Gelation properties of salt-extracted pea protein induced by heat treatment

Gelation is one of the most important properties of plant proteins. In this paper, a low denaturation salt extraction method was used to extract pea (Pisum sativum L.) protein isolate from commercial pea flour. The gelation properties of this isolate were examined and compared to commercial products. The pea protein isolate followed the three-step process of gelation that is generally accepted for heat-induced gelation of globular proteins. The minimum gelation concentration of salt-extracted pea protein isolate (PPIs) was 5.5% while that of commercial pea protein isolate (PPIc) was 14.5%. The gelling point was in the range of 82–86 °C for 14.5% PPIs, 0.3 M NaCl at natural pH (5.65). With increasing heating rate, the gelling point tended to increase. Higher heating and cooling rates resulted in decreased final G′ (storage modulus) and G″ (loss modulus) values, indicative of decreased gel strength. A higher protein concentration resulted in higher G′ and G″ values and it was found that there was a power law relationship between protein concentration and G′ and G″. Tan delta (δ) values decreased with increasing protein concentration and at concentrations of 5.5% and above, tan δ remained constant which means the critical concentration for gel formation was 5.5%. The values of G′ and G″ for PPIs were greater than those of PPIc, and tan δ of PPIs was smaller, indicative of a stronger gel network. DSC data showed that PPIc had undergone denaturation whereas PPIs had not (ΔH = 15.81 J/g protein). Although rheometer data showed that the final G′ value of commercial soy protein isolate (SPIc) was smaller than that of PPIs, the gel prepared with SPIc was visually stronger than that of PPIs. The rheological data obtained with small amplitude oscillatory testing was not consistent with the actual observations. Overall, the low degree of denaturation of the PPIs resulted in a stronger gel than that of PPIc making the PPIs a more attractive food ingredient.     

Influence of pH and hydrocolloids addition on yam (Dioscorea alata) starch pastes stability

Yam tubers (Dioscorea alata) are a non-traditional starch source that could be used as food ingredient. The stability of yam starch pastes (6/100 g suspension) submitted to different pH conditions during gelatinization and the effect of hydrocolloids addition (guar and xanthan gums) on starch syneresis under refrigeration were analyzed. Changes in pH (3, 5, 6) or the addition of gums (0.1–0.5/100 g suspension) did not affect the starch gelatinization temperature nor the gelatinization enthalpy as determined by differential scanning calorimetry. Rheological behavior was characterized by amylograph profiles and oscillatory rheometry. Amylograms showed that yam starch pastes maintained a high viscosity under heat treatment and mechanical stirring in neutral to slightly acidic conditions. Brabender viscosity increased when gums were added; the effect of guar gum on viscosity was more marked than that of xanthan gum. During refrigerated storage exudate production was observed of pastes without gums. Xanthan gum, at a concentration of 0.5/100 g suspension, showed higher effectiveness than guar gum to reduce exudate production during refrigerated storage. The addition of hydrocolloids could allow yam starch to be used in foods requiring low temperatures.     

Roles of components of rice-based fat substitute in gelation

The roles of maltodextrin, protein and fat in the gelation of the rice-based fat substitute were investigated. Rheological properties and gel strength of samples were measured. Results showed that the rising rates of both G′ and G″ of rice starch-based fat substitute were higher than those of rice-based fat substitute when temperature was lower than 35 °C. G′ was equal to G″ at 60 °C for 25% rice-based fat substitute whilst it was at 35 °C for rice starch-based fat substitute. The presence of rice protein and fat increased G′ and G″. Gel strength of rice-based fat substitute was lower than that of starch-based fat substitute at 25% solid content. However, gel strength of a fat substitute increased in the presence of protein. SEM showed that protein particles aggregated to gel. Therefore, protein in rice-based fat substitute gelled firstly, followed by maltodextrin which formed the main matrix of the rice-based fat substitute gel. Fat benefited for gel formation.     

Effects of somatic cell counts in milk on physical and chemical characteristics of yoghurt

The quality of plain stirred yoghurt produced from whole milk with somatic cell counts (SCC) at low (147,000 cells mL⁻¹), intermediate (434,000 cells mL⁻¹) and high (1,943,000 cells mL⁻¹) levels was examined. Each milk treatment was obtained from selected cows, according to its SCC status and milk composition. Yoghurt samples were analysed on days 1, 10, 20 and 30 after production. Analyses included pH, acidity, fat, lipolysis (expressed as free fatty acids, FFA), proteolysis and apparent viscosity. Viscosity of high SCC yoghurt was higher (P<0.05) than the low SCC yoghurt on days 10, 20 and 30 of storage. High SCC yoghurt also had higher FFA content (P<0.05). SCC did not affect pH, acidity, fat content and proteolysis of the yoghurt (P>0.05). Results indicate that SCC in milk increases the lipolysis in the resulting yoghurt during storage for 30 d.     

Ultrasonic processing influences rheological and optical properties of high-methoxyl pectin dispersions

The effect of high-intensity ultrasound on the rheological and optical properties of high-methoxyl pectin dispersions was studied. Pectin solutions (1.15 wt% pectin, 41.4 wt% sucrose) were treated with high-intensity ultrasound at intensity levels ranging from 0 to 40 W cm⁻² for various times (0–60 min). Samples were adjusted to pH 1.5 to initiate gelation and their dynamic rheological properties (G′, G″) were recorded as a function of time using a rotational rheometer. Ultrasonically pretreated pectin dispersions formed weaker gels with increasing sonication power and time. After 4 h, log G′ of the pectin dispersion that was pretreated at the highest intensity level (40 W cm⁻² and 30 min) was approximately five times lower than log G′ of the untreated dispersion. The change in phase angle [arctan (G′/G″)] with time indicated that the rate of gelation decreased as ultrasonic intensity and application time increased. The turbidity of ultrasonically pretreated pectin dispersions decreased by 50% yielding more transparent gels. Results were attributed to an overall reduction in the average molecular weight of pectin due to cavitational effects. A power law model was fitted to the flow curves of ultrasonically pretreated pectin dispersions to determine both flow behavior index n and consistency coefficient K. With increased sonication power and application time, n increased from 0.6 to 0.97 indicating that the flow behavior changed from viscoelastic to Newtonian.     

The cooling rate effect on the microstructure and rheological properties of blends of cocoa butter with vegetable oils

The elasticity (G′) and yield stress (σ¹) of blends of cocoa butter (CB) in vegetable oils (i.e., 30% CB/canola and 30% CB/soybean oil) crystallized at temperatures (T_Cr) between 9.5 °C and 13.5 °C and two cooling rates (1 °C/min and 5 °C/min) were determined, evaluating their relationship with parameters associated with the formation and structural organization of the crystal network [i.e., solid fat content (SFC), Avrami index, crystallization rate, fractal dimension (D)]. The results showed that T_Cr and cooling rate had a different effect for each blend on the three-dimensional organization of the crystal network, and on the proportion and size of β′ and β crystals. Thus, under low supercooling conditions at both cooling rates, the crystallized CB/canola oil blend was formed by a mixture of small β′ and large β crystals that provided high G′ and σ¹ at low SFC (i.e., 20.5–20.9%) and D (i.e., 1.66–1.72) values. The CB/soybean oil blend achieved similar G′ and σ¹ independent of cooling rate only at high supercooling. In this case, the crystal network was formed mainly by small β′ crystals with SFC (i.e., 25.4–26.3%) and D (i.e., 2.86–2.79) values higher (P < 0.05) than in the CB/canola oil blend at low supercooling.     

The rebodying of stirred yoghurt: interactions between proteins

The aim of the present study was to identify the nature of bonds established between protein particles after stirring that are responsible for the texture improvement of stirred yoghurts, called rebodying. Using a constant model yoghurt at pH 4.4, the effects of changes in the physicochemical conditions at stirring were studied on the subsequent rebodying. Short term rebodying was measured as the changes in viscoelastic properties at 4 degrees C during 20 h after stirring, while long-term rebodying was measured as the viscosity changes during 28 d storage at 4 degrees C. Moreover, stirred gels obtained from either set gels that were allowed time or not for ionic equilibration were compared. Increasing or decreasing ionic strength did not change the properties of stirred gels. Calcium chloride addition significantly decreased G'0 h, G'20 h and tan20 h but did not induce changes in the gel microstructure as observed by confocal scanning microscopy. Yoghurt rebodying could not be explained by fulfilling ionic equilibrium. Moreover, N-ethyl maleimide addition had no effect on the stirred yoghurt. Attractive electrostatic and disulphide interactions were not involved in the gel rebodying and increasing calcium concentration in the set gel limited rebodying.     

Preparation and rheological properties of a dairy dessert based on whey protein/potato starch

Gels were prepared by heating different mixtures of whey protein concentrates (WPC) potatoes and ι-carrageenan. The influence of these ingredients on the strength of the obtained gels was investigated. The rheological properties of these gels were also measured during cooling from 90 to 10 °C at variable frequencies and strains. Analysis of variance showed a highly significant (P<0.0001) relationship between the concentrations of the different ingredients and the gel strength. The storage modulus (G^′) was generally higher than the loss modulus (G″) at different temperatures. Generally, the log (G^′) and log (G″) increased with the increase in the applied frequency, which suggested a weak gel entanglement network. Desserts were also prepared using 4% WPC and 3% potato starch (PS), 0.1% ι-carrageenan, 10% sucrose, 3% milk fat and 3% cocoa powder, by heating at 100, 110 or 120 °C for 30 min, packaged hot and stored for 28 days at room temperature (20 ± 5 °C). Samples of fresh and stored desserts were taken at 0, 7, 14, 21, 28 days of storage and their gel strengths were measured. Also, the effect of heat treatment during preparation on G^′, G″ and phase angle (δ) of the different desserts was followed. The effects of the heating time and temperature on the gel strength and rheological properties were relatively small.     

Effect of storage temperature on rice thermal properties

Study on the gelatinization kinetics of rice showed that gelatinization process was divided into two steps: swelling of the amorphous region and disruption of the crystalline region. Higher temperature storage (37 °C) resulted in an increase in the breaking point temperature suggesting that energy for the disorder of these two regions of starch in rice stored at 37 °C was higher than the rice stored at 4 °C. Storage-induced changes in rice led to significant increases in DSC peak temperature (p < 0.05) and significantly broadened peak width (p < 0.01) for rice stored at 37 °C compared to rice stored at 4 °C. As the peak temperature of rice stored at 37 °C was not influenced by the “annealing” treatment in contrast with the increased peak temperature of rice stored at 4 °C after the “annealing” treatment, the results indicate that the ageing process (37 °C storage) has already re-ordered the rice grain structure and that the annealing process under these conditions has no further effect on starch thermal properties. Because starches isolated from rice grain stored at 4 °C and 37 °C had similar thermal properties, this implies that the effects of storage on thermal properties are associated with the interactions between starch and non-starch components following storage. The gelatinization endotherm shifted to a lower temperature (p < 0.01) and a narrowed peak width was achieved after cellulase and protease treatments of stored rice, which indicates that the changes in cell wall remnants and proteins are responsible for the changes in rice thermal properties during storage. Scanning electron microscopy was applied to visualize the treatments of cellulase and protease on rice.     

Combined effects of high pressure,moderate heat and pH on the inactivation kinetics of Bacillus licheniformis spores in carrot juice

The objective of this research was to evaluate the combined effect of high pressure processing (temperature, pressure and time) and product (pH) related variables on destruction kinetics of spores of Bacillus licheniformis in carrot juice. A 3-level factorial experimental design was used with the microbial spores inoculated into carrot juice at the natural pH (6.2) and acidified pH (4.5 and 5.5), pressure (400, 500 or 600 MPa), temperature (40, 50, and 60 °C) and time (0–40 min) conditions. D values found varied from 0.6 to 14.1 min based on the temperature, pressure and pH level combinations. The corresponding temperature and pressure dependency of D values were in the range 23.3 to 31 °C and 241 to 465 MPa, respectively. The destruction pattern was also dependent on pH, with lower pH contributing to higher destruction rate. Conventional log-linear model and Weibull model were used to describe the survivor curves and for predicting processing time to achieve a 5D spore reduction. The survivor curves exhibited slightly upward concavity and therefore better described by a Weibull than the log-linear model. Treatment combinations showed significant (p ≤ 0.05) effects on D and z values of log-linear model and rate parameter (α) of Weibull model. The 5D spore count reduction times estimated using Weibull model parameters were longer than those from the log-linear model, generally demonstrating an over-treatment. Overall, the pH reduction of low acid foods showed a significant enhancement of rate of destruction of B. licheniformis spores.     

Characteristics of stirred low-fat yoghurt as affected by high pressure

The effects of high pressure on the physicochemical, chemical, microbiological and sensory characteristics of stirred low-fat yoghurt were studied. Laboratory-made yoghurts were treated at high pressure (100–400 MPa) for 15 min at 20°C. No significant changes in pH and total organic acids were observed after pressuring the yoghurt. Pressures over 200 MPa prevented post-acidification of the yoghurt during chilled storage. Pressurized yoghurts exhibited higher viscosity and amino acid contents than did the untreated controls, and the differences were maintained after chilled storage. High-pressure treatments at 300 and 400 MPa reduced the number of viable cells of lactobacilli to below the legal minimum permitted in many countries. Significant differences in sensory characteristics between untreated and pressurized yoghurts (200–300 MPa) were detected after chilled storage.     

Characterisation of a high acyl gellan polysaccharide using light scattering and rheological techniques

Two high acyl gellan samples, one obtained from the aerobic fermentation using milk permeate-based medium, and the other, a commercial gellan (LT100, CPKelco), were converted to the sodium form (referred to as Na-31461 and Na-LT100, respectively) using a cation exchange resin. The Na-gellan solutions were characterised using rheological and static light scattering techniques. The shear rate dependence of viscosity master flow curve obtained for Na-LT100 solution gave an exponent value (1−n) of the Cross equation (∼0.76) typical of flexible random coil polymers. A higher exponent value (∼0.82) was obtained for Na-31461 suggesting that the molecules had a lower degree of flexibility than that of Na-LT100. The intrinsic viscosity ([η]) and molar mass (M_w) values for Na-31461 ([η]∼3950 ml/g, M_w∼5.2×10⁵ Da) were approximately twice that of Na-LT100 ([η]∼2360 ml/g; M_w∼2.5×10⁵ Da). The mechanical spectra (G′, G″) of Na-LT100 solution was characteristic of entangled random coil polymers whereas a weak gel characteristic (G′>G″) was obtained for Na-31461 solution. All these suggest that Na-LT100 molecules were single-stranded, while Na-31461 molecules were double-stranded. Both Na-LT100 and Na-31461 formed gels when the Na⁺ concentration was further increased. The complex moduli of Na-LT100 and Na-31461 superimposed closely at all Na⁺ concentrations implied that the conformation of Na-gellan molecules (from single to double-stranded to three-dimensional gel network) were highly dependent on Na⁺ concentration.     

Effect of protective atmospheres on physicochemical,microbiological and rheological characteristics of sliced Mozzarella cheese

Mozzarella cheese slices packaged in bags of polyvinylidene chloride under vacuum (V) and using a gas mixture of equal parts of CO₂ and N₂ (G) were stored at 4 °C for 8 weeks and analysed at different storage times by physicochemical, microbiological and rheological characterization. Expected values of moisture, total nitrogen, soluble nitrogen at pH 4.6, and chloride contents and of pH were observed. The degradation of α_s1-casein was greater than β-casein degradation, while no significant differences were observed due to the packaging methods. Coliform microorganisms were not detected, while levels of moulds and yeasts counts were acceptable. Expected values of total mesophile counts were obtained. The temperature at crossover moduli (T_c) was determined from temperature sweeps carried out by rheometry. Greater values of T_c were observed in samples G. The influence of temperature on complex viscosity was studied by an Arrhenius-type equation. Activation energy values were obtained from the solid-like region (20–40 °C) and liquid-like region (40–60 °C). A more rapid change in viscosity with temperature was observed when storage time increased and when storage method V was used.     

Effect of exopolysaccharides and proteolytic activity of Lactococcus lactis subsp. cremoris strains on the viscosity and structure of fermented milks

Two exopolysaccharide (EPS)-producing Lactococcus lactis subsp. cremoris strains (B35 and B891) were used to study the effect of the kinetics of EPS production and bacterial proteolytic activity on the structure of milk gels and the viscosity of stirred milk gels. Strains were grown at 20 °C in milk containing either yeast extract or casitone and at 30 °C in either milk alone or milk containing casitone. Lactococcal counts, pH decrease and production and molecular characteristics (molar mass and radius of gyration) of both EPSs were followed during milk fermentation. The level of proteolysis in the fermented milks was determined after 24 h of incubation. The results obtained showed that the yield of EPS and the timing of EPS production during milk-gel formation were the most important factors that influenced the structure of the milk gels and the viscosity of the stirred product. The proteolytic activity of the strains did not seem to play any significant role.     

Effect of soy protein substitution for sodium caseinate on the transglutaminate-induced cold and thermal gelation of myofibrillar protein

The influence of soy protein isolate (SPI) substitution for sodium caseinate (SC) on the properties of cold-set (4 °C) and heat-induced gels of pork myofibrillar protein (MP) incubated with microbial transglutaminase (TG) was investigated. The strength of cold-set MP–SC gels (formed in 0.45 M, NaCl, 50 mM phosphate buffer, pH 6.25) increased with time of TG incubation, but those gels with more than 66% SPI substituted for SC had a >26% reduced strength (P < 0.05). Upon cooking, both incubated and non-incubated protein sols were quickly transformed into highly elastic gels, showing up to 6000 Pa in storage modulus (G′) at the final temperature (72 °C). However, no differences (P < 0.05) in G′ were observed between heated samples with SPI and SC. Myosin heavy chain, casein and soy proteins gradually disappeared with TG incubation, contributing to MP gel network formation. Both cold-set and heat-induced gels had a compact protein matrix, attributable to protein cross-linking by TG.     

Rheological and calcium equilibrium changes during the ripening of Cheddar cheese

The small deformation rheological properties and the calcium (Ca) equilibrium of Cheddar cheese were investigated as a function of ripening time. The proportion of insoluble Ca as a percentage of the total Ca decreased from ∼72 to 57% between 3 days and 9 months; most changes occurred within the first 4 weeks. During ripening, the storage modulus (G′) of the cheese increased at low temperature, but decreased rapidly at high temperature. At temperatures >40 °C, the loss tangent increased to reach a maximum at a temperature of ∼70 °C in young cheese and there was a steady decline in this temperature during ripening. The maximum loss tangent values increased substantially during the first 4 weeks and then showed little change. Changes in the insoluble Ca content significantly correlated with pH 4.6 soluble nitrogen (pH 4.6 SN). Partial correlation analysis indicated that the insoluble Ca content was more significantly correlated with the rheological properties than was pH 4.6 SN.     

Effect of temperature,pH and presence of nisin on inactivation of Salmonella Typhimurium and Escherichia coli O157:H7 by pulsed electric fields

The aim of this investigation is to evaluate the concurrent influence of temperature (4–50 °C), pH (3.5–7.0), and the presence of nisin (up to 200 μg/mL) on the inactivation of two PEF-resistant Gram-negative, pathogenic bacteria, Salmonella Typhimurium STCC 878 and Escherichia coli O157:H7, using a PEF treatment of 30 kV/cm and 99 μs. A response surface model using a central composite design was developed for the purpose of understanding the individual effects and interactions of these factors.The models showed that temperature was the factor with the greatest influence on the PEF inactivation in the two strains investigated. Increasing the treatment temperature from 4 to 50 °C increased the lethality of PEF up to at least 4 Log₁₀ reductions for both microorganisms at all pH levels investigated. PEF lethality varied with the square of the pH observing the highest microbial PEF sensitivity at pH 5.25 at all temperatures. The addition of nisin to the treatment medium did not influence the PEF lethality independently of the temperature.PEF induced 1.0–1.5 Log₁₀ cycles of damaged cells at pH 3.5 for Salmonella Typhimurium STCC 878 and at pH 5.25 for E. coli O157:H7, independently of the temperature or the presence of nisin in the treatment medium.The application of PEF at 50 °C permitted the achievement of 5 Log₁₀ reductions of Salmonella Typhimurium STCC 878 and E. coli O157:H7 in a range of pH from 4.2 to 6.7 and from 4.5 to 6.0, respectively. Therefore, the application of PEF at moderate temperatures has great potential for achieving effective control of Gram-negative pathogenic microorganisms in the range of pH found in most foods.     

Thermo-mechanical properties of egg albumen–cassava starch composite films containing sunflower-oil droplets as influenced by moisture content

The effect of moisture content on the thermo-mechanical and structural properties of egg albumen–cassava starch composite films containing sunflower oil droplets was studied using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Composite films were prepared by cold gelation, dried in a moisture controlled incubator (83.5%RH) at 25 °C for 8 days and aged at different relative humidity at room temperature (21 ± 1 °C) for 7 days to obtain composite films with moisture contents of 4%, 7%, 11%, 17% and 46% (dry weight basis). In DMA thermograms the magnitude of G′ and G″ increased with increasing temperature in high-moisture samples, decreased and then again gradually increased for intermediate-moisture samples, and decreased in low moisture samples. DSC thermograms indicated two distinct peaks (at 49–53 °C and 79.8 to 132.4 °C) which were attributed to phase transitions and protein denaturation. SEM images indicated that the microstructure of the composite matrix changed with moisture content and heating temperature. Our study confirms that moisture content plays a key role in the thermo-mechanical properties and microstructure of egg albumen–cassava starch composite films containing sunflower oil.     

Effect of heat treatment and casein to whey protein ratio of skim milk on graininess and roughness of stirred yoghurt

The aim of this work was to study how heat treatment and casein (CN) to whey protein (WP) ratio of skim milk affect physical characteristics of stirred yoghurt. Different heat treatments (95 °C/256 s, 110 °C/180 s and 130 °C/80 s) were applied to the yoghurt milk with the CN to WP ratios of 1.5:1, 2:1, 3:1 and 4:1. Physical properties, including graininess and roughness, of stirred yoghurt were determined during storage at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress decreased, when heating temperature or CN to WP ratio increased.     

Studies on tenderization and preparation of enrobed pork chunks in microwave oven

This study was conducted to develop technology for tenderization and production of enrobed pork chunks in a microwave oven. Meat chunks from shoulder cuts of pork were cured in a solution containing salt, sodium tripolyphosphate and sugar for 48 h at 4 ± 1 °C, enrobed with cream based batter and cooked in a microwave oven at 900 MHz. These enrobed chunks (control) were compared with tenderized (treated) enrobed chunks, for which 7.5% cucumis extract, 9% ginger extract and 0.50% papain powder were incorporated in the standard curing solution. Ginger and papain treatments caused significant (P < 0.01) increases in the moisture content and pH of the cooked products compared to the control and cucumis treated samples. Shear force values were significantly (P < 0.01) lower and overall acceptability scores were significantly (P < 0.01) higher in all treated samples compared to the control. During storage at 4 ± 1 °C, the moisture contents and sensory qualities decreased, but TBARS values and microbial counts increased significantly (P < 0.01). Although all products were acceptable up to 15 days of storage, the ginger treated samples had significantly (P < 0.01) lower TBARS and microbial loads and higher sensory attributes than the control, cucumis and papain treated samples. The magnitude of the storage changes were less and acceptability ranking was higher for the ginger treated samples compared with the others. The results indicated that microwaves could be used for the preparation of enrobed pork chunks. The overall acceptability and shelf life of microwave cooked enrobed pork chunk can be further improved using ginger extract in the curing solution.