Hydration properties of soybean hull fractions: An undervalued agroindustrial waste

DELGADO, JUAN FRANCISCO; SALVAY, ANDRÉS G.; PORFIRI, MARÍA CECILIA

Mar del Plata

Congreso; XVI Congreso Argentino de Ciencia y Tecnología de Alimentos; 2017

Asociación Argentina de Tecnólogos Alimentarios - AATA

Soybean is an important commodity in Argentina and soy hull is an undervalued agroindustrial by-product that has many interesting biopolymers. Thus it is necessary a complete description of its general properties. The objective of the present work was to study the hydration properties of different fractions obtained from soy hull.Soy hull was ground to a particle size of <500 microns obtaining soy hull flour (HF). HF was dispersed in 0.1 M HCl (1:15, 90 °C, 45 min) and centrifuged (7000×g, 15 min, 4 °C). Supernatant (SN) and precipitate (PP) were separated. SN was adjusted to pH 3.5, dispersed in equal volume of 2-propanol, incubated for 4 hs and centrifuged. The pellet was dispersed in 70% 2-propanol, stirred for 30 min and centrifuged. The washing was repeated with 70% 2- propanol and with absolute 2-propanol. The PP was dispersed with distilled water, stirred for 15 min and centrifuged. The washing was repeated with distilled water and with ethanol 96%. Pellets from SN and PP were dried at 40 °C, obtaining hull soluble polysaccharides (HSPS) and hull insoluble polysaccharides (HIPS), respectively. Water content and retaining intensity of samples previously hydrated at 90 % relative humidity (r.h.) at 20 °C were analysed by thermal-gravimetrical analysis (TGA Q500, TA Instruments, USA). Samples were heated from 20 to 150 ºC at 1 ºC min-1 and then at 10 ºC min-1 up to 900 ºC. Hydration kinetics at 90 % r.h. as well as water sorption isotherms were studied at 7, 24, and 37 °C in order to obtain thermodynamics parameters. TGA thermograms showed that in first period studied (20-150 ºC) the HSPS fraction was more retained revealing a higher water binding force. The composition of each powder was reflected in the second period of thermograms (150-900 ºC), were higher amount of proteins in HSPS fraction could be appreciated through a greater degradation rate (0.34 % ºC-1) than HF and HIPS fractions (0.22 and 0.19 % ºC-1, respectively) at 242 ºC. The HIPS fraction showed an intense and narrow peak at 324 °C in derivative of weight with respect to temperature (1.0 % ºC-1) due to its more defined composition (mainly polysaccharides).Hydration kinetics data at 90 % r.h. were satisfactory fitted using both bi-exponential formula and the Pilosof-Boquet and Bartholomai (PBB) model. Both models show smaller water content at equilibrium in HIPS fraction. All samples showed an increase in the hydration rate at increasing temperature, revealing an endothermic process for water uptake.All isotherms showed hysteresis, more pronounced at 7°C. HP and HIPS had a little monolayer, while HSPS a bigger one. HSPS and HIPS showed distinct hydration curves, and HF was not the sum of both: for water activity (aw) <0.6 hydration curve of HF was similar to HIPS and for aw >0.8, it was most related to HSPS.So, we contribute to the understanding of the hydration properties of soy hull fractions, promoting the use of this unexplored material in the manufacturing of foods.