H.D. Goff

Fluorescence microscopy to study galactomannan structure in frozen sucrose and milk protein solutions

Abstract Locust bean gum (LBG) and guar were labeled with rhodamine isothiocyanate, and their location within a solution of sucrose or sucrose plus skim milk powder after freezing and during temperature cycling between −18 and −10°C was visualized with fluorescence and brightfield microscopy. LBG was observed to produce a structured gel-like network around the ice crystals, which became more distinct with repeated temperature cycling. Such a network was not evident with guar gum. Image analysis of the ice crystal size distributions showed that LBG had provided much greater resistance to ice recrystallization under these conditions than guar. The same trends were evident in sucrose plus skim milk powder solutions, but the overall increase in ice crystal size was less than it was without skim milk powder. Both the LBG and guar promoted the formation of a phase-separated protein region. Carrageenan successfully reduced the extent of protein/galactomannan phase separation, but solutions containing carrageenan recrystallized to a greater extent than those without carrageenan. It was suggested that a reduction in recrystallization resulted from the formation of a continuous polymer network and structural heterogeneity in the unfrozen phase.

Cryo-gelation of galactomannans in ice cream model systems

Locust bean gum (LBG) and guar gum (GG) are two galactomannan stabilizers that help maintain smooth textures in ice cream by slowing down ice crystal growth during constant and fluctuating temperatures. LBG and GG were dissolved in sucrose solutions without or with milk solids-not-fat (MSNF), fat, and/or emulsifier. Solutions were temperature cycled at subzero temperatures and measured after each cycle with a controlled stress rheometer to obtain yield stress and frequency sweep data. LBG solutions developed weak gel structures with temperature cycling, especially in the presence of MSNF, but GG solutions did not. Fat droplets interfered with the formation of LBG weak gel networks while emulsifiers did not change the rheological properties of emulsions. The ability of a polysaccharide to cryo-gel with temperature cycling and protein/stabilizer incompatibility leading to phase separation both helped create elastic structures. More realistic ice cream model emulsions containing fat showed different rheological responses, emphasizing caution in comparing model systems to real systems.

Rheological properties of dextran related to food applications

Chemical, physical and mechanical properties of dextran were evaluated to assess the potential use of this microbial polysaccharide in food applications. Dextran demonstrated high solubility characteristics and promoted low solution viscosities. Newtonian behaviour was observed at concentrations 1.5% w/w. Dilute and concentrated solution behaviour indicated the conformation of dextran in solution is dependent on both molecular weight and polysaccharide concentration. At low concentrations dextran (mol. wt 500 000) demonstrated properties typical of a ‘random coil’ polysaccharide. Increased concentration resulted in the polymer chain adopting a more compact coil geometry. Non-Newtonian behaviour observed in ‘native’ dextran solutions (>1.5% w/w) is attributable to the formation of inter-chain entanglements through polymer size effects and unique branching properties. Dextran (mol. wt 500 000) demonstrated two critical concentrations (c* = 4.7% and c** = 19% w/v); quasi-elastic light scattering measurements verified the coil overlap region (c* ~ 4.6% w/v). Dynamic oscillatory evaluations indicated dextran (mol. wt 500 000) solutions exhibited dilute solution characteristics at concentrations <20% (w/w) and obeyed the empirical Cox—Merz rule. ‘Native’ dextran solutions (10–15% w/w) demonstrated mechanical spectra typical of concentrated polysaccharide solutions. Calorimetric analysis of ternary dextran:sucrose:water solutions demonstrated that dextran addition modified the frozen system behaviour of sucrose solutions. Increasing dextran concentration effectively increased the onset of melting temperatures (Tg) by as much as 12°C.

Determination of protein surface concentration for emulsions containing a partially crystalline dispersed phase

This study was undertaken to determine the effect of dispersed phase crystallinity on the determination of protein surface concentration. Butteroil (25%) or soya oil (25%) was emulsified with whey protein isolate (1%) or sodium caseinate (1%). The emulsions were centrifuged at several temperatures (5, 20, 40°C) and speeds (5000, 10 000, 15 000g) for a variety of times (20, 40, 60 min) and the protein surface concentration determined for the isolated fat droplets. The isolated fat phase was also resuspended and the particle size distribution determined by light scattering. For the soya oil emulsions no difference was seen in the protein surface concentration or particle size distribution for any of the centrifugation conditions. For the butteroil emulsions the speed and time of centrifugation had little effect but the temperature appeared critical. When centrifuged at 5°C the butteroil emulsions underwent pronounced destabilization in the centrifuge and the resulting protein surface concentrations determined were very low. Centrifugation of an emulsion for protein surface concentration determination should be conducted at a temperature which eliminates the solid fat content of the dispersed droplets.

Influence of stabilizers and freezing rate on the stress relaxation behaviour of freeze-concentrated sucrose solutions at different temperatures

Abstract Stress relaxation behaviour of frozen sucrose solutions in the presence of xanthan gum, gelatin and guar gum was investigated using a thermomechanical analyzer. Freezing rate and stabilizer type had a significant influence on the calculated relaxation time (τ) and asymptotic modulus (E a ) values at temperatures within and above the glassy state. Following slow (2°C min −1 ) and rapid freezing (liquid nitrogen) the temperature corresponding to a large reduction in τ was found to correlate well with previously reported T g onset of annealed and non-annealed systems, respectively (−48°C, rapid; −41°C, slow). At −20°C stabilized systems appeared to alter significantly the mechanism of structural relaxation leading to increased τ and short-term viscoelasticity properties determined from E a values, xanthan and gelatin exerting the greatest effect. It is postulated that above the critical concentration stabilizer action may evolve from modification of the kinetics of the unfrozen phase facilitated by changes in free volume distribution specifically altering the rheological and viscoelastic response of systems at sub-zero temperatures.

Properties of dextran as a cryoprotectant in ice cream

Abstract In order to determine the usefulness of dextran as a stabilizer in frozen dairy products, ice cream mixes with varying concentrations of this microbial polysaccharide were prepared and tested by sensory, physical and mechanical means. Although previous results had shown that aqueous solutions of dextran exhibited remarkably low viscosities, the addition of dextrans to ice cream mixes resulted in much higher values. This is thought to result from interactions among dextrans, dairy proteins and, perhaps, other stabilizers. Dextran addition effectively increased the T g of ice cream mixes, indicative of its value as a cryoprotectant. Also, dextran significantly reduced the sensory perception of ‘iciness intensity’ in heat-shocked ice cream samples. This resulted from the ability of dextran to slow ice crystal growth during storage and thermal abuse, as indicated by reduced ice crystal size as compared to a control formulation. However, dextran also produced unwanted firmness and melting characteristics in ice cream that could influence commercial acceptability.