The reaction temperature is higher than the gelatinization temperature of starch and can be easily controlled to produce products with the desired molecular chain with high recovery efficiency. By choosing the type of alcohol, acid concentration, type of starch and starch concentration, dextrins with selective polymerization values over a wide range can be produced [101-103].
The process of hydrolysis of starch by acid in alcohol medium takes place in a two-stage model, the first stage is slow and the second stage is fast. The recovery efficiency is > 0% and the average particle size decreases slightly. The degree of polymerization gradually decreases with increasing number of carbon atoms in alcohol and the product has a solubility 4-9 times higher than that of native starch. Starch after acid treatment in alcohol shows internal cracks or capillaries in some granules, and the number of cracked or capillary granules increases with increasing number of carbon atoms in alcohol. The degree of molecular decomposition after acid treatment in alcohol is directly related to the rotation of internal structure of starch granules [104-107].
The hydrolysis of starch by acid in alcohol medium was proposed by Small to transform natural starches into soluble ones. To compare the effects of different alcohols on the degree of starch hydrolysis, Robyt [98] modified corn and potato starches by HCl acid in different alcohol mediums, namely methanol, ethanol, 2-propanol and 1-butanol. The results showed that both starches were hydrolyzed most strongly in methanol medium, followed by ethanol. This result was explained by the authors as due to
methanol has a smaller molecular size so it diffuses easily and carries H + ions into
deeper into the starch molecular chain than ethanol. As a result, more H + ions attack the starch chain, thus having a greater ability to break down the starch chain.
b. Properties of acid-modified starch
- Acid-modified starch changes many properties compared to unmodified starch and adds new properties, only resembling the original starch in physical form, there is no direct change in granular form but only a small change in birefringence. Acid-modified starch compared to unmodified starch
The original starch has properties such as relative crystallinity, reduced ionic affinity, smaller characteristic viscosity, higher gelatinization temperature, higher osmotic pressure due to smaller molecular weight, when gelatinized in hot water, the granules swell less. In warm water with a temperature lower than the gelatinization temperature, the solubility is higher. Acid-modified starch has many changes in properties compared to unmodified starch and has new properties, only similar to the original starch in physical form, only a small change in birefringence without a direct change in granule shape, only a small change in grain birefringence [140].
- When hydrolyzed by acid, the relative crystallinity of starch granules will change. Normally, the relative crystallinity in starch granules is from 15 to 45% depending on the origin and type of starch [15]. Kawaljit Singh Sandhu [2] studied the change in relative crystallinity of potato starch when hydrolyzed by H 2 SO 4 acid and discovered that the relative crystallinity of modified starch was higher than that of natural potato starch, this increase was proportional to the degree of hydrolysis. Similar results also appeared when hydrolyzing corn, rice, and cassava starch with different acids. Most authors believe that the acid agent preferentially attacks the amorphous region first, then the crystalline region, causing the proportion of amorphous region to gradually decrease and increasing the relative crystallinity. In addition, starch molecules located in the amorphous region after being broken down have the ability to combine with each other by hydrogen bonds and form a spiral shape. These spirals are tightly packed together and form new crystalline regions [122].
- Acid-modified starch has a large change in the structure of the starch chain, causing the gelatinization temperature to change. In general, the starch with a higher degree of cleavage has a higher gelatinization temperature. Jacobs and colleagues [11] studied the thermal transfer process of potato starch and acid-modified wheat starch using a DSC thermal analyzer. The results showed that the initial and peak temperatures of acid-modified starch were higher than those of native starch. Similar results were obtained when studying the thermal properties of barley starch [105], rice [104],
corn [102]. This increase in gelatinization temperature may be due to the increase in the proportion of crystalline regions in the starch granules after hydrolysis and because the crystalline regions have a higher melting temperature than the amorphous regions [122]. It can also be seen that when broken down into small molecules, although the starch chains are shorter, they are not uniform. The modified starch molecules have a shorter size so they are easier to arrange more tightly, hindering the hydration and swelling of the starch. On the other hand, it is also possible that at that time in the granule, the level of order of the micelles has increased, the starch chains in the amorphous regions are hydrolyzed, so the micelles link together, creating quite large chain arrays and therefore the gelatinization temperature increases.
- The basic shape of the starch granules after acid modification can change depending on the hydrolysis conditions, the size can be changed and the surface is rougher. H + ions penetrate the shell by diffusion and proceed to break down the starch molecules in the granules, creating shorter chain molecules, but the number of short chain molecules will increase, increasing the volume of the microgranules, so the size of the microgranules increases at the initial stage in the whole sample. However, if the hydrolysis process continues to be maintained until the entire outer shell of the starch granules
Once the first layer is completely hydrolyzed and the next layers begin to hydrolyze, the size of the starch microgranules also begins to decrease [123].
- The viscosity of acid-modified starch paste is reduced. The reduction in the viscosity of acid-modified starch paste is due to the destruction of the forming zone between the granule micelles and the weakening of the granule structure, leading to the destruction of the granules even when the granules are not swollen significantly. According to Mukerjea [107], the reason for the reduction in the viscosity of starch paste is due to its very high solubility in boiling water, which also means that its intermittent phase is reduced. The intermittent phase is a suspension of swollen granules and their fragments in an aqueous solution of starch substances (continuous liquid phase). This intermittent phase increases the viscosity of starch paste [140].
- The alkalinity index of acid-modified starch increases. The alkalinity index is the amount of 0.1 M alkali consumed to dissolve 10 g of dry starch at boiling temperature in 1 hour. The alkalinity index is related to the aldehyde group. When the molecular size
In small starches, the number of aldehyde groups increases. This reflects an increase in the alkali index during hydrolysis or an increase in fluidity [108- 110].
- Acid modified starch has high film strength. Therefore, it is suitable for applying gel-forming and film-forming properties to products. The gel strength of starch is increased by adjusting the production conditions. Therefore, if under normal conditions the transformation is carried out with 0.1 N H 2 SO 4 during the reaction time, it will form modified starch with high gel strength [111, 112].
c. Application as excipients in pharmaceuticals
Modified starch products in HCl acid solution are used as excipients in pharmaceuticals, and are also widely used in the food industry, textile industry and paper industry [118].
Recently, pharmaceutical companies have paid great attention to the widespread application of various types of modified starches in many stages of tablet and capsule manufacturing technology. Following this trend, the amount of modified starches used in the pharmaceutical industry as adjuvants has also increased. Adjuvants in tablets serve to give the initial mixture the necessary technological properties to provide an accurate dosage, mechanical strength, disintegration properties and stability for tablets during storage. Depending on the different purposes of use, adjuvants are divided into small groups [120, 122] (Table 1.3).
Analysis of the data in Table 1.3 shows that starch is found in almost all groups, thus it plays an extremely important role in the pharmaceutical industry. Modified starches are attracting the attention of researchers to develop pharmaceuticals with new composition and pharmacological properties. The reasons for the interest of these starches are their ability to bind, swell, and lubricate [12]. The increasing trend of manufacturing tablets, the diversity in structure and properties of starches produced from various unrefined raw materials, the ability of natural starches and modified starches
Concentrate and stabilize many drug components, ability to increase gelling and suspension properties of starch by direct modification methods. Ability to increase drug stability under conditions of cooling and thawing cycles, high temperatures and in highly acidic environments.
Table 1.3. Excipients for tablets
Function
Ingredient | |
Adhesion | Starch: tapioca starch, potato starch, corn starch, rice starch; distilled water, ethanol, syrup, carboxymethyl cellulose, gelatin, hydroxymethyl cellulose, PVP, PVA, algic acid |
Bulking agent Air-forming agent Wetting agent | Starch: wheat flour, potato, corn flour, rice flour; pectin, gelatin, methyl cellulose, agar- agar. Sodium bicarbonate mixed with citric acid or tartaric acid. Starch: wheat flour, potato, corn flour, rice flour; glucose. |
Lubricants Anti-stick agents Fillers | Steric acid, calcium sterate, magnesium sterate. Starch, soluble ice, steric acid. Starch, glucose, lactose, gelatin, cellulose and derivatives its extract, dextrin, amylopectin, sorbitol, mannitol, pectin. |
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Tablets are the most widely used form of medicine due to their many advantages. However, in terms of biopharmaceuticals, tablets are a form of medicine with many factors affecting the ability to release drugs during the preparation process from the formulation stage to the manufacturing process. Common tablets can be divided into two types: those that require disintegration and those that do not require disintegration. To release drugs, most common tablets require a process that reflects the physical structure of the tablet, thereby creating particles or particles called the disintegration process. For tablets that do not require disintegration, they often contain drugs and soluble excipients that dissolve quickly in the mouth or digestive juices.
The process of drug release in tablets includes: disintegration and partial release of drug, dissolution of drug into digestive fluid environment, and drug absorption.
The substance passes through the membrane into the circulatory system. In the process of disintegration, dissolution and absorption, whichever process is the slowest will be the rate-limiting step in the absorption of the drug into the circulatory system.
Usually, tablet disintegration occurs first and is a prerequisite for the dissolution and absorption of the drug. This is especially true for poorly soluble drugs. The dissolution rate of the drug from intact tablets is usually very low due to the very small contact area of the drug with the digestive fluid. The process of disintegration of the tablet into granules will increase the contact surface area of the drug with the dissolution medium and thus increase the dissolution rate of the drug [130,147].
1.4.3.6. Starch oxidation
Oxidized starch is synthesized by reacting starch with a certain amount of oxidizing agent under controlled temperature and pH conditions. The oxidation method and product recovery are also different when using different oxidants. Many oxidizing agents have been used to oxidize starch such as periodate, chromic acid, potassium permanganate, nitrogen dioxide, hydrogen peroxide, sodium hypochlorite, hydrogen peroxide... each has its own advantages and disadvantages.
a. Oxidation with periodate
The periodate ion reacts with the 2,-glycol group of starch to break the carbon-carbon bond with the formation of two carbonyl groups. The carbonyl groups are capable of forming hemiacetal structures and various hydrated aldehyde structures both intra- and inter-molecularly.
As a result, dianaldehyde starch does not give a carbonyl absorption band in the infrared region. The D-glucose end groups of starch contain more than one α -glycol group and oxidation of periodate yields 2 moles of formic acid and 1 mole of formaldehyde from the reducing end group and an additional mole of formic acid from the non-reducing end group.
Determination of formic acid production is a method for estimating the average degree of polymerization of starch.
The oxidation of α -glycol by periodate proceeds by a cyclic ester mechanism. Recently a method has been developed to classify the different types of carbonyl groups in oxidized cellulose according to their oxidation rates. Periodate oxycellulose contains six different types of carbonyl groups, application of this method to starch dialdehyde elucidated its fine structure [15, 15].
b. Oxidation with chromic acid
Oxidation of starch with chromic anhydride in a non-solvent system or in 0.2 M sulfuric acid gives rise to carboxyl, ketone, and aldehyde groups. However, potassium dichromate at pH = 0.7 gives a product containing 60% 2,-dialdehyde groups and only 4-5% carboxyl groups. These results are consistent with the recently accepted mechanism of oxidation with chromic acid, one of the first mechanisms to be explained.
In dilute solution, dichromate ion is hydrolyzed into acidic chromate ion. Cr 2 O 7 2- + H 2 O 2HCrO 4 2- (29)
In the presence of acid, the glycol groups react with chromate to form an ester.
cyclic, similar to that for periodate. Like periodate esters, chromate esters decompose to give two carbonyl – aldehyde groups on starch. The carboxyl groups are generated by oxidation of the aldehyde groups.
However, chromic acid can form acyclic esters with alcohols and then decompose to give carbonyl groups. Periodate, on the other hand, although it can form acyclic esters, cannot accept ions in the ester decomposition step required for conventional alcohol oxidation [15].
c. Oxidation with permanganate
The effect of permanganate on starch is known. Carbonyl and carboxyl groups are generated at the greatest rate at pH 10 and possibly at pH = 1.
The most likely mechanism for the oxidation of alcohol groups by permanganate in alkaline media involves the removal of an atom
hydrogen from the alcohol anion, although transfer of the hydride ion from the anion is also a compelling possibility [15].
d. Oxidation with nitrogen dioxide
Nitrogen dioxide (N 2 O 4 ) oxidizes the C-6 primary alcohol group of starch to a carboxyl group with specific characteristics, so that after hydrolysis, 26% of the product is crystalline D-glucuronic acid lactone. The oxidation process can be affected by N 2 O 4 vapor, N 2 O 4 solution in carbon tetrachloride, or nitric acid containing added nitrite. Although nitrogen dioxide is introduced into olefins by a free radical mechanism, the presence of bound nitrogen in the early stages of cellulose oxidation by N 2 O 4 vapor indicates the formation of nitrite esters via nitroso ion (NO + ).
N 2 O 4 NO + + NO 3 -
or HNO 3 + HONO NO + + H 2 O + NO 3 -
TB -CH 2 OH + NO + TB -CH 2 ONO + H + (30)
The alkyl nitrite undergoes thermal decomposition to give the aldehyde and is subsequently oxidized to give the carboxylic acid.
If oxidation proceeds via an intermediate ester both nitrite or nitrate at secondary hydroxyl groups may also occur, the effect of sodium borohydride on starch nitrogen dioxide oxygenation indicates the presence of 2- or 3-ketone groups [15].
e. Oxidation with hypochlorite
Compared with the above starch decomposition methods, the starch oxidation method with sodium hypochlorite is the most common process, giving products with smaller mass, shorter chain size, uniform particles, creating good conditions for the modification process with chemical agents. The reaction is usually carried out by the suspension method, the medium is sodium hypochlorite solution and the dispersed particles are starch. After the reaction, the starch is washed and dried. The properties of the product are determined by the reaction conditions [132, 133].