# Production of Starch Esters by Roasting Potato Starch with Unripe Apple Juice
## 1 Introduction  
Starch is the second most abundant organic compound in nature following cellulose. It is produced by higher plants and accumulated as a storage substance in grains (e.g., maize, wheat, rice, and sorghum) or tubers and rhizomes (e.g., potato and cassava) and also in seeds, stems, leaves, and fruits. It occurs in the form of granules, the shape and size of which are typical of the plant species it derives from [1]. However, the applicability of native starch in various industry branches is limited due to its low resistance to the effects of shear forces and high temperatures and pressures and low solubility and susceptibility to the effects of amyloid enzymes. Its broader applications require modification of its properties to desirable ones (capability for pasting, swelling, and film formation; solubility; swelling power; retrogradation; rheological properties; etc.) [2]. Modified starch is extensively used in both the food processing and non-food industries (textile, paper, pharmaceutical, cosmetic, plastics, and glue industries and others) [3], and its modification entails physical, chemical, or biochemical factors and combinations thereof. The morphology of a starch granule and its high reactability with various chemical compounds make the application of chemical agents one of the most common strategiesfor its modification [4]. Among the chemically modified starches, the acetylated starch (E1420), obtained upon starch reaction with acetic acid or acetic acid anhydride in the alkaline medium, has been most extensively harnessed in the food industry. This modified preparation offers stabilizing and thickening properties and is applied, most of all, to produce instant noodles, pastry and bakery products, sauces, soups, powdered concentrates, yogurts, and cottage cheeses [5]. Apart from starch acetates, the food industry makes use of other modified starch preparations acting as stabilizers, thickeners, or binding substances, like starch monophosphate (E1410) or cross-linked starches, including distarch phosphate (E1412), acetylated distarch phosphate (E1414), and acetylated distarch adipate (E1422) [6]. The chemical modification is the most common method for imparting new properties to starch. However, its application may be severely hampered given the toxicity of the resulting by-products and their adverse effects on the natural environment. Therefore, an intensive search is underway for chemical substances not compromising human health and safe for the environment, on the one hand, and effective in modifying starch preparations, on the other hand. These include organic acids, like citric acid and malic acid, which are used for starch esterification and cross-linking and considered to be safe food additives (E330 and E296, respectively). Investigations into the production of starch citrates have been conducted for 50 years [7], and the properties of these modified starch preparations, differing as affected by reaction conditions, the botanical origin of starch, or its degree of substitution, have been extensively described in the scientific literature [8]. In the food industry, starch citrates are mainly applied to enrich food with resistant starch (RS4), characterized by a reduced susceptibility to the effects of amyloid enzymes and exhibiting properties of dietary fiber [8]. Starch citrates are produced using, among others, malic acid, which is a safe and non-toxic compound whose structure and properties resemble those of citric acid [9]. Similarly to citrates, malates are classified as resistant starch preparations and produced primarily to enrich food with this valuable component [10]. Organic acids are produced on the industrial scale by means of chemical, enzymatic, or biotechnological methods. However, they can be found in most fruits and vegetables. Malic acid (L-malic acid) occurs commonly in fruits and is a major organic acid of apple juice [11].