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The current study investigates the amount of charged groups induced by sulfuric acid hydrolysis on the surface of starch nanocrystals. Conductometric titration revealed the existence of both weak and strong acidic groups. Strong acidic groups (sulfate ester groups) were present at low amounts (13.2 mmol kg−1) and their concentration increased with increasing duration of hydrolysis. The amounts of weak acidic groups (carboxylic acid) were greater (32-38 mmol kg−1) and the duration of hydrolysis appeared to have no effect. Negative zeta potential values were measured in the studied pH ranges (2.3 and 10). Zeta potential decreased for pH values fewer than 4, remained constant between 4 and 8, then decreased for higher values. Absolute zeta potential values were low (−6 to −29 mV) which could explain the poor suspension stability and the great tendency for particles to aggregate and settle in an aqueous medium. The increase in the pH of the suspension resulted in a lower aggregation as the mean apparent size of the particles decreased in this case. The increase in ionic strength had a similar effect, although the mechanisms involved were hypothesised to be quite different. [ABSTRACT FROM AUTHOR]

Starch nanocrystals (SNC), because of their interesting barrier properties, are of great interest for the formulation of biodegradable nanocomposites. Conventional production process by acid hydrolysis results in a very low yield and a heterogeneous suspension which limits the possibility of scaling up. A continuous process that includes a microfiltration step for isolating starch nanocrystals during the hydrolysis step may be a solution to increase SNC production yield. The main objectives of this study were, first, to quantify the transmission of SNC through membranes under different operating conditions and then to identify the preponderant fouling phenomenon by using simple linear fouling models. Filtration tests were run using a lab-scale microfiltration unit. It was equipped with ceramic membranes with a nominal pore sizes of 1.4 µm and 0.8 µm. Suspensions of nanocrystals at 0.01, 0.02 and 0.04 % (w/w) were filtered in a concentration mode under different transmembrane pressures (50, 100, and 150 kPa). Particle size analysis by Dynamic Light Scattering (DLS) showed that microfiltration is an effective tool for suspension fractionation. Mean diameter of SNC particles was reduced to less than 300 nm in permeate. The transmission through the membrane depended on operating conditions and reached 37 %. The permeate flow rate decreased with time to reach a steady value. The decrease rate was higher when the feed concentration increased. Constant pressure filtration models were used to identify the preponderant fouling phenomenon during filtration and highlighted that the decrease in permeate flow was mainly due to a cake formation on membrane surface. [ABSTRACT FROM AUTHOR]