materials Article Preparation of Edible Non-wettable Coating with Soybean Wax for Repelling Liquid Foods with Little Residue Tianyu Shen, Shumin Fan *, Yuanchao Li, Guangri Xu and Wenxiu Fan * School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China; [email protected] (T.S.); [email protected] (Y.L.); [email protected] (G.X.) * Correspondence: [email protected] (S.F.); [email protected] (W.F.) Received: 17 June 2020; Accepted: 20 July 2020; Published: 24 July 2020 Abstract: Liquid food adhesion on containers has increased food waste and pollution, which could be effectively alleviated with a superhydrophobic surface. In this research, the superhydrophobic coating was fabricated with edible soybean wax on different substrates by a spraying method. The coated surface showed excellent superhydrophobicity due to its microstructure formed by self-roughening, which could repel a variety of viscous liquid food with the apparent contact angle of 159 2 . ± ◦ The coated surface was still liquid-repellent after hot water immersion (45 ◦C), abrasion test with sandpaper, water impact, finger touch and immersion into yogurt. The liquid-repellent coating with soybean wax, which is natural and green, is promising for application in the food industry to reduce waste. Keywords: non-wettable coating; edible; soybean wax; residue 1. Introduction With the development of the economy, people’s living standards improved, especially in food demand. In daily life, viscous liquid food (such as yogurt, honey, milk, coffee, et al.) residue remain adhering to the container after drinking, which has given us a great deal of inconvenience and resulted in huge waste (up to 15% of liquid food products) [1]. Food adhesion problems could be effectively alleviated by using a superhydrophobic surface. Superhydrophobic surfaces have become a popular topic due to their application in oil/water separation [2–4], anti-corrosion [5], drag reduction [6], reproductive medicine and cryobiology [7] et al. Superhydrophobic surfaces, characterized by high apparent contact angles (> 150◦) and low contact angle hysteresis (the difference between the advancing and receding contact angles, which could lead to a low sliding angle), have tremendous practical applications, including self-cleaning and drag reduction. The non-wetting coatings could eliminate liquid food residue, avoiding the adhesion of viscous liquid to packaging material [8–14]. Superhydrophobic coatings are fabricated through surface texture and low solid surface energy [15] or modification of surface microstructure without using low surface-energy reagents [16,17]. Microscale structure, nanoscale structure and hierarchical structure turned out to be important for surface texture fabrication. So far, most of the studies to fabricate water-repellent surfaces use nanoparticles combined with fluorine-containing reagents. Vahabi et al. [18] prepared flexible non-wettable films by creating coatings of polyurethane and fluorinated silica particles on a substrate, which repelled varieties of liquids. Pan et al. [19] fabricated superhydrophobic stainless-steel wire meshes by creating coatings of cross-linked poly(dimethylsiloxane) and fluorodecyl polyhedral oligomeric silsequioxane. The fluorocarbon materials could decompose to perfluorooctanoic acid (PFOA), whose toxicity is biocumulative and persistent to humans. Thus, the fluorocarbon materials, classified as contaminants, are unfit for the preparation of edible non-wettable coatings. Materials 2020, 13, 3308; doi:10.3390/ma13153308 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 11 Materials(PFOA),2020 whose, 13, 3308 toxicity is biocumulative and persistent to humans. Thus, the fluorocarbon materials,2 of 11 classified as contaminants, are unfit for the preparation of edible non-wettable coatings. To make a non-wettable coating on packaging material, the most important factor is safety: not allowingTo make toxicity a non-wettable when contacting coating food. on A packagingsuperhydrophobic material, surface the most could important be prepared factor by spraying is safety: nota mixture allowing of toxicitya non-polar when compound contacting food.and solvent A superhydrophobic [20,21]. Wang surface et al. [22] could prepared be prepared water-repellent by spraying acoatings mixture using of a non-polarbeeswax and compound carnauba and wax. solvent The coatings [20,21]. were Wang non-wettable et al. [22] prepared towards water-repellentdifferent liquid coatingsfood, which using was beeswax edible and carnaubanatural. Liu wax. et Theal. [23] coatings created were liquid-repellent non-wettable coatings towards using different rice liquid bran food,wax and which candelilla was edible wax and by natural. a one step Liu etspraying al. [23] createdmethod liquid-repellent on polypropylene coatings (PP) using substrates, rice bran which wax andcould candelilla be used wax for byeliminating a one step sprayingliquid waste method of food on polypropylene containers. Zhang (PP) substrates, et al. [24] which fabricated could bea usedsuperhydrophobic for eliminating paper liquid surface waste with of food good containers. transparency Zhang and et stability al. [24] fabricatedproperties a via superhydrophobic mixture coating paperof beeswax surface and with carnauba good transparency wax. The wax and mixture stability was properties emulsified via mixtureand coated coating on a of paper beeswax surface, and carnaubafollowed by wax. annealing The wax at mixture different was temperatures. emulsified and Su coatedbmicrometer on a paper structure surface, was followed generated by annealing on the base at diofff erentmicrometer temperatures. spherical Submicrometer wax particles. structure Yang was generatedet al. [25] on theprepared base of micrometerthree kinds spherical of edible wax particles.superhydrophobic Yang et al. surfaces [25] prepared by fumigating three kinds lard, of ediblefood grade superhydrophobic paraffin and beeswax surfaces on by fumigatingcalcined Fe lard,foil. foodAll samples grade para showedffin and excellent beeswax superhydrophobicity on calcined Fe foil. All and samples effectively showed repelled excellent starch superhydrophobicity slurries and liquid andfoods eff withoutectively repelledobservable starch residues. slurries The and used liquid sample foods withoutcould be observable recovered residues. by re-fumigation. The used sample These couldresearches be recovered have promoted by re-fumigation. the preparation These of researches edible non-wettable have promoted materials, the preparation which should of edible be non-wettableenvironmentally materials, friendly whichand should should not be contai environmentallyn toxic chemicals friendly and unapproved and should additives. not contain toxic chemicalsSoybean and wax unapproved is made additives.from natural soybeans, which are abundant. Thus, the soybean wax is cheap,Soybean natural wax and isbiodegradable made from natural without soybeans, environm whichental pollution, are abundant. which Thus, has great the soybean advantages wax in is cheap,terms of natural health and and biodegradable environmental without protection environmental [26,27]. The pollution, main ingredient which has of greatsoybean advantages wax is intriacylglyceride terms of health composed and environmental of stearic acid. protection Herein, superhydrophobic [26,27]. The main ingredientcoatings were of soybeanfabricated wax with is triacylglyceridesoybean wax by spray composed coating of stearicof a wax acid. suspension Herein, in superhydrophobic ethanol. This low-cost coatings and weresimple fabricated non-wettable with soybeancoating exhibited wax by spray excellent coating liquid of a waxrepelling suspension properties in ethanol. against This a variety low-cost of andnon-Newtonian simple non-wettable viscous coatingfood liquids exhibited or hot excellent water solution liquid repellingon different properties food packaging against a materials variety of (glass, non-Newtonian paper, plastic viscous and foodceramics). liquids The or coating hot water provided solution excellent on different resistan foodce packagingto hot water materials immersion, (glass, sandpaper paper, plastic abrasion, and ceramics).water impact, The finger coating touching provided and excellent yogurt immersio resistancen. toThus, hot waterthe non-wettable immersion, coating sandpaper with abrasion,soybean waterwax could impact, be used finger to touchingreduce liquid and yogurtfood residue immersion. on different Thus, thesubstrates, non-wettable including coating glass with slide, soybean paper, waxplastic could and beceramic. used to reduce liquid food residue on different substrates, including glass slide, paper, plastic and ceramic. 2. Materials & Methods 2. Materials & Methods 2.1. Materials 2.1. Materials Ethanol was purchased from Tianjin Hengxing Chemical Reagent Manufacturing co, LTD, Ethanol was purchased from Tianjin Hengxing Chemical Reagent Manufacturing co, LTD, Tianjin, Tianjin, China. Soybean wax was purchased from Hubei Xinghe Chemical co, LTD, Hubei, China. China. Soybean wax was purchased from Hubei Xinghe Chemical co, LTD, Hubei, China. Milk, fruit Milk, fruit juice, Coca Cola, honey and black tea were purchased from local supermarkets, Xinxiang, juice, Coca Cola, honey and black tea were purchased from local supermarkets, Xinxiang, China. Glass China. Glass slides, paper, plastic and ceramic were available at local markets, Xinxiang, China. slides, paper,