Undercooling associated with slow freezing and its influence on the microstructure and properties of rice starch gels

It is well accepted that high undercooling or supercooling usually produces numerous small ice crystals. This paper shows that if heat transfer is not rapid enough, high undercooling causes non-homogenous sized ice crystals. Three freezing regimes (i.e. fast, slow and slow with undercooling) were used in this study. Fast freezing produced numerous homogeneously small ice crystals embedded in a thin rice starch gel matrix. This microstructure caused low % syneresis and hardness versus slow freezing’s rather homogenous distribution of fewer large ice crystals embedded in a thicker gel matrix resulting in high % syneresis and hardness. However, slow freezing with undercooling produced non-homogenous clusters involving small and large ice crystals embedded in a very thick gel matrix. Starch retrogradation before ice formation played an important role in this frozen structure. The information gained from this study enhances understanding of the behavior of starch-based food during freezing and storage.     

The effect of hydrolysed tragacanth gum and inulin on the probiotic viability and quality characteristics of low‐fat yoghurt

This study investigates the effect of supplementation of tragacanth gum (GT, 0.05% w/v), low molecular weight gum tragacanth (LMWGT, 0.5% w/v) and inulin (0.5% w/v) on the viability of Bifidobacterium bifidum and the quality parameters of low‐fat yoghurt during a three‐week storage period. The count of probiotics was found to be 7.8 log cfu/g in inulin, and LMWGT enriched yoghurts at the end of the storage period. The minimum water holding capacity and the maximum syneresis values were also obtained in the low‐fat yoghurt enriched with GT throughout the storage time. The samples containing inulin and LMWGT revealed sensory attributes that were judged superior compared to those in GT.     

Influence of curd cutting programme and stirring speed on the prediction of syneresis indices in cheese-making using NIR light backscatter

An NIR reflectance sensor, with a large field of view and a fibre-optic connection to a spectrometer for measuring light backscatter at 980 nm, was used to monitor the syneresis process online during cheese-making with the goal of predicting syneresis indices (curd moisture content, yield of whey and fat losses to whey) over a range of curd cutting programmes and stirring speeds. A series of trials were carried out in an 11 L cheese vat using recombined whole milk. A factorial experimental design consisting of three curd stirring speeds and three cutting programmes, was undertaken. Milk was coagulated under constant conditions and the casein gel was cut when the elastic modulus reached 35 Pa. Among the syneresis indices investigated, the most accurate and most parsimonious multivariate model developed was for predicting yield of whey involving three terms, namely light backscatter, milk fat content and cutting intensity (R² = 0.83, SEy = 6.13 g/100 g), while the best simple model also predicted this syneresis index using the light backscatter alone (R² = 0.80, SEy = 6.53 g/100 g). In this model the main predictor was the light backscatter response from the NIR light back scatter sensor. The sensor also predicted curd moisture with a similar accuracy.     

Characteristics of enzymatically-deesterified pectin gels produced in the presence of monovalent ionic salts

Pectin methylesterases (PMEs) from Valencia orange (p-PME) and Aspergillus aculeatus (f-PME) were used to produce pectin gels in the presence of 0.2 M monovalent ionic salts. At pH 5.0, pectin gels were induced following enzymatic deesterification of high methoxy pectin, with greater deesterification observed using p-PME compared to f-PME. Under these conditions, the deesterification limit of f-PME ended up with a pectin of DE 30.5–31.9 which did not gel at the PME reaction completion, while p-PME reduced the pectin's DE to 16.0–17.2, resulting in gel formation. The pectin gel induced by KCl was significantly stronger than the NaCl-induced gel, but LiCl did not induce pectin gelation. The gel strength was influenced by both DE and species of monovalent cation. The KCl-induced gels released less water than NaCl-induced gels. A synergistic effect on gel strength was observed from the pectin treated with a combination of (p + f)-PMEs, producing even more stable gels. These results indicated that the pectin gelation of our system would be enhanced both by using larger monovalent cation and by lowering the DE value, which would presumably be attributed to the different action patterns recognized for p- and f-PMEs. This pectin gelation system could provide a useful alternative to acid-sugar or calcium cross-linked gels in food and other industrial applications.     

Clean label starches as thickeners in white sauces. Shearing, heating and freeze/thaw stability

The linear viscoelastic properties and syneresis of freshly prepared and freeze/thawed white sauces prepared with different native starches (corn, waxy corn, potato and rice) at different shearing speeds were studied. Viscosity changes during processing were also measured using a starch pasting cell attached to a rheometer. The freeze/thaw cycle produced a significant increase in the viscous and elastic modulus and the appearance of syneresis in the corn and potato starch sauces, while the waxy corn and rice starch sauces were only slightly affected. Syneresis was significantly reduced upon subsequent heating. Greater shearing speed produced a significant decrease in viscoelasticity. Viscosity profiles revealed that the increase in shearing speed decreased the starch gelatinization temperature and swelling capacity and increased starch breakdown.     

Interaction between flaxseed gum and meat protein

Thermal properties, dynamic rheological properties, texture and microstructure of salt-soluble meat protein and flaxseed gum (SSMP-FG) mixtures were investigated. Two transitions, 57.0 °C (T_SSMP1) and 63.2 °C (T_SSMP2), were observed for SSMP without FG with differential scanning calorimetry (DSC). Addition of 2% FG to SSMP increased T_SSMP1 and T_SSMP2 by 1.9 °C and 5.9 °C, respectively. Two transitions, 53 °C (T_SSMP1′) and 75 °C (T_SSMP2′), were also observed for SSMP without FG with dynamic rheological measurement. Addition of 2% FG to SSMP increased T_SSMP1^′ and T_SSMP2^′ by 9 °C and 14 °C. These results indicated that addition of FG increased thermal stability of SSMP. Addition of FG also increased the storage modulus G′, gel strength, decreased syneresis, and changed the microstructure of SSMP gels with texture analyser and scanning electron microscope (SEM), respectively, suggesting that an interaction between FG and SSMP may have occurred. The results of addition of destabilizer to SSMP gels indicated that electrostatic forces seemed to be the main force involved in the formation and stability of protein–polysaccharide gel.     

Characterization of Galactomannan Stabilised Yogurt Drink Using Dynamic Rheology

A stabilised yogurt drink (SYD) was produced by mixing yogurt and water with the galactomannans guar (GG) and locust bean (LBG) gum at concentrations of 0.02, 0.06, 0.10, 0.14, and 0.20 g/100 g. The resulting colloid mixtures' viscous behaviour and stability was investigated. Rheological characterization using oscillatory methodology revealed that dynamic properties of elastic (G′), viscous (G″) modules and complex viscosity (η*) appreciably increased with increasing addition of both gums. Changing shear stress demonstrated complex flow behaviour and revealed the shear dependency of the SYD's flow behaviour. Syneresis decreased with increasing viscosity giving 93% improvement at 0.2 g/100 g gum addition.     

The relationship between rheological parameters and whey separation in milk gels

The relation between whey separation of rennet-induced gels and rheological properties of those gels is reasonably well understood. A low fracture stress and a high value for the loss tangent at low frequencies have been correlated with a tendency to exhibit syneresis in rennet gels. In contrast, little is known about the relationship between mechanical properties of gels and whey separation in acid-induced milk gels, such as yoghurt, although this continues to be a major defect. In recent work, it has been found that conditions such as high milk heat treatment, fast rates of acidification and high incubation temperatures all gave high levels of whey separation compared with gels made from unheated milk that were incubated at low temperatures and where the rate of acidification was slow (i.e. when bacterial cultures were used instead of the acidogen, glucono-δ-lactone). The tendency to exhibit whey separation in acid gels made from heated milk was related to a low fracture strain and an increase in the loss tangent (observed even at high frequencies) during the gelation process (a high value indicates conditions favouring relaxation of bonds). Excessive rearrangements of particles in the gel network before and during gelation were implicated as being responsible for whey separation and rheological conditions that appeared to indicate this defect are described. It was also concluded that techniques that measure the spontaneous formation of surface whey should be distinguished from those that measure the expression of whey from networks under pressure as the latter tests only measure gel rigidity.     

Structure and rheology of the κ-carrageenan/locust bean gum gels

The structure and interaction of κ-carrageenan and locust bean gum (LBG) has been studied using rheology, cryo-SEM, conductivity and syneresis characterization. The rheological behaviour of the binary system has been characterized using both compression and shear measurements. Elimination of slip in the shear measurements yields G′ values of the order 10,000–30,000 Pa for a 1% κ-carrageenan gel in 0–0.2 M added KCl. These values are higher than previously reported. No synergistic peak was found with the addition of LBG as has been previously reported. The measured modulii for these gels yields a Poisson's ratio of 0.5. Compression rupture stress and strain were also monitored. The rupture measurements do show a synergistic peak indicating that the interaction does occur and is important at high strain amplitudes. The gel points as determined by conductivity for these systems show a decrease in temperature with increasing LBG concentration, which is consistent with rheological measurements. Syneresis results are reported for the range of κ-carrageenan/LBG ratios. The syneresis shown by the mixtures is the same as that shown by the same concentration of κ-carrageenan. Structures of the gels as determined by cryo-SEM are also reported. Characteristic length scales in these systems are of the order of tens of microns and show little change with LBG concentration. The reduction in the characteristic length scale with increasing LBG concentration is discussed in terms of the rheological behaviour.