An unusual manifestation of phase separation in mixtures of disordered gelatin with konjac glucomannan

The behaviour of konjac glucomannan (KGM) and the galactomannans locust bean gum (LBG), guar gum and fenugreek gum at a concentration of 0.2 wt% in mixtures with type B gelatin in the solution state at 40 °C was examined by visual inspection. At gelatin concentrations of ∼2 wt% and above, the mixtures with LBG and guar gum formed cloud-like flocs, attributed to association of unsubstituted “smooth” regions of the mannan backbone in response to segregative interactions with gelatin. Fenugreek gum, which has no such smooth regions, did not form flocs, but gave a dispersion of small, gel-like particles. Mixtures of gelatin with KGM showed the unusual, striking phenomenon of forming a single gel-like lump which remained suspended in the surrounding liquid, neither rising nor sinking. The single lumps formed by mixtures of 0.2 wt% KGM with 5.0 wt% gelatin were studied in detail. The gelatin content, as determined by macro-Kjedahl analysis and comparison of the area of differential scanning calorimetry (DSC) cooling and heating peaks with calibration curves for gelatin alone, was ∼1.35 wt%. Rheological characterisation by low-amplitude oscillatory measurements and creep-recovery experiments showed that, despite its gel-like appearance, the lump had the properties typical of a solution of densely entangled polysaccharide coils, including close Cox–Merz superposition of steady-shear viscosity (η) and complex dynamic viscosity (η*). The concentration of KGM, determined by comparison of the maximum value of η* in the Newtonian region at low frequency with calibration curves for KGM alone, was 3.4 wt%. The composition of the surrounding liquid was obtained by calculations of mass balance, giving values of 5.20 wt% gelatin and 0.024 wt% KGM. The “p-factor” determined as the ratio of water to gelatin in the solution divided by the ratio of water to KGM in the lump was 0.65, well within the range found for other biopolymer mixtures. It is concluded that the gelatin–KGM mixtures show normal liquid–liquid phase separation, but that the phases have exactly the same density, preventing resolution into two liquid layers, and that enthalpically unfavourable interactions are instead minimised by one of the phases assuming spherical geometry.