Effect of high pressure processing on rheological and structural properties of milk–gelatin mixtures

There is an increasing demand to tailor the functional properties of mixed biopolymer systems that find application in dairy food products. The effect of static high pressure processing (HPP), up to 600 MPa for 15 min at room temperature, on milk–gelatin mixtures with different solid concentrations (5%, 10%, 15% and 20% w/w milk solid and 0.6% w/w gelatin) was investigated. The viscosity remarkably increased in mixtures prepared with high milk solid concentration (15% and 20% w/w) following HPP at 300 MPa, whereas HPP at 600 MPa caused a decline in viscosity. This was due to ruptured aggregates and phase separation as confirmed by confocal laser scanning microscopy. Molecular bonding of the milk–gelatin mixtures due to HPP was shown by Fourier-transform infrared spectra, particularly within the regions of 1610–1690 and 1480–1575 cm⁻¹, which reflect the vibrational bands of amide I and amide II, respectively.     

Dynamic viscoelastic behaviour of high pressure treated gluten–soy mixtures

Hydrated gluten and soy mixtures with concentrations of gluten–soy=20:80, 40:60, 60:40 and 80:20% were subjected to high pressure treatment at 700 MPa for 50 min at 20 and 60 °C. The treated samples were subsequently analysed for viscoelastic properties and electrophoretic patterns. A quadratic canonical polynomial model was used for the mixture design. Following high pressure treatment, the samples formed solid-like gel structures. In general, in the gels having high concentrations of gluten, both storage and loss moduli tended to increase with increasing pressure and temperature whereas, in the gels having high concentrations of soy, both moduli appeared to increase only slightly with increasing severity of the treatments. These results meant that the combined effect of temperature and pressure was much greater on the large complex gluten molecule than on the smaller soy globulins.     

Effects of high hydrostatic pressure on the structure of bovine α-lactalbumin

The effects of high hydrostatic pressure (HHP) processing (at 200 to 600 MPa, 25 to 55°C, and from 5 to 15 min) on some structural properties of α-lactalbumin was studied in a pH range of 3.0 to 9.0. The range of HHP processes produced a variety of molten globules with differences in their surface hydrophobicity and secondary and tertiary structures. At pH values of 3 and 5, there was a decrease in the α-helix content concomitant with an increase in β-strand content as the pressure increased. No changes in molecular size due to HHP-induced aggregation were detected by sodium dodecyl sulfate-PAGE. All samples showed higher thermostability as the severity of the treatment increased, indicating the formation of a less labile structure related to the HHP treatment.     

The effect of high hydrostatic pressure on the microbiological quality and physical–chemical characteristics of Pumpkin (Cucurbita maxima Duch.) during refrigerated storage

Pumpkins were processed at high hydrostatic pressure (HHP) ranging from 350 to 550 MPa for 0.5 min to 30 min. Two different nonlinear mathematical models were compared to fit the inactivation kinetics. The second model consistently produced better fits to the inactivation data than the first model (Weibull model). According to the inactivation of microorganisms, pumpkin was subjected to 450 MPa/15 min and 550 MPa/10 min. The microbiological and physicochemical changes in pumpkin subjected to (HHP) and thermal-treated (854 °C/5 min) were compared during 4 °C storage. The total plate counts (TPC) treated with thermal processing, 450 MPa/15 min and 550 MPa/10 min were 5.12, 4.02 and 1.71 log₁₀ CFU/g, respectively on the 60th day. The growth of microorganisms caused the increase in ΔE, decrease in hardness in other treatments. Treatment of 550 MPa for 10 min had little effect on color during storage. There were no significant changes in the L^⁎, a^⁎ and b^⁎ values (p > 0.05). The hardness of pumpkin treated with 550 MPa/10 min decreased by 32.28% after 60 days. A greater retention of the original color, Vc and antioxidant capacity and increased total phenols were observed in 550 MPa/10 min-treated samples immediately after processing. During storage, color changed, Vc content, total phenols and the antioxidant activity were decreased. While the soluble solids content (SSC), sugars and pH value of pumpkin with HHP or thermal treatment did not show significant change immediately during 60-day storage. Based on these results, working at 550 MPa for 10 min ensures physicochemical and high standard of sanitation parameters in pumpkin.Industrial relevancePumpkin (Cucurbita maxima Duch.) is one of the popular vegetables, and fresh-cut pumpkin requires strict processing treatment and storage conditions to protect its quality. HHP is one promising novel non-thermal technique and is likely to replace thermal processes. A better knowledge of effects of storage temperature on the quality of HHP-treated pumpkin and its storage time prediction through microbiological quality and physical–chemical characteristics analysis of these changes is necessary. The available data would provide technical support for commercial application of the HHP technique in fresh-cut pumpkin processing.     

Influence of milk pre-heating conditions on casein–whey protein interactions and skim milk concentrate viscosity

The viscosity of concentrates (50–55% total solids) prepared from skim milk heated (5 min at 80 or 90 °C) at pH 6.5 and 6.7 was examined. The extent of heat-induced whey protein denaturation increased with increasing temperature and pH. More denatured whey protein and κ-casein were found in the serum phase of milk heated at higher pH. The viscosity of milk concentrates increased considerably with increasing pH at concentration and increasing heating temperature, whereas the distribution of denatured whey proteins and κ-casein between the serum and micellar phase only marginally influenced concentrate viscosity. Skim milk concentrate viscosity thus appears to be governed primarily by volume fraction and interactions of particles, which are governed primarily by concentration factor, the extent of whey protein denaturation and pH. Control and optimization of these factors can facilitate control over skim milk concentrate viscosity and energy efficiency in spray-drying.     

Effects of high hydrostatic pressure on microstructure,physicochemical properties and in vitro digestibility of oat starch/β-glucan mixtures

Oats do not contain gluten protein, and oat dough structure is formed mainly through the hydrogen bonding of starch and β-glucan. As a non-thermal processing technology, high hydrostatic pressure (HHP) is mainly used to modify starch and protein in food processing. This study investigated the effects of HHP treatment on the morphological, structural, thermal, pasting and in vitro digestion properties of oat starch/β-glucan mixtures. Results showed that β-glucan interconnects with amylose through hydrogen bonding and has a protective effect on the crystalline region of oat starch. Effect of HHP treatment on the crystal structure of mixture system goes through crystal structure perfection stage, crystallisation disintegration and gelatinisation stage. After 300–400 MPa treatment, the changes in particle surface were not obvious, the phase transition temperature, the ΔH_gel and the PT of mixtures increased, while the particle size, viscosity and BD values decreased. After 500–600 MPa treatment, mixtures were completely gelatinised, most of the particles swelled and deformed, the particle size increased significantly. The principal component analysis results show that the complexes were distributed in the same region with similar properties after the 300–400 MPa and 500–600 MPa treatments, respectively.     

Effects of high hydrostatic pressure on color of spinach purée and related properties

BACKGROUND: Changes in instrumental color parameters, chlorophyll a and b, activity of chlorophyllase, Mg‐dechelatase, peroxidase and polyphenol oxidase, total phenolic compounds and pH of spinach purée were assessed after high hydrostatic pressure (HHP) (200, 400 and 600 MPa for 5, 15 and 25 min) treatments at room temperature. RESULTS: HHP treatments induced a better retention of visual green color (?a* and L* values) and chlorophyll contents of spinach purée. As for chlorophyll degradation‐related enzymes, the results indicated that chlorophyllase activity decreased at all pressures; however, Mg‐dechelatase activity was dramatically activated after HHP treatment at 400 and 600 MPa. Peroxidase exhibited higher resistance to HHP; however, polyphenol oxidase, which is responsible for enzymatic browning, was suppressed progressively with increase in pressure level from 200 to 600 MPa. In addition, the pH value of HHP‐treated spinach purée was increased to be close to neutral pH, which could effectively inhibit chlorophyll degradation. No significant differences (P > 0.05) were found after extending the treatment times at the same level of pressure. CONCLUSION: HHP treatments effectively prevent chlorophyll degradation and enzymatic browning in spinach purée and retain a better original fresh green color of spinach compared with conventional thermal treatment. Copyright © 2012 Society of Chemical Industry     

Addition of gelatin enhanced gelation of corn–milk yogurt

Corn–milk yogurt set by a combination of sodium caseinate plus gelatin at concentrations of 0, 0.2, 0.4 and 0.6% (w/v) were studied. The quality of the gels was determined by measurement of acidity, syneresis, texture profile analysis, viscoelasticity, structure scanning electron microscope and microbiology. Texture profile analysis (TPA) showed that increasing levels of gelatin increased hardness, adhesiveness and springiness as well as the acidity of the products. Viscoelastic behaviour displayed similar trends to the TPA characteristics, the storage modulus was less frequency dependent than the loss modulus giving a loss tangent of 0.2 in the high gelatin systems, which might indicate a true gel system. The microstructure was dense and spongy-like with small air cells, in particular, those having a high concentration of gelatin (0.6%, w/v) gave a very firm structure which might impair palatability. The addition of a commercial gelatin at 0.4% (w/v) gave good acceptability for the product (little syneresis of the gels produced). While the gelatin used for this study had a bloom value of 246 g the authors acknowledge that a different commercial gelatin may well result in a different concentration being required.     

Microstructure of acid–induced skim milk–locust bean gum–xanthan gels

The microstructure of acid skim milk gels (14% w/w milk protein low heat powder) with or without addition of locust bean gum (LBG), xanthan gum (XG) and LBG/XG blends was determined by transmission electron microscopy (TEM), phase-contrast light microscopy (PCLM) and scanning electron microscopy (SEM). Three polysaccharide concentrations (0.001%, 0.02% and 0.1%, w/w) were used for binary mixtures. In the case of ternary mixtures, three LBG/XG weight ratios were used (4/16, 11/9 and 16/4) at 0.02% total polysaccharide concentration. Control acid skim milk gels were structured by a homogeneous network of casein particles (0.1–0.7 μm in diameter) and clusters immobilizing whey in small pores (1–5 μm in diameter). Filamentous structures and small aggregates were observed at the surface of casein particles. Low concentration of LBG or XG (0.001% w/w) did not affect markedly the microstructure of acid skim milk gels. Conversely, LBG or XG at 0.02 or 0.1% concentration and LBG/XG blends at the three ratios selected had a great influence on the gel microstructure. Although the size and surface structure of the casein particles were not modified by the presence of polysaccharides, the primary casein network appeared very compact with a decrease of pore size and a large increase in the porosity of the network at the supramolecular level (sponge-like morphology). The effect is stronger for gels containing LBG and XG used at higher concentration and less apparent for gels containing LBG/XG blends. Skim milk/XG gels were highly organized into fibrous structures whereas skim milk/LBG gels were more heterogeneous. These structures were discussed in the light of volume-exclusion effects (demixing) and specific interactions between casein micelles and polysaccharides. At the three weight ratios, skim milk/LBG/XG gels displayed both jagged “coral-like”, “veil-like” and filamentous structures. These structures could originate from a secondary network constituted by the known LBG/XG synergistic interactions.     

Effect of κ-carrageenan on milk protein polysaccharide mixtures

The effect of κ-carrageenan (0, 0.025, 0.05%) on phase separation between polysaccharides (0.36% of locust bean gum (LBG), guar gum, or xanthan gum) and milk proteins (from 10.5% skim milk powder) in solution was studied. Xanthan gum was seen to be the most incompatible with milk proteins, followed by guar gum and LBG. Casein micelles were more incompatible with all polysaccharides than whey proteins. Whereas at either concentration κ-carrageenan inhibited visual phase separation, it was seen by transmission electron microscopy that samples with κ-carrageenan showed microscopic phase separation. Samples with 0.05% κ-carrageenan and either LBG or guar gum and all samples with xanthan gum could be described rheologically as weak gels, while those with no or 0.025% κ-carrageenan and either LBG or guar gum could be described as concentrated solutions. Thus, no correlation was seen between the inhibition of macroscopic phase separation by κ-carrageenan and the formation of a weak gel in solution.     

The effect of protein concentration and heat treatment temperature on micellar casein–soy protein mixtures

The objective of this study was to investigate the effect of concentration and temperature on the rheological properties of soy proteins (SP) and micellar casein (MCN) systems. Individual and mixed (1:1) protein systems of 2–15% concentration were prepared and heat treated for 5 min at 40–90 °C. After cooling to 20 °C, their rheological properties were determined using steady-shear rheology. Zeta potential and particle size measurements were also conducted. Both proteins were negatively charged under all experimental conditions, but the absolute values of zeta potential and thus the stability of the protein solutions decreased with temperature and concentration. For SP solutions, viscosity and apparent yield stress increased with concentration. Shear thinning behavior was prevalent, becoming more pronounced with increasing concentration. Heat treatments at T ≥ 80 °C induced glycinin denaturation, followed by aggregation and network formation when C ≥ 7.5%. Heat treatment did not significantly affect viscosity of MCN systems, while increasing concentration resulted in a significant increase in apparent viscosity and apparent yield stress. Most MCN systems exhibited Newtonian flow behavior, with the exception of systems with C ≥ 12.5% treated at T ≥ 80 °C, which became slightly shear thickening. Mixed SP–MCN systems mimicked the behavior of SP, with most values of rheological parameters intermediate between SP and MCN-only systems. Mixtures of 7.5–12.5% concentration treated at 90 °C displayed local phase separation, low viscosity and apparent yield stress, while 15% mixtures treated at 90 °C showed protein aggregation and incipient network formation. The data generated in this study can be used to develop a range of protein based products with unique flow characteristics and storage stability.     

Time-dependent flow properties of starch–milk–sugar pastes

The time-dependent flow properties of starch–milk–sugar (SMS) pastes have been studied. The flow properties were assessed from the measurement of the shear stress versus time of shearing at constant shear rate. Corn and wheat starches were used in this study, while the sugars were glucose, sucrose, and fructose. The Weltman model was used to evaluate the flow properties of SMS pastes prepared under different conditions. SMS pastes heated at 95 and 85 °C exhibited a thixotropic behavior, while pastes heated at 75 °C behaved like a rheopectic fluid. It was noted that the thixotropy occurred at high shear stress (above 50 Pa), and the rheopexy occurred at low shear stress (below 45 Pa). The degree of thixotropy, as assessed by the Weltman model parameters, increased significantly with starch concentration, and with less pronounced effect with sugar concentration. The effect of sugar type on the degree of thixotropy of SMS pastes heated at 95 °C decreased in the following order: fructose>sucrose>glucose. The type of starch played a role in the time-dependent flow properties of the SMS paste, with a general conclusion that wheat starch had a greater degree of thixotropy than corn starch.     

Comparison of chitosan–gelatin composite and bilayer coating and film effect on the quality of refrigerated rainbow trout

The effect of chitosan–gelatin coating and film on the rancidity development in rainbow trout (Oncorhynchus mykiss) fillets during refrigerated storage (4 ± 1 °C) was examined over a period of 16 days. Composite and bilayer coated and film wrapped fish samples were analysed periodically for microbiological (total viable count, psychrotrophic count) and chemical (TVB-N, POV, TBARS, FFA) characteristics. The results indicated that chitosan–gelatin coating and film retained their good quality characteristics and extend the shelf life of fish samples during refrigerated storage .The coating was better than the film in reducing lipid oxidation of fillets, but there was no significant difference between them in control of bacterial contamination.     

Okara treated with high hydrostatic pressure assisted by Ultraflo® L: Effect on solubility of dietary fibre

Okara is an abundant and inexpensive by-product from soybean, rich in total dietary fibre (> 55% dry weight), but poor in soluble dietary fibre (SDF, < 5% dw). A combined method of high hydrostatic pressure (HHP) aided by the food grade enzyme Ultraflo® L was used for SDF maximization. At atmospheric pressure, incubation time was not a key factor, and a ratio of 1:40 (enzyme:Okara, v/w) was able to saturate the enzyme within 120–150 min. When HHP plus Ultraflo® L were applied, a synergy between both treatments was observed. Thus, at 600 MPa, 0.025% Ultraflo® L and 30 min treatment, soluble carbohydrate added up to 15.64 ± 0.32%, consisting of two peaks (9.14 ± 0.18 and 0.57 ± 0.01 kDa), determined by HPLC-ELSD. The combination of HHP plus Ultraflo® L on Okara improved the solubility of the dietary fibre, making it more suitable to be used in functional foods.Industrial relevanceThere is an increasing interest in finding new prebiotics from food industrial waste. Okara has been simultaneously treated with Ultraflo® L and HHP, taking into account the industrial costs of the procedure, and adjusting the quantity of enzyme and time of treatment to the minimum, by a Response Surface Methodology and an enzymatic activity assay. Besides, an HPLC-ELSD direct analysis (High Performance Liquid Chromatography with Evaporative Light Scattering Detector) was employed to monitor soluble fibre, as an easy and fast analytical method.     

Inactivation of Staphylococcus aureus and Escherichia coli by the synergistic action of high hydrostatic pressure and dissolved CO₂

This study focused on the synergistic inactivation effects of combined treatment of HHP and dissolved CO₂ on microorganisms. The aim was to reduce the treatment pressure of the traditional HHP technology and make it more economically feasible. The combined treatment showed a strong bactericidal effect on Staphylococcus aureus and Escherichia coli in liquid culture, which usually have high levels of barotolerance under pressure alone. To identify the influence of CO₂, a new setup to dissolve, retain and measure the concentration of CO₂ was constructed. The results demonstrated that an inactivation rate of more than 8 log units was obtained for E. coli both at 300 MPa with 1.2 NL/L CO₂ and at 250 MPa with 3.2 NL/L CO₂, while only 2.2 and 1.8 log reductions were observed at 300 MPa and 250 MPa, respectively, for the HHP treatments alone. For S. aureus, the inactivation rate of more than 7 log units was found at 350 MPa with 3.8 NL/L CO₂, while only a 0.9 log reduction was achieved at this pressure in the absence of CO₂. The SEM photographs showed seriously deformed cells after the synergistic treatments. In contrast, the cells treated with individual HHP maintained a relatively smooth surface with invaginations. Propidium iodide staining and fluorescence observation was performed after pressure treatments. The results demonstrated that the combination of CO₂ with HHP also promoted pressure induced cell membrane permeabilization greatly. It was deduced that the enrichment of CO₂ on the cell surface and its penetration into the cells at high pressure accounted for the membrane damage and cell death.     

Effects of gelatin origin,bovine-hide and tuna-skin,on the properties of compound gelatin–chitosan films

With the purpose to improve the physico-chemical performance of plain gelatin and chitosan films, compound gelatin–chitosan films were prepared. The effect of the gelatin origin (commercial bovine-hide gelatin and laboratory-made tuna-skin gelatin) on the physico-chemical properties of films was studied. The dynamic viscoelastic properties (elastic modulus G′, viscous modulus, G″ and phase angle) of the film-forming solutions upon cooling and subsequent heating revealed that the interactions between gelatin and chitosan were stronger in the blends made with tuna-skin gelatin than in the blends made with bovine-hide gelatin. As a result, the fish gelatin–chitosan films were more water resistant (∼18% water solubility for tuna vs 30% for bovine) and more deformable (∼68% breaking deformation for tuna vs 11% for bovine) than the bovine gelatin–chitosan films. The breaking strength of gelatin–chitosan films, whatever the gelatin origin, was higher than that of plain gelatin films. Bovine gelatin–chitosan films showed a significant lower water vapour permeability (WVP) than the corresponding plain films, whereas tuna gelatin–chitosan ones were only significantly less permeable than plain chitosan film. Complex gelatin–chitosan films behaved at room temperature as rubbery semicrystalline materials. In spite of gelatin–chitosan interactions, all the chitosan-containing films exhibited antimicrobial activity against Staphylococcus aureus, a relevant food poisoning. Mixing gelatin and chitosan may be a means to improve the physico-chemical performance of gelatin and chitosan plain films, especially when using fish gelatin, without altering the antimicrobial properties.     

The effect of agitation on the nucleation of α-lactose monohydrate

Nucleation in an industrial crystallisation process determines how many crystals are formed and defines the final particle size distribution. This parameter plays a critical role in determining the success of an industrial lactose crystallisation, impacting yield, throughput and product quality. Previous studies on lactose crystallisation have reported that mixing can influence the nucleation kinetics of α-lactose monohydrate. This work looked to authenticate this by measuring the induction time required for nucleation of supersaturated lactose solutions across a range of agitation rates. Increasing agitation increased the rate of nucleation at a given supersaturation. The results show that this is a result of an increase in the frequency of activated molecular collisions and not a change in the critical nucleus size, which remains constant at a defined supersaturation.     

The effect of high pressure homogenization and endogenous pectin-related enzymes on tomato purée consistency and serum pectin structure

The influence of mechanical tissue disintegration techniques (i.e. blending and high pressure homogenization) and the stimulation of endogenous pectin-related enzymes (i.e. pectin methyl-esterase and polygalacturonase) on tomato purée consistency, serum composition and serum pectin structure were investigated. Serum pectin structure was characterized in terms of degree of methyl-esterification, acetylation, neutral sugar composition and molecular weight (M_w) distribution.Endogenous pectin methyl-esterase and polygalacturonase stimulation resulted in the lowest purée consistency and highest serum yield. However, when such purée was homogenized, a higher purée consistency and a low serum yield were observed. Moreover, the M_w of serum pectin was exceptionally high for the homogenized purées. The low methyl-esterified, linear and remarkably high M_w tomato serum pectin of the homogenized purées partly explains their increased consistency. This work demonstrated that high pressure homogenization can at least partially restore the consistency of tomato purée despite an initial consistency loss ascribed to enzymatic pectin degradation.Industrial relevanceThe synergistic action of endogenous pectin-related enzymes causes serum pectin de-polymerization that consequently results in consistency loss of tomato purée. Nevertheless, intense high pressure homogenization showed to influence serum pectin molecular weight and at least restore the consistency loss. This means that a high tomato purée consistency can still be achieved regardless of the initial action of endogenous enzymes in the tomato pieces or puréed tomato food system using high pressure homogenization. This offers an additional processing alternative in the production of tomato-based dispersions with a targeted functionality.