The extraction kinetics of water soluble extracts (WSE) using the subcritical water extraction are shown in Fig. 1b. Looking at the kinetic curves we can presume that hydrolysis of the sunflower seed material occurs at temperatures higher than 100 C, since yield of WSE keep increasing over longer periods of time.
The highest WSE extraction rate is obtained at 160 C, where the maximal yield of 30.0 ± 0.8% is reached after 60 min of extraction.
Comparing the two lower temperature curves (60 C and 100 C), it can be observed that higher yields of WSE are achieved at 60 C.
This is probably due to higher solubility of WSE at lower temperature. M/S ratio seems to have a lower impact on extraction yield of WSE than Te, since identical curves were obtained at 130 C for all M/S, with the only exception for the M/S = 1/10 g/mL curve, which has a bit lower values of extraction yield throughout the whole process.
It seems that for the applied M/S range, higher M/S increases the overall yield of WSE, but only by moderate
amounts.
Increase of WSE yield at TeP130 C can be explained by two occurring hydrolysis reactions: (1) hydrolysis of protein (producing water-soluble peptides and amino acids) or (2) hydrolysis of carbohydrates (producing water-soluble oligosaccharides and simple sugars).
Both of these reactions can contribute to the total amount of WSE produced, but it is possible that one contributes
more than the other, since hydrolysis of different types of bonds is preferred at different conditions. For instance,
in the article reported by Rogalinski, Liu, Albrecht, and Brunner (2008), which studied the hydrolysis kinetics of cellulose, starch and BSA in Sub-CW from 210 C to 310 C, it was proven that the two carbohydrate biopolymers were much more susceptible towards hydrolytic degradation at the studied reaction conditions than the protein one.
Peptide bonds present within the protein structure are therefore much more stable than glycosidic bonds present within the carbohydrates.
Also, as already mentioned, other products may have been formed during extraction.