First, preservatives
A preservative is a chemical that inhibits the growth and reproduction of microorganisms in food. If used according to the quantity stipulated by the state, it can not only prevent the mildew of the food, but also prevent the food from spoiling or spoiling, and can prolong the storage time without causing any harm to the consumer.
The use of preservatives must control a certain amount of use, and should have the following characteristics:
(1) Preservatives added to foods are first of all non-toxic, harmless and have no side effects to the human body;
(2) Long-term use of foods with preservatives should not cause any pathological changes in the body tissues, nor can it affect the development and growth of the second generation;
(3) After adding a preservative, it must not have any influence or decomposition on the quality of the food;
(4) After the food has been added with preservatives, it is not possible to mask the quality of the inferior food or change any sensory traits.
Second, commonly used preservatives
Preservatives permitted in China include benzoic acid and its sodium salt, sorbic acid and its potassium salt, ethyl p-hydroxybenzoate and propyl ester. The first two applications are widely used.
Benzoic acid and its sodium salt
Benzoic acid, also known as benzoic acid, is a white, mercerized scale or needle crystal with a melting point of 122 ° C and a boiling point of 249.2 ° C. Sublimation begins at 100 ° C. Under acidic conditions, it can be distilled with steam, slightly soluble in water, easily soluble in organic solvents such as chloroform, acetone, ethanol, ether, etc., and its chemical properties are relatively stable.
Sodium benzoate is easily soluble in water and ethanol, hardly soluble in organic solvents, and reacts with acid to form benzoic acid.
Benzoic acid and its salts use range: soy sauce, vinegar, juice, jam, glucose, canned food, maximum dosage 1g / kg; sparkling wine, soft drinks, low-salt pickles, noodles, candied fruit, hawthorn cake, fruit flavor Dew, use up to 0.5g per kilogram.
Benzoic acid combines with glycine to form hippuric acid when it enters the body, and is excreted from the urine, no longer stimulating the kidney; sorbic acid enters the body and participates in metabolism, and finally produces CO2 and H2O, which are excreted, due to sorbic acid and its salts. The price of the class is generally not commonly used, and most of it is used in export foods.
2. Sorbic acid and its potassium salt
Sorbic acid is a colorless, odorless needle crystal having a melting point of 134 ° C and a boiling point of 228 ° C. Sorbic acid is hardly soluble in water, and is easily soluble in organic solvents such as ethanol, ether and chloroform. It can be distilled with steam under acidic conditions and is chemically stable.
Potassium sorbate is easily soluble in water, is insoluble in organic solvents, and forms sorbic acid by acid. The two preservatives of benzoic acid and sorbic acid are mainly used for the preservation of acidic foods.
Sorbic acid and its salts are allowed to use up to 1 g per kg in soy sauce, vinegar and jam, and 0.5 g/kg for low-salt pickles, noodles and candied fruit.
3. Determination of benzoic acid and its sodium salt
Determination method: (1) Neutralization method (alkali titration) - wide application (2) ultraviolet spectrophotometry (3) thin layer chromatography (4) gas chromatography (5) high pressure liquid chromatography
UV spectrophotometry:
The benzoic acid in the sample can be distilled by steam distillation in an acidic solution, separated from the non-volatile components in the sample, and then oxidized with a strong acid to oxidize and decompose other organic substances other than benzoic acid, and the oxidized solution is distilled again. Other impurities except benzoic acid were substantially decomposed, and the extinction value was measured according to the absorption wavelength of benzoic acid at 225 nm.
The sample is added with 1 ml of phosphoric acid by steam distillation. The obtained distillate is mainly benzoic acid, and other acids, and then 0.2N of K2Cr2O7 and 4N of H2SO4 are added to oxidize other acidic substances, and then distilled to obtain no impurities. Benzoic acid, measured at 225 nm.
The reason for the use of ultraviolet light in benzoic acid is that the formic acid forms a p-Ï€ conjugated system with benzene, which is suitable for measurement by ultraviolet spectrophotometer.
This experiment requires blanking, no phosphoric acid in the blank, because benzoic acid can not be distilled out in alkaline solution (sample +1N NaOH 5ml) → distilled benzoic acid (g / kg) = C / W * 1000
Gas chromatography:
After extraction with diethyl ether, the separation was measured using a hydrogen flame ionization detector and then quantified in comparison with the standard series.
High pressure liquid chromatography:
After the sample is processed, it is injected into the high-performance liquid chromatograph, and the measured components are separated by the difference in the partition coefficient between the stationary phase and the mobile phase, and the measured component is determined by the ultraviolet detector at a specific wavelength. Absorbance, qualitative and quantitative compared to standards.
4. Determination of sorbic acid and its potassium salt
Determination method: (1) colorimetric method (thiobarbituric acid colorimetric method) (2) ultraviolet spectrophotometry (3) thin layer chromatography (4) gas chromatography (5) high pressure liquid chromatography.
The thiobarbituric acid colorimetric method will be described below.
1, the principle
The sorbic acid in the sample is distilled in an acidic solution by steam, and then oxidized to malondialdehyde and other products by K2Cr2O7. The malondialdehyde reacts with thiobarbituric acid to form a red substance, the color depth and the sorbic acid content. In direct proportion. (measured at 530 nm)
2, the operation method
(1) Sample preparation
Weigh about 100g of sample → add 250ml of distilled water → beat on high-speed masher → constant volume 500ml → filter → collect filtrate
(2) Extraction of sorbic acid
Accurately take 20ml of each filtrate separately into two 250ml distillation bottles → one bottle plus 1ml phosphoric acid, anhydrous sodium sulfate 20g, water 70ml, glass beads 3 → another bottle plus 1N NaOH 5ml, anhydrous sodium sulfate 20g 70ml of water, 3 pieces of glass beads→distillation→receive the distillate with a 100ml volumetric flask containing 10ml of 0.1N NaOH→when the distillate is collected to 80ml, stop the distillation, wash the condensation tube with a small amount of water→constant volume→ respectively absorb 10ml solution → separately placed in two 100ml volumetric flasks → constant volume with 0.01N NaOH → sample solution, blank measurement
(3) Determination
Accurate aspirate liquid, blank solution 2ml → in 25ml colorimetric tube → add water 3ml → add 2ml (K2Cr2O7 + sulfuric acid) mixture → water bath at 100 ° C for 5 minutes → add 0.5% thiobarbituric acid 2ml → boiling water bath heating 7 Minutes → Cooling → Constant volume → Colorimetric at 530nm in a 1cm cuvette
(4) Standard curve drawing
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