Optimum alkaline treatment parameters for the extraction of cellulose and production of cellulose nanocrystals from apple pomace

•Cellulose nanocrystals were produced from apple pomace cellulose.•Characterization of nanocrytals were determined.•Alkaline concentration, temperature and time were optimized for cellulose extraction.•Cellulose were converted into nanocrystals by acid hydrolysis and ultrasonication.•High purity cellulose and low particle size cellulose nanocrystal were gained. Cellulose nanocrystals (CNCs) were produced from cellulose extracted from apple pomace (AP). In the alkaline treatment step for cellulose extraction, three variables, the NaOH concentration (6–12%), extraction time (30–240 min), and temperature (30–90 °C) were optimized through response surface methodology (RSM) for highest yield, highest purity (α-cellulose), and whiteness index (WI). The optimum conditions were determined as 10.23% NaOH, 69.82 °C, and 161.54 min, where the cellulose yield was found as 27.96 ± 0.78%, α-cellulose content was 85.31 ± 0.91% and WI was 47.79 ± 0.65%. The acid hydrolysis and ultrasonication treatments were applied to the cellulose to produce CNCs. Morphological analysis of CNCs was revealed the formation of a needle–like structure with averages of 7.9 ± 1.25 nm in diameter and 28 ± 2.03 nm in length. Meanwhile, the XRD diffraction and thermal analysis showed that CNCs have 78% crystallinity index higher than α-cellulose (69%), and is thermally less stable.