阅读说明：本技术 一种食品中蛋白质含量的检测方法 (Method for detecting protein content in food ) 是由 刘芳 石林 赵玉民 靳会娟 张静茹 于 2020-05-28 设计创作，主要内容包括：本发明公开了一种食品中蛋白质含量的检测方法,属于食品检测技术领域,包含以下步骤：步骤S1：取样品与催化剂、消解液混合加热沸腾后,置于超声波震荡器中超声震荡；步骤S2：将步骤S1得到的样品溶液进行凯氏定氮法蒸馏；步骤S3：馏出液用0.1mol/L的盐酸溶液滴定,记录颜色变化时,盐酸溶液的用量。本发明采用超声波与凯氏定氮法相结合,能够快速准确测得食品中蛋白质的含量,与凯氏定氮法测得结果几乎一样,准确性高,速度快,具有积极的推广意义。(The invention discloses a method for detecting protein content in food, which belongs to the technical field of food detection and comprises the following steps of S1, mixing a sample with a catalyst and digestion liquid, heating, boiling, placing in an ultrasonic oscillator, ultrasonically oscillating, S2, distilling the sample solution obtained in the step S1 by a Kjeldahl method, and S3, titrating the distillate by 0.1 mol/L of hydrochloric acid solution, and recording the dosage of the hydrochloric acid solution when the color changes.)

1. A method for detecting the protein content in food is characterized in that: comprises the following steps:

step S1: mixing the sample with a catalyst and a digestion solution, heating to boil, and placing in an ultrasonic oscillator for ultrasonic oscillation;

step S2: distilling the sample solution obtained in the step S1 by a Kjeldahl method;

and step S3, titrating the distillate with 0.1 mol/L hydrochloric acid solution, and recording the dosage of the hydrochloric acid solution when the color changes.

2. The method for detecting the protein content in food according to claim 1, wherein the method comprises the following steps: in the step S1, the catalyst is formed by mixing copper sulfate, potassium sulfate, 98% sulfuric acid solution and mercury oxide, wherein the copper sulfate: potassium sulfate: sulfuric acid solution with a concentration of 98%: the mercury oxide is prepared from the following components in parts by weight: 7.5-8:35-40:0.07-0.1.

3. The method for detecting the protein content in food according to claim 1, wherein the method comprises the following steps: in step S1, the digestion solution is a 35% hydrogen peroxide solution.

4. The method for detecting the protein content in food according to claim 1, wherein the method comprises the following steps: in step S1, sample: catalyst: the weight portion ratio of the digestion solution is 1: 12-15:20-25.

5. The method for detecting the protein content in food according to claim 1, wherein the method comprises the following steps: in the step S1, the time of the ultrasonic oscillation is 15-30 min.

6. The method for detecting the protein content in food according to claim 1, wherein the method comprises the following steps: in the step S1, the boiling time is 8-10 min.

Technical Field

The invention relates to the technical field of food detection, in particular to a method for detecting protein in food.

Background

With the continuous improvement of the life quality of people and the increasing emphasis on body health, the requirements of consumers on the quality aspects of food such as palatability, nutrition and the like are also improved. Proteins are the material basis of life, are important components constituting cell tissues of living bodies, and are also important nutritional indexes in food. The method for determining the content of the protein in the food has great significance for evaluating the nutritional value of the food, reasonably developing and utilizing food resources and the like, can provide data for reasonable diet, master the variation of the nutritional value and the quality of the food, and ensure the requirements of different people on the protein. The method for measuring protein in food comprises the following steps: kjeldahl method, salicylic acid colorimetric method, biuret colorimetric method, Folin-phenol reagent method, ultraviolet absorption method, etc. The most common method at present is the Kjeldahl method, which comprises the steps of digestion, distillation and titration, has the advantages of wide application range, higher sensitivity, better recovery rate, no need of expensive instruments and the like, but is time-consuming to operate, for example, the digestion time of a high-fat-assistant high-protein sample needs more than 5 hours. Therefore, in terms of detecting aging, the kjeldahl method is increasingly difficult to meet the market demand.

Patent document No. CN108205045A discloses a method for detecting the protein content in regenerated protein fibers. Weighing a certain amount of regenerated protein fibers which are dried to constant weight, putting the regenerated protein fibers into a digestion tube, adding a certain amount of copper sulfate, potassium sulfate and concentrated sulfuric acid, digesting for a period of time in a digestion furnace with constant temperature, cooling, testing on an automatic Kjeldahl azotometer, and converting by a set protein conversion coefficient to obtain the content of the protein.

The patent document with publication number CN104122367A discloses a rapid detection technology for protein content in liquid starch, which comprises (1) digesting by sucking 10 ml of liquid corn starch uniformly stirred by a pipette, transferring into a 500 ml Kjeldahl flask or a 300 ml nitrogen-fixing test tube, adding 10 ml of copper sulfate, potassium sulfate mixed catalyst and 25 ml of concentrated sulfuric acid and 3 glass beads in turn, (2) distilling, after cooling the decomposed liquid, washing a glass funnel and a bottle neck with distilled water, diluting to 200 ml, transferring the Kjeldahl flask onto a distilling rack, connecting a 500 ml conical flask as a receiver at the lower end of a condensing tube, (3) titrating the liquid in the conical flask with 0.1 mol/L of sulfuric acid standard solution, so that the solution is changed from blue-green to grey-purple, namely, to obtain an end point, (4) simultaneously performing a blank test, and (5) calculating that X = (V1-V0) × C × 0.014.014 0.014 × 6.25.25/[ × (1-X1) ] × 100.

Although the two methods improve the defects of complex operation, long test period and the like of the conventional Kjeldahl method to a certain extent, the digestion heating time is still long, and needs to be further improved.

Disclosure of Invention

In view of this, the present invention provides a method for detecting protein content in food.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows:

a method for detecting the protein content in food comprises the following steps:

step S1: mixing the sample with a catalyst and a digestion solution, heating to boil, and placing in an ultrasonic oscillator for ultrasonic oscillation;

step S2: distilling the sample solution obtained in the step S1 by a Kjeldahl method;

and step S3, titrating the distillate with 0.1 mol/L hydrochloric acid solution, and recording the dosage of the hydrochloric acid solution when the color changes.

Further, in step S1, the catalyst is a mixture of copper sulfate, potassium sulfate, a 98% sulfuric acid solution, and mercury oxide, where the ratio of copper sulfate: potassium sulfate: sulfuric acid solution with a concentration of 98%: the mercury oxide is prepared from the following components in parts by weight: 7.5-8:35-40:0.07-0.1.

Further, in step S1, the digestion solution is a hydrogen peroxide solution with a mass fraction of 35%.

Further, in step S1, the sample: catalyst: the weight portion ratio of the digestion solution is 1: 12-15:20-25.

Further, in the step S1, the time of the ultrasonic oscillation is 15-30 min.

Further, in the step S1, the boiling time is 8-10 min.

The protein is used as an important physicochemical index in food, the content of the protein in the food is accurately measured, the understanding of the quality of the food has very important significance, data can be provided for reasonable diet, the change of the nutritional value and the quality of the food can be mastered, and the requirements of different people on the protein can be ensured. The protein determination method is a plurality of methods, and the protein detection method adopted by the inventor is a classical constant Kjeldahl method which is one of the most accurate methods for determining total organic nitrogen and simpler and more convenient to operate. The method has the advantages of wide application range, high sensitivity, high recovery rate, no need of expensive instruments and the like, but the operation is time-consuming, and if high lipid is helpful and the digestion time of a high protein sample is more than 5 hours. To speed up the detection efficiency, the digestion process is modified by those skilled in the art. For example, patent publication No. CN107356702A discloses a method for detecting crude protein in castor seeds or castor cake, which comprises adding copper sulfate, potassium sulfate, and concentrated sulfuric acid into a sample to be detected, and mixing them uniformly to make the sample completely soaked by the concentrated sulfuric acid; and adding hydrogen peroxide, heating, fully digesting, and detecting the content of crude protein in the obtained digestion solution. Zhanghua, rapid protein detection and method improvement discussion [ J ], Chinese science and technology journal database science, 2018,1:129, aiming at long digestion time of the Kjeldahl method, a protein mixed digestion solution and a mixed catalyst are designed to improve the digestion speed. The sample is heated and digested together with concentrated sulfuric acid and a catalyst, so that protein is decomposed, carbon and hydrogen are oxidized into carbon dioxide and water to escape, and organic nitrogen in the sample is converted into ammonia and sulfuric acid to be combined into ammonium sulfate. Then, alkali is added for distillation, ammonia is distilled out, and after absorption with boric acid, titration is carried out with a standard hydrochloric acid or sulfuric acid solution. The protein content can be calculated according to the standard acid consumption. However, these studies are improved, complicated in operation and long in digestion time by heating. To overcome the above disadvantages, there are many methods for rapidly determining proteins, such as: biuret method, ultraviolet spectrophotometry, dye binding method, salicylic acid colorimetry, refraction method, optical rotation method, near infrared spectroscopy and the like. However, these methods have different limitations in terms of flexibility, application range, etc., and are susceptible to interference from environmental factors and their own factors. Therefore, a new digestion method is needed to be researched, so that the accuracy of the kjeldahl method detection is ensured, the digestion time is shortened, and the detection efficiency is improved.

The invention has the beneficial effects that: the sample, the digestion solution and the catalyst are heated and digested together to decompose organic matters in the food, wherein carbon and hydrogen are oxidized into carbon dioxide and water to escape, nitrogen in the protein is converted into ammonia and combined with sulfuric acid to generate ammonium sulfate to be left in the sulfuric acid, then alkali is added according to a Kjeldahl method for distillation to evaporate the ammonia, the ammonia is absorbed by boric acid and titrated by sulfuric acid or hydrochloric acid standard titration solution, and the consumption of the standard titration solution is multiplied by a coefficient, namely the protein content.

In the digestion process, the sample, the catalyst and the digestion solution are heated and boiled to decompose protein, and carbon and hydrogen are oxidized into carbon dioxide and water to escape. Then the conversion of organic nitrogen into ammonia and the combination of sulfuric acid into ammonium sulfate is accelerated by ultrasonic oscillation. The digestion method provided by the invention reduces the heating time in the digestion process in the Kjeldahl method, reduces the energy consumption and improves the digestion efficiency. The ultrasonic wave has strong capability of digesting the sample, utilizes ultrasonic energy to improve the reaction rate, and the frequency is 15-20 KHz.

The hydrogen peroxide solution is decomposed to accelerate the oxidation of organic matters. The catalyst is formed by mixing copper sulfate, potassium sulfate, 98% sulfuric acid solution and mercury oxide, wherein the copper sulfate and the potassium sulfate increase the digestion temperature and accelerate the decomposition of organic matters, and the mercury oxide and hydrogen peroxide have synergistic effect to accelerate the oxidation of the organic matters and shorten the digestion time.

The method disclosed by the invention combines ultrasonic waves and a Kjeldahl method, can quickly and accurately measure the content of protein in food, is almost the same as the result measured by the Kjeldahl method, is high in accuracy and speed, and has positive popularization significance.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention are clearly and completely described below. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the described embodiments of the invention, are within the scope of the invention.