In the production of biopolymers, the processing operations (e.g. extraction and drying) involve some degradation of the polysaccharide-causing structural and functional changes in final products. In this study, short-term heat treatment (75–115 °C, 15 min) influence on commercial carrageenans' — furcellaran, κ-carrageenan, ι-carrageenan and a κ/λ-carrageenan — structure, molecular weight and gel rheology was studied. Compared with other carrageenans, commercial furcellaran that had undergone multiple heatings at high temperatures during production was found to be susceptible to polymer degradation. Heat caused the desulphation and degradation of furcellaran galactans and the molecular weight was significantly decreased, causing a drop in viscosity and gel hardness. The loss of the network cross-linking of furcellaran gels was confirmed by scanning electron microscopy. Carrageenan gel storage modulus values decreased with the increase in the temperature of the treatment. The greatest decrease in storage modulus values occurred with κ/λ-carrageenan gels, followed by ι-carrageenan > furcellaran > κ-carrageenan.
Isothermal microcalorimetry was used to study the exothermic heat flow caused by the absorption of water into carrageenans and a furcellaran, leading to their thermodynamic instability under moderate storage conditions (35 °C, τ = 12 h), where τ is the reaction time (h). The net heats evolved by furcellaran, ι-carrageenan, κ-carrageenan, and a mixture of κ/λ-carrageenan were 43.5 J/g, 45.9 J/g, 31.6 J/g, and 28.1 J/g, respectively. The pronounced exothermic behavior of furcellaran was attributed to the enthalpic association of water molecules with the thermally degraded carbohydrate matrix. The responses of a heat treatment of carrageenans at 55 °C, 85 °C, and 105 °C for 15 min on exothermic heat Q (J/g) at different water activities (a w = 0.26, 0.51, 0.76, and 1.0) were measured. The dependence of the net recorded heat Q on water activity can be satisfactorily approximated by the equation Q = a[1 − (1 − a w) b ], where a and b are the coefficients found by the nonlinear least-squares method. It was concluded that carrageenans affected by excessive heat treatment should be preferably stored with limited water access.
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