echemi logo
Product
  • Product
  • Supplier
  • Inquiry
    Home > Chemicals Industry > Chemical Technology > Cognition of the basic procedures of food physical and chemical testing (2)

    Cognition of the basic procedures of food physical and chemical testing (2)

    • Last Update: 2021-09-16
    • Source: Internet
    • Author: User
    Search more information of high quality chemicals, good prices and reliable suppliers, visit www.echemi.com

    3.


    Sample pretreatment is an important task in food physical and chemical testing


    (1) Organic matter destruction method

    The organic destruction method is mainly used for the determination of inorganic elements in food


    1.


    The organic matter is destroyed by high-temperature burning


    The advantage of dry ashing is that the blank value is low (therefore, the method does not add or adds a few reagents), can enrich the tested components, destroys thoroughly, and is easy to operate; the disadvantage is that it takes a long time and high temperature may cause some The loss of volatile elements such as arsenic, lead and brocade, coupled with the absorption of the measured component by the crucible, will lead to low measurement results and recovery rates


    2.


    Destroy the organic matter by strengthening the heating of the oxidant, that is, add concentrated nitric acid, concentrated sulfuric acid, perchloric acid , potassium permanganate , hydrogen peroxide and other strong oxidants to the sample, and heat and digest it in an acidic solution to completely decompose and oxidize the organic matter.


    Compared with dry ashing, wet digestion has the advantages of fast decomposition speed, short time required, less storage in the container, and less volatilization of certain volatile elements (because the heating temperature is relatively low); the disadvantage is Someone is watching at any time (because a large amount of foam is produced in the early stage of digestion, it is easy to overflow, and the digestion temperature needs to be adjusted at any time), and the blank value is too high (a larger amount of reagents need to be added)


    (2) Solvent extraction method

    In the same solvent, different substances have different solubility


    1.


    The method of leaching a certain component to be tested in a solid sample with an appropriate solvent is called leaching method, also known as liquid-solid extraction method


    (1) Selection of extractant Generally speaking, the extraction effect conforms to the principle of similar compatibility, so the extractant should be selected according to the polarity of the extract


    (2) Extraction method

    ①Oscillation dipping method: cut the sample into small pieces, place it in a suitable solvent system, immerse and shake for a certain period of 550℃), until the residual gray is white or off-white and reach a constant weight, the resulting residue is ready for measurement Inorganic ingredients


    The advantage of dry ashing is that the blank value is low (therefore, the method does not add or adds a few reagents), can enrich the tested components, destroys thoroughly, and is easy to operate; the disadvantage is that it takes a long time and high temperature may cause some The loss of volatile elements such as arsenic, lead and brocade, coupled with the absorption of the measured component by the crucible, will lead to low measurement results and recovery rates


    2.


    Destroy the organic matter by strengthening the heating of the oxidant, that is, add concentrated nitric acid, concentrated sulfuric acid, perchloric acid, potassium permanganate , hydrogen peroxide and other strong oxidants to the sample, and heat and digest it in an acidic solution to completely decompose and oxidize the organic matter.
    , It escapes in a gaseous state, and the component to be tested is transformed into an inorganic state and exists in the digestive juice for determination
    .

    Compared with dry ashing, wet digestion has the advantages of fast decomposition speed, short time required, less storage in the container, and less volatilization of certain volatile elements (because the heating temperature is relatively low); the disadvantage is Someone is watching at any time (because a large amount of foam is produced in the early stage of digestion, it is easy to overflow, and the digestion temperature needs to be adjusted at any time), and the blank value is too high (a larger amount of reagents need to be added)
    .
    In the digestion process, a large amount of harmful gases such as sulfur dioxide and carbon dioxide are often produced , so the operation must be carried out in a fume hood
    .

    (2) Solvent extraction method

    In the same solvent, different substances have different solubility
    .
    The solvent extraction method is the method of completely or partially separating each component of the sample by using the difference in solubility in a specific solvent
    .
    Commonly used inorganic solvents include water, dilute acid, and dilute alkali; organic solvents include ethanol, ether, chloroform, acetone, n-hexane, petroleum ether and so on
    .
    This method is often used for the determination of vitamins, pesticides and aflatoxins
    .
    There are many solvent extraction methods, and the commonly used methods are leaching and solvent extraction
    .

    1.
    Extraction

    The method of leaching a certain component to be tested in a solid sample with an appropriate solvent is called leaching method, also known as liquid-solid extraction method
    .
    Such as the extraction of sugar in solid foods with water, and the extraction of fats and oils in meat products with petroleum ether
    .

    (1) Selection of extractant Generally speaking, the extraction effect conforms to the principle of similar compatibility, so the extractant should be selected according to the polarity of the extract
    .
    For components with weak polarity (such as organochlorine pesticides), solvents with low polarity (such as n-hexane , petroleum ether ) can be used for extraction; for components with strong polarity (such as aflatoxin B1), solvents with high polarity (such as aflatoxin B1) can be used.
    Such as a mixed solution of methanol and water) extraction
    .
    The boiling point of the extractant should be 45-80°C.
    If the boiling point is too low, it is easy to volatility, and if the boiling point is too high, it is not easy to concentrate, and it is also disadvantageous to the extracted components with poor thermal stability
    .
    In addition, it is required that the extractant used can dissolve a large amount of the extracted substance without destroying the properties of the extracted substance
    .
    The extractant should be non-toxic or low-toxic
    .

    (2) Extraction method

    ①Oscillation dipping method: Cut the sample into small pieces, place it in a suitable solvent system, immerse and shake for a certain period of time, and then extract the measured components from the sample
    .
    This method is simple and easy to implement, but the recovery rate is low
    .

    ②Mashing method: Put the chopped sample into a masher, add extractant and mash for a certain period of time, so that the tested components are extracted
    .
    This method has a higher recovery rate, but more interference impurities dissolve out
    .

    ③ Soxhlet extraction method: Put a certain amount of sample into the Soxhlet extractor, add a solvent and heat it to reflux for a certain period of time, so that the tested components are extracted by the solvent, such as the extraction of fat in food
    .
    This method requires less solvent, complete extraction, and high recovery rate, but the operation is cumbersome and a special Soxhlet extractor is required
    .
    The Soxhlet extraction device is shown in Figure 1-2
    .


    Figure 1-2 Soxhlet extraction device diagram

    2.
    Solvent extraction method

    This method, also known as solvent stratification, uses the difference in the partition coefficient of a component in two immiscible solvents to transfer it from one solvent to another, thereby separating it from other components.
    Method
    .
    For example, when determining the content of saccharin sodium or benzoic acid in beverages, first extract the saccharin sodium or benzoic acid with ether (under acidic conditions), and then volatilize the solvent ether, and finally use chromatography or colorimetry to determine
    .
    Solvent extraction method is simple, fast, good separation effect, and widely used, but the extraction solvent is usually volatile, flammable and toxic
    .

    ①Selection of extraction agent: First, the extraction solvent should be immiscible with the original solvent, and have the maximum solubility for the tested component, but the minimum solubility for impurities
    .
    After extraction, the tested component enters the extraction solvent and is separated from the impurities remaining in the original solvent
    .
    Secondly, we should consider the difficulty of separating the two solvents and whether it will produce foam, emulsification and other issues
    .

    ②Extraction method: Extraction is usually carried out in a separatory funnel.
    It usually takes 4 to 5 times to achieve the purpose of complete separation.
    The operation is shown in Figure 1-3
    .

    Figure 1-3 Schematic diagram of extraction operation

    When a solvent with a density less than water is used to extract substances with a small partition coefficient or easily emulsified after shaking from an aqueous solution, the continuous liquid extractor is more effective than the separatory funnel
    .

     

     

    This article is an English version of an article which is originally in the Chinese language on echemi.com and is provided for information purposes only. This website makes no representation or warranty of any kind, either expressed or implied, as to the accuracy, completeness ownership or reliability of the article or any translations thereof. If you have any concerns or complaints relating to the article, please send an email, providing a detailed description of the concern or complaint, to service@echemi.com. A staff member will contact you within 5 working days. Once verified, infringing content will be removed immediately.

    Contact Us

    The source of this page with content of products and services is from Internet, which doesn't represent ECHEMI's opinion. If you have any queries, please write to service@echemi.com. It will be replied within 5 days.

    Moreover, if you find any instances of plagiarism from the page, please send email to service@echemi.com with relevant evidence.