Starch is a long chain of molecules connected to glucose, the general formula of starch is (C6H10O5)n, hydrolyzed to the disaccharide stage for maltose, and completely hydrolyzed to get glucose.

Starch is a nutrient stored in the plant body, widely present in many plant seeds, roots, stems and other tissues, of which rice contains 62% -86% starch, wheat contains 57% -75% starch, corn contains 65% -72% starch, potato contains 12% -14% starch. Potato contains 12%-14% starch.

Structure of starch

Most natural starches are composed of a mixture of two polysaccharide types, consisting of straight-chain starch and branched-chain starch.

1.1 Straight chain starch
Straight chain starch is D-glucose through the α-1,4 glycosidic bond connected to the chain molecules, from the stereo conformation, it is not linear, but by the hydrogen bonding within the molecule so that the chain curls and coils into a left helix.

In the crystalline state, it is believed by X-ray mapping analysis that straight-chain starch contains three sugar groups per strand in each loop of the double-helix structure, and six sugar groups in each loop of the single-helix structure.

There is only one reducing end group and one non-reducing end group on the straight-chain starch chain. In solution, straight-chain amylopectin can take a helical structure, a partially broken helical structure, and an irregular curled structure. The relative molecular mass of straight-chain amylopectin has been measured by different methods to be 32,000-160,000 or even greater.

The proportion of straight-chain starch in starch indicates the distribution of molecular size, and the average degree of polymerization varies with the starch obtained. The molecular weights of the straight-chain starches of potato starch and tapioca starch are higher than those of the straight-chain starches of corn.

1.2 Branched-chain starch
Branched starch is a complex macromolecule with branches of D-glucopyranose linked by α-1,4 glycosidic bond and α-1,6 glycosidic bond.

The overall structure of branched starch is also different from straight-chain starch, it is dendritic, the branched chain is not long, and it contains 20-30 glucose groups on average.

Branched starch has a highly branched structure, consisting of short chains of linear straight-chain starch. The molecule of branched starch is larger than that of straight-chain starch, with a relative molecular mass of 100000-1000000, equivalent to a polymerization degree of 600-6000 glucose residues.

Branched chain starch molecule shape such as sorghum spike, small molecules are very much, estimated at least in more than 50, each branch contains an average of about 20-30 glucose residues, each branch is also D-glucose to α-1,4 glycosidic bonding into the chain, curled into a helix, but the molecular junction is α-1,6 glycosidic bonding, α-1,6 glycosidic bonding, but the molecular junction is α -1,6 glycosidic bonds at the molecular junction, and the spacing between branches is 11-12 glucose residues.

Properties of starch

2.1 Physical properties of starch
Starch is white powder, generally insoluble in organic solvents, soluble in dimethyl sulfoxide and N,N’-dimethyl formamide. Starch is highly hygroscopic and its granules are permeable.
Pure branched-chain starch is soluble in cold water, while straight-chain starch is insoluble in cold water, and natural starch is also completely insoluble in cold water.

2.1.1 Pasteurization of starch
Starch is insoluble in water at room temperature, but when the temperature of water reaches above 53℃, its physical properties change obviously. The property of dissolving and splitting like this starch to form a uniform paste solution at high temperature is called the dextrinization of starch.

The pasting of starch can be divided into three stages: reversible water absorption stage; irreversible water absorption stage; starch granule disintegration stage.

Different starch pasting temperature is different, even if the same starch due to the different size of the granules, the paste temperature is not consistent, the general use of the temperature of the beginning of the paste and the temperature of the completion of the paste to indicate the temperature of starch pasting.

In many cases, starch coexists with sugar, protein, fatty acids, and water. Starch pasting, the viscosity of the starch solution, and the nature of the starch and gel depend not only on the temperature, but also on the type and amount of other combinations that coexist.

2.1.2 Aging of starch
When pasted α-starch is left at or below room temperature, it becomes opaque or even coagulates into a precipitate, a phenomenon known as aging.

The essence of starch aging is that the pasteurized starch molecules are automatically arranged into sequences again, forming compact, highly crystallized micelles of insoluble starch molecules.

After aging, starch loses its affinity for water and is difficult to be hydrolyzed by amylase, and thus is not easily digested and absorbed by the human body.

The control of starch aging is of great importance in the food industry. The aging process can be regarded as the inverse process of pasting, but aging cannot make starch completely recover to the structural state of raw starch, and the aging starch has a lower degree of crystallization than raw starch.

Different sources of starch, aging is not the same degree of difficulty, this is due to the aging of starch and the proportion of straight-chain starch and branched-chain starch, in general, straight-chain starch is easier to aging than branched-chain starch, the more straight-chain starch, the faster the aging of branched-chain starch is almost unchanged.

Aging and starch water content is closely related, water content of 30% -60% of the starch is easy to aging, less than 10% or in a large number of water starch is not easy to aging; aging effect of the optimal temperature between 2-4 ° C, greater than 60 ° C or less than -20 ° C does not occur; starch in the acidic or alkaline conditions are not easy to Starch is not easy to be aged under acidic or alkaline conditions.

2.2 Chemical properties of starch
Starch decomposes under the action of heat, oxidizing agent, acid, alkali, enzyme and so on, and many kinds of decomposition products are obtained, such as amylose dextrin, oxidized starch, acid-treated starch, maltose, glucose and so on.

Starch molecules have a large number of hydroxyl groups, and the hydroxyl groups in starch molecules can undergo the following reactions:

2.2.1 Esterification reaction
The esterification reaction of starch can be used to produce sulfate ester, phosphate ester, acetate ester, starch xanthate ester and so on.

2.2.2 Etherification reaction
The etherification reaction of starch can be used to produce carboxymethyl starch, hydroxyethyl starch, hydroxypropyl starch, starch acryl ether and so on.

2.2.3 Other reactions
Starch can have cross-linking reaction with multifunctional compounds, and can be grafted and copolymerized with many monomers to produce grafted compounds; starch can also be hydrolyzed; in addition, starch hydroxyl can be used to generate a variety of starch derivatives.

2.3 Other properties of starch

2.3.1 Granular properties

The granular properties of starch include adsorption in the cohesive state, cohesion, hygroscopicity, rewettability and so on.

2.3.2 Paste or slurry properties
The viscosity change of starch when it is heated or cooled, including the stability of paste viscosity, water retention, coagulation, and the performance of protecting colloid or emulsification during low-temperature storage and freezing and thawing.

2.3.3 Film properties
The properties of starch film mainly include solubility in cold or hot water, hygroscopicity, permeability, plasticity, elasticity and toughness. Generally speaking, straight-chain starch has excellent film-forming property and film strength, and branched-chain starch has better adhesion.