[In-depth analysis] Research on the reasons for unqualified sulfur dioxide in food sampling (with common detection methods for SO 2O in food)
Food additives have promoted the development of the food industry and are the soul of the modern food industry. However, the abuse of food additives is not uncommon. Sulfur dioxide residues often appear in the sampling unqualified items announced on the official websites of local governments. In particular, "sulfur dioxide" has been added to the sampling items of many food sub-categories in the detailed rules of the 2023 national drawing. In the sampling unqualified items announced by the government in 2023, we can see that the unqualified categories of sulfur dioxide have also changed significantly. How much do you know about sulfur dioxide? Today we come to a scientific interpretation of sulfur dioxide in food
01 The detection of sulfur dioxide in food
Category of unqualified products
According to the sampling data released by the scientific grain officer on the government's official website, the results announced by the 19 provincial/municipal market supervision and administration bureaus in 2023 showed that:
From January to September 2023, a total of 281 batches of sulfur dioxide unqualified products were detected in food, and the unqualified product categories included 10 food categories. Compared with the same period in 2022 (212 batches unqualified from January to September), the total number of unqualified batches increased significantly by 69 batches, an increase of 32.5% year-on-year.
The categories of unqualified products increased from 3 to 10, an increase of 7 categories. Among them, condiments, fried food and nut products, biscuits, tea and related products, grain processed products (wet flour products), candy products, and sugar (brown sugar and borneol sugar) are all new product categories for the detection of sulfur dioxide in the 23-year national pumping rules.
It can be seen that the changes in the national pumping rules in 23 years have helped the regulatory sampling identify the risk of sulfur dioxide exceeding standards in many products.
Substandard product category
According to sub-categories, the unqualified sub-categories of sulfur dioxide in 2023 contain 25 sub-categories, an increase of 19 sub-categories compared with the 6 sub-categories in 2022. Among them, pickles, other spices and seasonings, peanuts, peppers, dried fruit products, substitute tea and other sub-categories are the product categories of the newly added sulfur dioxide detection project in the 2023 national pumping rules. It is recommended that enterprises increase the quality control of sulfur dioxide for these products. At the same time, the change in the number of unqualified batches of dried vegetable products decreased from 196 batches in 22 years to 74 batches in 23 years (122 batches lower), a year-on-year decrease of 62.2%. It can be seen that the 22-year regulatory sampling has played a significant role in controlling the abuse of sulfur dioxide in dried vegetable products.
From the high-frequency vocabulary of sulfur dioxide substandard product varieties in 2023 and 2022, it can be seen that high-risk varieties have also changed. In 2022, lily (dried), yam (dried), and daylily (dried) accounted for 84.9% of the total failure rate. In 2023, yam (dried), bamboo shoots (pickled vegetables), lily (dried), and dried chili (powder) accounted for 40.6% of the total failure rate.
02
Where does the sulfur dioxide detected come from? So where does the sulfur dioxide in these products come from? The scientific grain officer conducted an analysis and found that sulfur dioxide generally comes from three aspects: the presence of natural background in food, the addition of sulfur dioxide (type) food additives to food, and the use of sulfur (food additives) to fumigate food.
natural background
Relevant research data show that some foods will naturally produce sulfur dioxide natural background or similar sulfides during growth and processing, resulting in the detection or detection of sulfur dioxide in foods exceeding the actively added value. For example, the products in the following table:
Food added sulfur dioxide, etc
"National Food Safety Standard, Food Additive Use Standard" (GB2760-2014) allows some foods to use sulfur dioxide, potassium metabisulfite, sodium metabisulfite, sodium sulfite, sodium bisulfite, and low sodium sulfite, which may produce sulfur dioxide residues after use.
sulfur fumigation
Sulfur is also a food additive allowed by the state, and fumigation with sulfur can also lead to sulfur dioxide residues.
03
International management and control of sulfur dioxide use
The Codex Alimentarius Commission (CAC), the European Union, the United States, Australia, New Zealand, Canada and other international organizations, countries and regions have regulations and standards that allow sulfur dioxide to be used in the corresponding food categories. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) conducted a safety assessment of sulfur dioxide and established an allowable daily intake (ADI) of 0-0.7 mg/kg bw. The Codex Alimentarius (CODEX STAN 212-1999) also sets a limit for sulfur dioxide in sugar, and the sulfur dioxide residue in white sugar should be less than 15 mg/kg.
04
Common detection methods for sulfur dioxide
oxidation method
Principle:
Under low temperature conditions, the free sulfur dioxide in the sample reacts excessively with hydrogen peroxide to form sulfuric acid, which is titrated with alkali standard solution. From this, the content of free sulfur dioxide in the sample can be obtained.
Reagents and materials:
1. Hydrogen peroxide solution (0.3%): absorb 1ml of 30% hydrogen peroxide (store in the refrigerator after opening), dilute to 100ml with water. Prepare on the day of use.
2. Phosphoric acid solution (25%): Take 295ml of 85% phosphoric acid and dilute it to 1000ml with water.
3. Sodium hydroxide standard titration solution [c (NaOH) = 0.01mol/L]: Accurately absorb 100ml of sodium hydroxide standard titration solution, and use carbon dioxide-free water bandwidth evaluation to 500ml. Store in a bottle with a sodium lime tube on a rubber bag and redistribute it every week.
4. Methyl red-methylene blue mixed indicator solution: prepared according to GB/T603-2002.
Instrument:
A -- short-necked pins.
B - tee connecting pipe
C -- snorkel.
D - straight pipe condenser
E -- elbow.
F - Vacuum Distillation Receiver
G -- pear bottle.
H -- gas scrubbers.
I - Right angle elbow (connected to vacuum pump or exhaust pipe)
2 - Vacuum pump or suction pipe (glass jetting pump)
Analysis steps:
1. Connect the sulfur dioxide measuring device properly, the I tube is connected to the vacuum pump (or exhaust pipe), and the D tube is filled with cooling water. Remove the pear-shaped bottle (G) and the gas scrubber (H), add 20ml of hydrogen peroxide solution to the G bottle and 5ml of hydrogen peroxide solution to the H tube. After adding 3 drops of each mixed indicator solution, the solution immediately turns purple, drop into the sodium hydroxide standard solution, so that its color just turns olive green, and then reinstall it properly, immerse the A bottle in a water bath.
2. Absorb 20.00ml of sample (liquid temperature 20 ° C), add it to the A bottle from the top of the C tube, then absorb 10ml of phosphoric acid solution, and also add it to the A bottle from the top of the C tube.
3. Turn on the vacuum pump (or exhaust pipe), so that the pumping air flow rate is 1000ml/min-1500ml/min, and the pumping air is 10min. Remove the G bottle and use the milliliters of the sodium hydroxide standard solution. Use water instead of the sample for a blank test, and the operation is the same as above. Under normal circumstances, the solution in the H tube should not change color. If the solution turns purple, it is also necessary to use the sodium hydroxide standard titration solution to titrate to olive green, and add the volume of the sodium hydroxide standard titration solution consumed to the volume of the sodium hydroxide standard titration solution consumed by the G bottle.
Calculation result:
The content of free sulfur dioxide in the sample is calculated as follows
In the formula:
X - the amount of free sulfur dioxide in the sample in milligrams per liter (mg/L);
C - Concentration of sodium hydroxide standard titration solution in moles per liter (mol/L);
V - the volume of standard titration solution of sodium hydroxide consumed in the determination of the sample, in milliliters (ml);
V0 - volume of standard titration solution of sodium hydroxide consumed in blank test, in milliliters (ml);
32 - the value of the molar mass of sulfur dioxide in grams per mole (g/mol);
20 - The volume of the absorbed sample, in milliliters (ml).
The resulting representation is an integer.
Precision: The pair difference between two independent measurements obtained under reproducible conditions shall not exceed 10% of the mean value of the arithmetic.
Direct iodimetry
Principle: Iodine can be used to redox with sulfur dioxide to determine the content of sulfur dioxide in the sample.
Reagents and materials:
1. Sulfuric acid solution (1 + 3): Take 1 volume of concentrated sulfuric acid and slowly inject it into 3 volumes of water.
2. Iodine standard titration solution 0.02mol/L: prepared and calibrated according to GB/T601-2016, accurately diluted 5 times.
3. Starch indicator solution (10g/L): After preparing according to GB/T603-2002, add 40g of sodium chloride.
Analysis steps: Take 50.00ml of sample (liquid temperature 20 ° C) in a 250ml iodine flask, add a small amount of crushed ice, add 1ml of starch indicator solution, 10ml of sulfuric acid solution, and rapidly titrate to light blue with iodine standard titration solution. Keeping 30s unchanged is the end point. Write down the volume (V) of the iodine standard titration solution consumed and replace the sample with water. Do a blank test. The operation is the same as above.
Calculation result:
The content of free sulfur dioxide in the sample is calculated as follows
In the formula:
X - the amount of free sulfur dioxide in the sample in milligrams per liter (mg/L);
C - concentration of iodine standard titration solution in moles per liter (mol/L);
V - Volume of standard titration solution for consumed iodine in milliliters (ml);
V0 - volume of standard titration solution for iodine consumption in blank test, in milliliters (ml);
32 - the value of the molar mass of sulfur dioxide in grams per mole (g/mol);
50 - The volume of the absorbed sample, in milliliters (ml).
The resulting representation is an integer.
Precision: The pair difference between two independent measurements obtained under reproducible conditions shall not exceed 10% of the mean value of the arithmetic.
Sodium mercury tetrachloride absorption-pararosaniline hydrochloride colorimetry (referred to as the first method of national standard)
I. Background technology
At present, the commonly used method for the determination of sulfur dioxide in food in China is the sodium tetrachloromercury absorption-pararosaniline hydrochloride colorimetry (referred to as the first method of the national standard), which contains high mercury content in the absorption liquid, is difficult to handle after use, and is easy to cause new pollution to the environment. However, the distillation method (the second method of the national standard) has problems and defects such as unstable judgment points and good sealing of glass instruments. The above two methods are cumbersome and not conducive to on-site rapid detection.
II. Content-related
This method is a method for rapid detection of sulfur dioxide in food. It uses the principle of sulfite and indicator (bright green) to make the indicator fade, and the amount of sulfur dioxide in food can be determined by the degree of color degeneration of the reagent. Or use a spectrophotometer to compare the color at 630nm, sulfite fades the bright green, and the absorbance and sulfite content satisfy the quadratic equation.
III. Specific implementation methods
1. Solution preparation
1.1 Sulfur dioxide absorption solution
0.05mol/L的EDTA-2Na20.0mL。
1.2 Indicator 1g/L bright green solution: Take 0.1g of bright green (AR) chemical and dissolve it in 100mL of distilled water.
2. Sample processing: (Take sugar as an example)
Take 1g of pulverized sample, add 9mL of sulfur dioxide absorption solution (use solution), ultrasonic extraction for 10min or water bath soaking for 30min (shaking upside down several times in the middle), and then filter through the filter paper. The filtrate is the sample treatment solution.
3. Detection method
Take 2mL of sample solution, add 2 drops of indicator, and determine the content of sulfur dioxide and sulfur according to the color of the solution according to the colorimetric card after 5 minutes.
4. Colorimetric card preparation
4.1 Preparation of sulfur dioxide standard stock solution (concentration 2500mg/L): Weigh 0.5000g of anhydrous sodium sulfite and dissolve in 100mL of distilled water.
4.2 Sulfur dioxide standard intermediate solution preparation (concentration 25mg/L): Pipette 1mL of standard stock solution and use sulfur dioxide absorption solution for bandwidth evaluation to 100mL.
4.3 Use sulfur dioxide intermediate solution to prepare standard solutions with concentrations of 0, 2, 4, 6, 8, and 10 mg/L. Take 2 mL of sulfur dioxide standard solution with different concentrations, add 2 drops of indicator, and after 5 minutes, the color of each solution corresponds to different concentrations of sulfur dioxide. The color of the solution corresponding to different concentrations of sulfur dioxide is made into a colorimetric card. It can be seen from the following figure that the concentration of So2 in the sugar sample is approximately between 0-2 mg/L.
4.4 Use a spectrophotometer to determine sulfur dioxide
Drawing 4.4.1 standard curve
Sulfur dioxide intermediate solution was used to prepare standard solutions with concentrations of 0, 2, 4, 6, 8 and 10 mg/L. Take 2 mL of different concentrations of sulfur dioxide standard solution, add 2 drops of indicator, and measure by colorimetric at 630 nm after 5 minutes.
Determination of sulfur dioxide in 4.4.2 samples
Take 2mL of the sample solution, add 2 drops of indicator, and place it in a colorimetric dish at 630nm after 5 minutes. The absorbance was measured to be 1.350. It can be seen that the concentration of sulfur dioxide in the sample treatment solution is lower than 2mg/L, and between 0 and 2mg/L, which is consistent with the rapid detection qualitative results.
5. Conclusion
This method is a method for the rapid detection of sulfur dioxide in food. It uses the principle of sulfite and indicator (bright green) to make the indicator fade. The general content of sulfur dioxide in the sample can be quickly determined by colorimetry. For samples that require accurate quantification, they can be sent to the testing laboratory for further accurate determination.
