography

Release of volatiles in oil emulsion systems containing various fat contents was determined using purge-and-trap dynamic headspace/gas chromatography. Oil emulsions were prepared using 0, 0.5, 1, 2, 4, or 8% oil, and a standard mixture was prepared using hydrocarbons, aldehydes, ketones, and alcohols. Fat content in oil emulsions reduced the volatility of added volatile standards, and the amount of volatiles released from oil emulsion correlated negatively with fat content (r²= 0.92 in total volatiles). Among these tested, the volatilization of ketones was least influenced by fat content. The release of nonpolar hydrocarbons was not influenced but polar compounds (aldehydes, ketones, and alcohols) were greatly influenced by water.     

Evaluation of oscillatory and shear strain behaviour for thermo-rheological plasticisation of non-ripened cheese curd: Effect of water,protein, and fat

Non-ripened cheese curd with different dry matter (34.5–47.0 g 100 g⁻¹), protein (13.4–30.4 g 100 g⁻¹), and fat (0.5–25.9 g 100 g⁻¹) contents were produced from pre-acidified milk (citric acid, pH 5.8) with different fat-to-protein ratios (0, 0.18, 0.33, 0.79, 1.60) and different curd cooking times (15 min, 30 min, 85 min). From small angle oscillatory temperature sweep and large strain capillary rheology shear experiments it was deduced that the protein content increases and fat decreases maximal loss tangent and storage modulus. This is discussed in relation to suggested changes in casein micelle voluminosity with temperature. A critical shear stress above which the flow behaviour changes from regular to melt fracture behaviour was observed. This is explained by shear jamming and the frequency dependency of the sol–gel transition.     

Casein molecular assembly affects the properties of milk fat emulsions encapsulated in lactose or trehalose matrices

Properties of spray-dried anhydrous milk fat emulsions stabilized by micellar casein (milk protein isolate—MPI) or non-micellar casein (sodium caseinate—Na-caseinate) with trehalose or lactose as encapsulants were studied. A lower concentration of Na-caseinate (0.33%) compared with MPI (1.26%) was sufficient to stabilize a 10% fat emulsion. Reconstituted emulsions showed larger droplet size than fresh emulsions, especially for MPI systems (from<1 μm to around 14 μm), which was attributed to lower shear resistance during atomization. Creaming behavior reflected changes in particle size. Powder surface free fat was affected by protein type and concentration. Trehalose systems (regardless of protein type) released significantly lower amounts of encapsulated fat upon crystallization compared with those containing lactose. Individual and hence, more mobile and flexible casein molecules, as opposed to aggregated and less mobile casein micelles, appear to result in superior co-encapsulation properties of Na-caseinate compared with MPI.     

Effect of Pretreatment with Dehulling and Microwaving on the Flavor Characteristics of Cold‐Pressed Rapeseed Oil by GC‐MS‐PCA and Electronic Nose Discrimination

Raw and dehulled rapeseeds were treated with microwave energy (800 W) from 1 to 8 min with 1‐min intervals at a frequency of 2450 MHz to investigate the influence of microwaving and dehulling pretreatment on the flavor characteristics of rapeseed oil extracted by pressing. Headspace solid phase microextraction was used to isolate the volatile compounds of rapeseed oil, which were then identified by gas chromatography‐mass spectrometry analysis. The results indicated that microwave and dehulling pretreatment of rapeseed can significantly influence the kinds and content of volatile compounds. The key flavor compounds in rapeseed oil were oxidized volatiles, heterocyclic compounds, and degradation products of glucosinolates. A pungent compound, 4‐isothiocyanato‐1‐butene, was reduced by 97% in rapeseed treated for 3 min with microwaves energy when compared to the rapeseed oil without any treatment. The pyrazine compounds in the oil appeared after 6 min of microwave pretreatment and give a pleasant roasting flavor when compared to crude oils. Principal component analysis was able to differentiate between oils obtained using 4 pretreatment processes based on volatile compounds and electronic nose. The results showed that dehulling pretreatment could improve the flavor, yet microwaving had a greater effect on the flavor of rapeseed oils.     

Fractionation by microfiltration: Effect of casein micelle size on composition and rheology of high protein,low fat set yoghurt

The effects of casein micelle size on rheological properties of high protein (5.6% crude protein), low fat (≤0.2%) set yoghurt were investigated. Microfiltration with 0.20 μm membranes was used to fractionate skim milk with an average casein micelle size of ∼174 nm into a retentate and a permeate containing “large” (∼183 nm) and “small” (∼129 nm) casein micelles, respectively. The permeate containing the small casein micelles was further concentrated with 0.10 μm membranes. Yoghurt milk bases with large or small casein micelles were subjected to heat treatment at two different temperatures; 95 °C or 75 °C for 5 min. Yoghurt milk base with small casein micelles gave set yoghurts with higher storage modulus (G′) and higher firmness than yoghurt milk base with large casein micelles. Increased gelation capacity can be attributed to an increased amount of κ-casein in the yoghurt milk base containing small casein micelles.     

Sensory stability and oxidation of fish oil enriched milk is affected by milk storage temperature and oil quality

The experiments evaluated the influence of fish oil quality and cold storage temperature on the oxidative stability of milk emulsions containing 1.0% w/w milk fat and 0.5% w/w of either a pure fish oil or a fish oil:rapeseed oil mixture. The results showed that it was possible to produce a pasteurised milk product enriched with the important n-3 PUFA from fish oil with acceptable sensory characteristics if (1) the emulsions were based on a mixture of fish oil and rapeseed oil and (2) the initial peroxide value (PV) of the added oil blend was below 0.5 meq kg⁻¹. The sensory analysis showed a clear distinction between emulsions based on oil with PV 0.1 and 0.5 meq kg⁻¹, whereas the PV and the gas chromatographic (GC) analysis of volatile oxidation products were not sensitive enough to reveal these differences clearly. The GC analyses showed that the onset of formation of the volatiles was earlier with increased storage temperature in the range of 2–9 °C.     

Characterisation of heat-set milk protein gels

Milk protein solutions [10% protein, 40/60 whey protein/casein ratio containing whey protein concentrate (WPC) and low-heat or high-heat milk protein concentrate (MPC)] containing fat (4% or 14%) and 70–80% water, form gels with interesting textural and functional properties if heated at high temperatures (90 °C, 15 min; 110 °C, 20 min) without stirring. Adjustment of pH before heating (HCl or glucono-δ-lactone) produces soft, spoonable gels at pH 6.25–6.6, but very firm, cuttable gels at pH 5.25–6.0. Gels made with low-heat MPC, WPC and low fat gave some syneresis; high-fat gels were slightly firmer than low-fat gels. Citrate markedly reduced gel firmness; adding calcium had little effect on firmness, but increased syneresis of low-heat MPC/WPC gels. The gels showed resistance to melting, and could be boiled or fried without flowing. Microstructural analysis indicated a network structure of casein micelles and fat globules interlinked by denatured whey proteins.     

Changes in volatile compounds and oil quality with malaxation time of Tunisian cultivars of Olea europea

While there has been considerable work examining the effect of malaxation time on different characteristics of olive oils, there have been few that deal with all the major aspects. Here, the influence of malaxation time was evaluated using major local Tunisian (cv. Chemlali and Chetoui) cultivars. Standard characteristics were measured as well as detailed analyses of volatile compounds were conducted. Headspace solid‐phase microextraction (HS‐SPME) was applied to the analysis of volatile compounds of virgin olive oils from Chemlali and Chetoui varieties with differing malaxation time. Twenty‐seven compounds were characterised by GC‐FID and GC–MS. Compounds belonging mainly to alcohols, esters, aldehydes, ketones and hydrocarbons chemical classes characterized the volatile profile. Significant differences in the proportion of volatiles from oils of different malaxation time were detected. The results suggest that besides genetic factors, malaxation time influences volatile formation. The main variables that were affected by malaxation time were the total amount of phenols and composition of the volatile compounds. At malaxation time of 30 min, Chemlali and Chetoui olive oils presented the highest total phenol content (244.19 and 877.63 mg kg^?1, respectively), while the lowest content was observed at 60 min from regardless of cultivars. In turn, this influenced the oxidative stability and peroxide value. It was also clear that the cultivars behaved differently and this prevented general conclusions being made for all of the quality characteristics.     

A potential of ultrasound on minerals,micro‐organisms,phenolic compounds and colouring pigments of grapefruit juice

The grapefruit juice was sonicated in an ultrasonic bath at 28 kHz frequency (amplitude 70%), for 0, 30, 60 and 90 min at 20 °C. This research was focused on the effects of ultrasound treatment on phenolic compounds, minerals, viscosity, lycopene, total anthocyanins, total carotenoids, micro‐organism analysis and sugars. A statistically significant increase was observed in total carotenoids, lycopene, sugar contents (sucrose, glucose and fructose) and phenolic compounds, whereas a decrease in viscosity and micro‐organisms were found in all the grapefruit juice samples sonicated for 30, 60 and 90 min as compared to control. However, maximum improvement was observed in the sonication treatment for 90 min. The results of this study suggest that ultrasound treatment may improve the quality of grapefruit juice.     

Antioxidant Extraction from Mustard (Brassica juncea) Seed Meal Using High‐Intensity Ultrasound

Brassicaceae oilseeds provide feedstocks for the biofuels industry, but value‐added coproducts are necessary to supply financial incentives for increased production. Our objective was to use high‐intensity ultrasound to optimize extraction of antioxidants from mustard (Brassica juncea) seed meal. The ultrasound‐assisted extraction (UAE) variables included temperature, solvent‐to‐material ratio, sonication duration, and EtOH concentration. Extracts were analyzed for total phenolics content (TPC), antioxidant activity, and sinapine content. Conventional extraction using water and 70% EtOH (v/v) at 80 °C for 3×30 min yielded 7.83 ± 0.07 and 8.81 ± 0.17 mg sinapic acid equivalents (SAE)/g meal, respectively. UAE extraction at 40 °C for 30 min yielded similar phenolics content (8.85 ± 0.33 mg SAE/g meal) as conventional hot ethanolic extraction, but required less time and lower temperature. The highest TPC (13.79 ± 0.38 mg SAE/g meal) was in the 7‐d aqueous extracts. Sonicated solutions of pure sinapine and sinapic acid showed 1st‐order reaction kinetics with greater degradation of isolated compounds than those present in extracts. Sinapine contained in extracts showed insignificant (P < 0.05) degradation after 30 min of sonication. Our research indicates that ultrasound treatment can assist the extraction of antioxidants from B. juncea meal by reducing both the temperature and time requirement without significant degradation of the primary antioxidants present.     

Characterisation of improved foam aeration and rheological properties of ultrasonically treated whey protein suspension

Suspensions of 10, 15 and 20% (w/v) whey protein concentrate (WPC) were treated with 20 kHz ultrasound for 5, 15 and 25 min at an amplitude of 20, 40 or 60%. The treated suspensions were whipped into foam and the aeration and rheological properties were investigated. With increasing ultrasound amplitude and treatment time, whey protein foam at 15% concentration produced the highest foaming capacity, while foam stability, storage modulus, loss modulus, consistency index and viscosity of foam increased with protein concentration. Foam viscosity correlated with foam stability with R² = 0.7425 and significant at P < 0.001.     

Mass transfer modelling in ultrasound assisted osmotic dehydration of kiwi fruit

In this study, the effect of ultrasonic pre-treatment on osmotic dehydration of kiwi slices was investigated. Kiwi fruit slices were subjected to ultrasonic pre-treatment in a sonication water bath at a frequency of 25 kHz for 20 min. Osmotic dehydration of ultrasonic pre-treated samples were conducted for a period of 300 min in 60 Brix sucrose solution. The kinetics of moisture loss and solute gain during osmotic dehydration were predicted by fitting the experimental data with Azuara's model and Weibull's model. The effects of ultrasound application on water loss, sugar gain, effective moisture diffusivity and solute diffusivity of the samples were analysed. The osmotic dehydration process showed a rapid initial water loss followed by a progressive decrease in the rates in the later stages. From the Azuara's model, the predicted equilibrium water loss value for ultrasound pre-treated sample was 58.4% (wb) at 60°C that was nearly 16% higher than the samples treated under atmospheric conditions. Fitting of Weibull model showed that the ultrasound pre-treated and untreated samples had shape parameter (β_w) ranging between 0.570–0.616 and 0.677–0.723 respectively. The lower values of shape parameter indicated that sonication caused accelerated water loss resulting faster dehydration rate. Results indicated that the effective moisture diffusivity and solute diffusivity was enhanced in ultrasonic pre-treated samples. The effective moisture diffusivity during osmotic dehydration of ultrasonic pre-treated samples was ranged between 5.460×10⁻¹⁰–7.300×10⁻¹⁰ m²/s and solute diffusivity was varied between 2.925×10⁻¹⁰–3.511×10⁻¹⁰ m²/s within the temperature range 25–60 °C. The enhanced moisture and solute diffusivity in ultrasound pre-treated kiwi slices was due to cell disruption and formation of microscopic channels.     

Equilibrium headspace analysis of volatile flavor compounds extracted from soursop (Annona muricata) using solid-phase microextraction

The influence of headspace solid-phase microextraction (HS-SPME) variables, namely, sample concentration, salt concentration and sample amount, on the equilibrium headspace analysis of the main volatile flavor compounds released from soursop was investigated. A total of 35 volatile compounds, comprising 19 esters, six alcohols, three terpenes, two acids, two aromatics, two ketones and an aldehyde, were identified. The results indicated that all response-surface models were significantly (p < 0.05) fitted for 10 target volatile flavor compounds. The results further indicated that more than 65% of the variation in the equilibrium headspace concentrations of target volatile flavor compounds could be explained by the final reduced models, with high R² values ranging from 0.658 to 0.944. Multiple optimization results showed that extraction using a 76.6% (w/w) sample concentration, 20.2% (w/w) salt and 8.2 g of blended soursop pulp was predicted to provide the highest overall equilibrium headspace concentration for the target soursop volatile flavor compounds.