Aeration of model gels: Rheological characteristics of gellan and agar gels

Model food sols containing gellan and agar at different solid concentrations were aerated. The rheological characteristics of the setted gels were determined. These were the compression characteristics (fracture strain/stress/energy, firmness and the total energy for compression) and the stress relaxation characteristics (extent of relaxation and relaxation time). Fracture strain of gels increased due to aeration indicating that the aerated samples exhibited a delayed fracture due to the presence of incorporated air. The extent of increase in fracture strain for aerated agar gels over the corresponding non-aerated samples is 1.7-23.1% while it is 4.3-15.1% for gellan gels. The extent of relaxation increased due to aeration while relaxation time (λ) decreased. Relaxation time for aerated agar and gellan gels is 9.6-40.7% and 19.6-37.6% lower than the non-aerated samples. At a low concentration of hydrocolloid, the effect of aeration was usually non-significant (at p ? 0.05) while prominent changes in rheological behaviour occurred at a high concentration. Aeration of food gels offered the possibility of rheological modifications to suit the consumer requirements in developing specialty fabricated gels.     

Effect of inulin on texture and clarity of gellan gels

The effect of inulin on the gelation of 0.5 wt% low acyl gellan in the presence of increasing concentrations of potassium chloride was investigated by large deformation compression experiments and visible light absorbance. The sugar and salt concentrations varied from 0 to 15 wt% and 40-100 mM, respectively. Stress and strain at failure along with Young’s modulus were calculated from each compression curve. Samples prior to compression were refrigerated at 5 °C for 24 h. Reduced gel strength and firmness were observed for all inulin and salt concentrations. Increasing amounts of inulin resulted in an increase in gel strength which was greater for higher potassium concentrations. Elasticity values did not exhibit great diversion. Inulin at concentrations of 5 and 10 wt% led to less turbid gels than those with only gellan. Samples containing 15 wt% inulin gave absorbance readings greater than 1.     

Rheological and sensory properties of fish gelatin gels as influenced by agar from Gracilaria tenuistipitata

Agar extracted from Gracilaria tenuistipitata and commercial agars were incorporated into fish gelatin at various levels (0, 5, 10, 15 and 20% gelatin substitution). G. tenuistipitata agar (GA) had lower failure stress (~16 kPa) than commercial agar (CA) (~20 kPa). However, the former showed higher failure strain (~30%) with lower melting temperature (65.9 °C). The critical linear stress and failure stress of agar/gelatin mixed gels increased with increasing agar levels (P < 0.05). At 15 and 20% of agar used, the mixed gels containing CA exhibited higher failure stress than those with added GA (P < 0.05). Two melting points of agar/gelatin mixed gels were observed, corresponding to the melting temperatures of gelatin and agar gels. Nevertheless, the incorporation of agar lowered the likeness score of gelatin gel. Thus, both GA and CA had the impact on rheological property and the selected sensory characteristics of fish gelatin, depending on the level of substitution.     

Texture and structure of high-pressure-frozen gellan gum gel

To determine the effects of sucrose and high-pressure-freezing, unsubstituted form-gellan gum gels with 0, 5, 10 or 20% sucrose were frozen at 0.1–686 MPa and −20 °C. Gels were frozen during pressurization at 0.1, 100, 600–686 MPa. However, at 200–500 MPa, gels did not freeze but froze during pressure release (pressure-shift-freezing). On pressure release, a sharp rise in sample temperature was observed for the samples between 200 and 500 MPa. This was a consequence of the exothermic freezing event. Thus, appearance and structure of gels frozen at 200–500 MPa were better than other treated samples due to quick freezing. However, when gels were frozen at 0.1–686 MPa, rupture stress decreased remarkably and strain increased. Texture of pressure-shift-frozen gel was somewhat better than that of gels frozen in freezers (−20, −30 or −80 °C) at atmospheric pressure. Consequently pressure-shift-freezing was more effective. It was found that the addition of sucrose to gels was effective in improving the quality of frozen gellan gum gels.     

Sensory quality of low-sugar orange gels with gellan, xanthan and locust bean gums

Effects of reducing the sucrose content (from 55 to 30 ^°Brix in the final product) and of the use of gellan gum or a mixture of gellan, xanthan and locust bean gums (3:1:1) on the mechanical characteristics (maximum rupture force and deformation at rupture) of orange gels prepared with 15% w/w fruit pulp, sucrose and different amounts of hydrocolloids (0.25, 0.4, 0.55 and 0.7% w/w) were studied by uniaxial compression. The concentration of aspartame needed to compensate for sweetness loss was determined by a paired-comparison constant-stimulus method. Sensory characteristics (texture, appearance and flavour) of low-sugar orange gels (30 ^°Brix) with 0.5% w/w aspartame and different amounts (0.55 and 0.7% w/w) of gellan or the mixture of gums were analysed by the free choice profile method in comparison with the high-sucrose reference material (55 ^°Brix and 0.4% w/w gellan). Use of the mixture of gums permitted the obtention of low-sugar orange gels showing mechanical characteristics similar to those of the reference gel, though some differences in texture were perceived. The low-sugar gels were slightly lighter in colour and slightly more bitter and refreshing than the reference sample. Received: 29 November 1995/Revised version: 13 May 1996     

Instrumental Texture,Syneresis and Microstructure of Yoghurts Prepared from Goat,Cow and Sheep Milk

Set-yoghurts from goat, cow, and sheep milk from middle lactation period were produced. In fresh yoghurts and after 14 days cold storage the following properties were analysed: hardness, adhesiveness, and extrusion force using instrumental texture analyzer, syneresis using drainage and centrifugal methods and microstructure using scanning electron microscope (SEM). Yoghurt from goat milk was characterized by lower hardness, adhesiveness, extrusion forces, and higher susceptibility to syneresis than yoghurts from cow and sheep milk. Microstructure of goat milk yoghurt was more delicate in comparison with microstructure of cow and sheep milk yoghurt. The composition and/or properties of goat milk for yoghurt production, or processing conditions need to be modified to obtain the proper texture and reduced syneresis in final product.     

结冷胶凝胶特性及在食品工业中的应用

目的:介绍新型微生物多糖--结冷胶的凝胶特性及其在食品工业中的应用现状和前景.方法:查阅今年来国内外的相关文献报道,并进行分析、整理和归纳.结果:结冷胶组织相容性和复配性能良好,具有良好的透明性,凝胶性能卓越,具有独特的胶凝和融化温度,可形成多种凝胶质构,在极低的用量下形成的预(弱)凝胶可以发挥良好的悬浮、稳定作用.结论:结冷胶作为一种新型凝胶剂,其优异的性能将在食品工业中得到广泛应用.     

酸度和钙质量分数对复原乳酶凝胶质地特性的影响

考察了不同酸度、钙质量分数调节后复原乳酶凝胶的质地特性,并对其质地变量进行了主成分分析。结果表明,不同工艺处理样品间的差异可以清晰地表现在两个新变量中。酸度、钙质量分数调整可对凝胶的表观黏度和持水力产生很大影响。此外,通过对照分析可以发现,不同前处理的复原乳样品具有与原料乳相近的凝胶特性。     

Textural properties and microstructure of low-fat and sodium-reduced meat batters formulated with gellan gum and dicationic salts

Instrumental texture characteristics of low-fat, reduced-sodium meat batters formulated with other salts (KCl and MgCl₂ or CaCl₂) with gellan gum were evaluated. Fat and sodium reduction through incorporation of gellan gum and either of the dicationic salts produced less rigid, more ductile structures. Inclusion of magnesium chloride resulted in better performance, whereas addition of calcium chloride resulted in less desirable properties. The dicationic salts level used probably inhibited the gellan gum thermoreversible properties, affecting its water holding properties. Microstructural differences between the dicationic salt treatments were apparently due to the effect of dicationic salt concentration on myofibrillar protein extraction and solubilization, and gellan gum gelation properties. Use of magnesium chloride in tandem with gellan gum in the studied low-fat, reduced-sodium meat batters effectively compensated for the structural differences caused by fat and sodium reduction.     

Colour and textural attributes of sucuk during ripening

The sensory (flavour, colour, and ease of cutting scores), colour (Hunter L, a, b, YI, total colour difference, hue angle, chroma, and browning index values) and textural (hardness, adhesiveness, springiness, cohesiveness, gumminess, chewiness, and resilience) attributes of sucuk were followed during the ripening period. Colour scores increased (P<0.05) from score 4 to 6 during the first 3 days of the ripening period and then decreased (P<0.05). The lightness (L values), yellowness (b), and hue angle of sucuk decreased (P<0.05) during the ripening period. The major colour changes occurred between the 5th and 9th days of the ripening period. Pearson correlation test indicates that a positive relationship (P<0.01) exists between a-values and sensory colour scores. The overall sensory scores changed (P<0.05) in parallel to the flavour scores. Hardness, gumminess, and chewiness of sucuk increased (P<0.05) during the ripening period. Strong relationships (P<0.01) were found between hardness and cutting scores. Adhesiveness values of sucuk decreased significantly (P<0.05) about 10 times from -9.3 to -92.6 during the ripening period. Springiness and cohesiveness values decreased during the ripening period, but not significantly (P>0.05). Cutting scores were related (P<0.01) to gumminess (0.921) and chewiness (0.922) values but not to resilience (P>0.05). It was observed that most instrumental colour and textural attributes were in agreement with sensory attributes. These results indicated that instrumental methods could be easily adapted and can be used for evaluation of quality attributes of sucuk during the ripening period.     

Gelation and microstructure of dilute gellan solutions with calcium ions

The viscoelasticity at 25 °C and microstructure of 0.02–0.07 wt% of low acyl gellan aqueous media were investigated for ratios of Ca²⁺ to gellan in the range of 0–38.8, using small amplitude oscillatory shear rheometry and confocal laser scanning microscopy (CLSM), respectively. The total ionic concentration (C_T = γ C_P + C_S, being C_P and C_S the gellan and calcium concentrations, respectively, and γ the mean activity coefficient) of the systems was found to be the triggering and critical factor for the gelation and elasticity of gellan systems. The gel point (T_gel) and storage moduli (G′) increased upon increasing C_T. However, G′ showed a maximum for C_T = 9.3 ± 1.2 meq/L, followed by a progressive reduction as C_T increased; this was primarily due to further addition of calcium, as C_P had a low contribution to C_T of the systems. CLSM demonstrated that the level of counter-ions was enough to induce the formation of a network, whose connection depended on C_P and whose reinforcement was ion dependent. Therefore, even at very low levels of gellan, it is possible to create a wide spectrum of viscoelastic behaviors going from structured liquids to strong gels through the specific combinations of gellan and cation concentrations.     

Sol-gel transition temperatures of high acyl gellan with monovalent and divalent cations from rheological measurements

The sol–gel transition temperatures of 0.1–1.0% high acyl gellan (HAG) with 0–200 mM NaCl or KCl and 0–20 mM CaCl₂ or MgCl₂ were determined using rheological measurements. Transition temperatures for monovalent cations, Na⁺ and K⁺, in the range of 50–80 °C were not significantly different (p > 0.5). Absence of thermal hysteresis was the salient feature. However, thermal hysteresis (∼4.4 °C) was observed for 0.1% HAG without added salt, but disappeared on increasing HAG and counterion concentrations. Few concentrations of HAG and added monovalent and divalent cations showed thermal hysteresis not higher than 2.5 °C. Transition temperatures for divalent cations were similar to those for monovalent cations although for considerably lower concentrations of Ca²⁺ or Mg²⁺. Increasing concentrations of monovalent and divalent counterions give rise to higher transition temperatures but not to higher storage moduli. This was interpreted as a lack of cross-link formation in the three-dimensional network structure of the gels. A single sol–gel transition diagram for monovalent cations is proposed, in which different zones associated with the presence of ordered and disordered conformations serve to identify the conditions in which HAG can exist in aqueous media.     

Effect of polymer ratio and calcium concentration on gelation properties of gellan/gelatin mixed gels

Gelation properties of gellan/gelatin mixed solutions were studied using dynamic viscoelastic testing at eight different ratios of gellan (1.6–0.2% w/v) to gelatin (0–1.4% w/v) and seven different calcium levels (0–30 mM). The gelation temperature and gelation rate of the mixed gels were significantly affected by the ratio of gellan to gelatin as well as concentration of calcium. Addition of calcium at low levels resulted in an increase in gelation temperature and gelation rate compared to gels with no added calcium. Further increases in calcium increased the gelation temperature, but caused a decrease in gelation rate of the mixed gels. In addition, the presence of gelatin generally had a negative influence on gelation rate, especially at high proportions and when the solution had a high gelling temperature, probably by physically hindering the growth and development of gellan crosslinks. It appeared that in the presence of calcium, gellan formed the continuous gel matrix, with gelatin present as a discontinuous phase. Gellan/gelatin mixtures can form gels over a wide temperature range by varying the ratio of the two polymers as well as the calcium concentration.     

Comparative assessment of textural properties and microstructure of composite gels prepared from gelatine or gellan with maize starch and/or egg white

Single component gels (SCG) were formed from gelatine, gellan, maize starch or egg white, while binary composite gel (BCG) and ternary composite gel (TCG) were formed by mixing gelatine or gellan with maize starch and/or egg white. Each type of SCG exhibited distinct textural and microstructure characteristics. Gelatine‐SCG was the softest to hold but has the strongest and the most elastic network compared with other SCG. The effects of gelatine or gellan proportions on the textural properties of composite gels were investigated using mixture design experiments. Gelatine and gellan yielded composite gels that showed different textural behaviours and microstructure. All BCG and TCG blends showed antagonistic effects; the composite gels were softer and weaker as compared to the SCG. Gelatine‐BCG and TCG were comparatively stronger than those of gellan due to their different gelling mechanisms, in which the former had yielded a denser network structures as compared to the latter.     

Dynamics of water in agar gels studied using low and high resolution 1H NMR spectroscopy

The aim of this work was to determine whether the decreased motion of water in agar gels (2.5–12.5% agar) resulted not only from the chemical interaction of the water molecules with the agar macromolecules but also from obstruction by the gel networks. Relaxation experiments were conducted using a 500‐MHz nuclear magnetic resonance (NMR) spectrometer. Diffusion experiments were conducted using a 23‐MHz NMR spectrometer with diffusion times ranging between 15 and 200 ms and a 500‐MHz NMR spectrometer with a fixed diffusion time of 10 ms. This study shows that the interaction of water with hydroxyl groups of agar macromolecules resulted in faster relaxation and slower self‐diffusion of water. The time‐dependent self‐diffusion coefficient of water provides clear evidence of the obstructive effects of the agar gel network on diffusion. This work is the first report on restricted diffusion of water in agar gel systems.     

Effect of microbial transglutaminase on the rheological and textural characteristics of black soybean packed tofu coagulating with Agar

The objective of this study was to investigate the effects of microbial transglutaminase (MTGase) on the rheological and textural characteristics of black soybean packed tofu containing agar as the coagulant. Results showed that the addition of MTGase increased the gelation temperature of soymilk, and produced a firmer and more elastic packed tofu with low cooking loss. Dynamic rheological and textural parameters of the tofu were significantly affected by enzyme concentration, incubation temperature and time. Both G′ and η∗ were positively correlated to the hardness, gumminess and adhesiveness of packed tofu, thus the rheological parameters (G′ and η∗) were good indicators of the texture of the packed tofu. Results suggested that good quality packed tofu could be produced by mixing 90 g black soymilk solid and 2 g agar powder in 1 L of water and incubating with 10 g MTGase for 30 min at 55 °C.     

Physicochemical and textural properties of heat-induced pea protein isolate gels

Chemical and thermal properties of pea protein isolates (laboratory prepared or native; PPIn and commercial; PPIc) and textural properties of heat-set gels obtained from pea protein isolates were compared with homologous soy protein isolates (laboratory prepared, or native; SPIn and commercial; SPIc). The protein banding pattern resulting from electrophoresis separation confirmed the presence of predominant storage proteins of pea and soy seeds in the respective protein products. PPIc and SPIc had lower nitrogen solubility than their native counterparts, likely due to their denaturated state which was further confirmed by the absence of distinct endotherms in these commercial materials compared to the laboratory prepared ones. Addition of NaCl at 1.0–2.0% (w/w) to PPIn and SPIn slurries increased thermal transition temperatures for both proteins.     

Effect of homogenization, microfiltration and pH on curd firmness and syneresis of curd grains

The impact of the independent variables, homogenization pressure (p₁), concentration factor of microfiltration (i) and pH on curd firmness (CF) and syneresis of curd grains was studied. Texture analysis was used to characterize CF of the rennet-induced gels. The analysis of a two-level factorial design revealed that i, p₁, pH and the interaction of i and pH had the most important influence on CF. Cutting time was therefore individually determined for each milk system using small amplitude oscillatory rheometry for generating comparable conditions for the syneresis experiments. Syneresis of curd grains with a diameter of 11 mm was followed at 35 °C close to semi-hard cheesemaking conditions. The permeate release during microfiltration was taken into consideration, allowing an evaluation of syneresis of grains made from concentrated and unconcentrated milk. It was shown that with increasing milk concentration less curd treatment time was needed to reach a certain syneresis value. Hence, total processing time in cheesemaking is decreased. Analysis of variance revealed that syneresis was affected by the individual variables. Kinetic parameters were satisfactorily estimated through regression (R²>0.98) and it was shown that milk composition and concentration due to microfiltration markedly influenced the endpoint of syneresis, RWR_max. The experiments demonstrate that microfiltration and homogenization can be combined to reach CF and syneresis comparable to untreated milk used in conventional cheesemaking. This meets one claim of the cheese industry when implementing both technologies in the manufacture process, since consistency and quality of the ripened cheese are expected to be unchanged.     

Influence of different exopolysaccharide-producing strains on the physicochemical, sensory and syneresis characteristics of reduced-fat stirred yoghurt

Reduced-fat stirred yoghurts were prepared by using ropy and nonropy exopolysaccharide (EPS)-producing strains or nonexopolysaccharide-producing strains. Yoghurts were evaluated for pH, titrable acidity, acetaldehyde, whey separation, viscosity, EPS concentration, sensory and microbiological analyses. Determinations of these parameters were made at 1, 7, 14 and 21 days of storage. Reduced-fat yoghurts made with EPS⁺ strains exhibited lower syneresis and acetaldehyde contents, but higher viscosity than those made with the EPS ⁻ strain. Physico-chemical results correlated with the sensory results in that panellists scored the EPS⁺ yoghurts as having an overall better mouthfeel, but a worse flavour than EPS ⁻ yoghurt.