Selective wax recovery from flax and wheat straw

[Display omitted] •Epicuticular waxes can be extracted by liquid nitrogen immersion cycles.•Presence of wax crystals directly increases the yield from the cyclic immersions.•GC–MS and DSC analyses prove the selectivity of the immersion cycles in liquid nitrogen. Straw is a good source of vegetal waxes, which are commonly extracted using hazardous and non-renewable solvents. In this study, one to six immersion cycles in liquid nitrogen (10–60 s immersion followed by 10–60 s rest time) were employed to remove vegetal waxes from wheat and flax straw. The wax yields after six immersion cycles in liquid nitrogen were 25.8 ± 1.0 and 18.8 ± 2.7 mg per 100 g wheat and flax straw, respectively. Scanning electron micrographs helped demonstrate the selectivity of the immersions in liquid nitrogen toward the epicuticular waxes. Prior to extraction, wheat straw presented higher crystal concentrations, related to better immersion cycle wax yields. Chemical and thermal analyses confirmed that the epicuticular waxes from wheat and flax straw were purer and presented different compound proportions than the respective cuticular waxes. Thus, the wheat and flax straw cuticular waxes comprised β-diketones (40% w/w) and aldehydes (36.7% w/w), respectively, as their major compounds. On the other hand, the wheat and flax epicuticular waxes comprised β-diketones (62.9–66.8% w/w) and esters (60.2–64.6% w/w), respectively. The thermodynamic and chemical analytical data of the waxes were in concordance with each other.