Cognition of the basic procedures of food physical and chemical testing (1)

Food physical and chemical testing is a process of testing and measuring the quality indicators of raw materials, semi-finished products and finished products in food production and processing using theories, methods and technologies of physics, chemistry, biochemistry and other disciplines

There are many types of food, complex ingredients, different sources, and different analysis and testing purposes, items and requirements.

1.

The sample is the object and carrier of the testing work, and various quality assurance measures such as human, machine, material, method, and environment are also based on the premise that the sample is well controlled

2.

(1) Collection of samples

Sample collection (abbreviated as sampling) refers to taking a certain amount of representative samples from a large number of samples to be tested for analysis and inspection

1.

To sample correctly, two principles should be followed: one is to collect samples with uniform texture and representative composition, which can reflect the composition, quality and sanitation of the overall sample; the other is to maintain the original physical and chemical indicators of the sample during sampling and avoid measuring the composition.

2.

There are two methods for sample collection: random sampling and representative sampling

Random sampling can avoid artificial tendencies, but for some samples that are difficult to mix uniformly (such as vegetables, viscous liquids, etc.

Regarding the sample collection method and the number of samples, if it is clearly stipulated in the national standard, the sample should be sampled according to the stipulated method; if there is no specific stipulation, the sample can be sampled according to the method described below

(1) Collection of uniform solid samples

①Completely packaged samples (such as bagged flour, grain, etc.

②Unpackaged bulk samples: Take a small amount of samples from different parts of the upper, middle and lower layers of each batch of samples with a double set of rotary samplers and mix them according to the quarter method until an average sample is obtained

Common sampling tools are shown in Figure 1-1

Figure 1-1 Common sampling tools

(2) Collection of uneven solid samples (such as meat, fish, fruits, vegetables)

①Meat: According to the purpose and requirements of the inspection, samples from different parts and mixed to represent the whole animal, or samples from the same part of many animals and mixed to represent the situation of each part

②Aquatic products: multiple samples of small fish and shrimps can be randomly taken, chopped and mixed, and then divided and reduced to the required quantity; for larger individuals, a small amount of edible parts can be cut from several individuals as above
.

③Fruits and vegetables: For small individuals (such as grapes), randomly select a number of individuals and chop and mix them to reduce to the required quantity; for larger ones (such as watermelon and radish), select a few according to the composition of maturity and individual size Individuals, each individual is divided into 4 or 8 parts according to the growth axis, and 2 parts diagonally are chopped and mixed, and reduced to the required quantity; leafy vegetables with bulky volume (such as small rapeseed, spinach), from After taking a certain amount from multiple packages (bundles or baskets), they are mixed, mashed, mixed, and divided to reduce to the required amount
.

(3) The collection of thicker semi-solid materials (such as sauce, animal fat, condensed milk, etc.
) is not easy to mix well.
You can press and determine the number of sampling pieces (barrels, cans) before unpacking and sampling.
The device takes out the test samples from the upper, middle and lower parts of each barrel (tank), then mixes them, and divides and reduces the average samples to the required number
.

(4) Collection of liquid samples For liquid materials stored in large containers (such as barrels, cylinders, tanks), generally, first determine the number of samples, and then use a sampling tube to siphon the upper, middle, and lower parts of each extraction package.
For 100-200mL samples, if there is no special abnormality in the sensory measurement, the samples taken from each package are fully mixed, and the aliquots are reduced to the required number.
If the sensory measurement of the individual package samples is abnormal, the samples can be kept separately
.
For the sample in a large pool, the siphon method can be used to layer the four corners and the center of the pool to sample, each layer is about 500mL, after fully mixing, the fraction is reduced to the required amount
.

(5) For the collection of small-package food, the outer packaging is a carton, and the inner packaging is a sample of boxes, bags, bottles, etc.
(such as bagged milk powder, bottled beverages).
If there is no prescribed inspection lot, generally press and confirm the sampling number first.
, And then according to the random sampling method to extract the corresponding packaging from the different positions of the sample stack, randomly select a small package from each package, and merge it into the original sample
.

3.
Requirements for sampling

(1) Sampling is generally divided into three steps, followed by obtaining the test sample, the original sample, and the average sample
.
A small amount of materials collected from various parts of a large batch of materials to be analyzed is called a test sample; many test samples are collectively called an original sample; the original sample is technically processed, and then a part of the sample for analysis and inspection is called an average sample
.

(2) The number of samples should meet the needs of the test items for the sample size, so as to effectively reflect the hygienic quality of the batch of food
.
The samples should be made in triplicate for inspection, re-inspection, and future reference; the quantity of each sample should not be less than 0.
5kg.
In case of samples with adulterants, the analysis items should be appropriately increased because the analysis items are not clear in advance
.

(3) Sampling tools should be clean to prevent contamination of samples
.

(4) Keep the sample free from contamination or change before testing
.
For example , samples for testing aflatoxin B ₁ should be protected from sunlight to prevent aflatoxin B _{1 from} decomposing
.

(5) Samples with very different organoleptic properties should not be mixed together, and should be packaged separately to indicate their properties
.

(6) Analysis should be carried out immediately after collecting samples to avoid changes
.

(7) Labels should be affixed to the equipment containing samples, indicating the sample name, sampling location, date, batch number, method, quantity, analysis items, and sampling person
.

(2) Sample preparation

1.
The purpose of sample preparation

Due to the large number of samples obtained by the general method, the large particles, and the uneven composition of each part of the food, in order to ensure the accuracy of the analysis results, the samples must be crushed, mixed, and reduced before the sample inspection.
That is the preparation of the sample
.
The purpose of preparation is to ensure that the sample is very uniform, so that any part taken during analysis can represent the average composition of the sample for inspection and meet the requirements of the sample for analysis
.

2.
The method of preparation

The method of sample preparation varies according to regulatory requirements and the characteristics of the food itself
.
The sample preparation methods include stirring, shredding, crushing, grinding or mashing, etc.
The tools used include electric mixers, pulverizers, tissue mashers, and mortars
.

(1) The solid sample is made into a uniform sample to be tested by cutting, grinding, mashing, grinding and other methods
.
Solid samples with low moisture content and high hardness (such as grain) can be ground with a mortar or grinder and mixed evenly; samples with high moisture content and soft texture (such as vegetables and fruits) can be made by mashing with a tissue masher Homogenization; samples with high moisture content and strong toughness (such as meat) can be placed in a meat grinder and mixed evenly
.

(2) Liquid, slurry or suspended samples Generally, the samples (such as liquid condiments, milk, fruit juice drinks) are fully stirred with a stirrer
.

(3) The edible part of the canned food is taken out, and the condiments (such as chili, spices, etc.
) are taken out and then prepared evenly with a tissue masher
.

3.
Matters needing attention

During the sample preparation process, the escape of ingredients should be prevented, and changes in the composition and physical and chemical properties of the sample should be avoided
.

(3) Preservation of samples

The samples should be analyzed on the same day after collection to prevent the loss of moisture or volatile substances and changes in the content of the components to be tested
.
If it cannot be analyzed immediately, it should be stored properly, and the samples should not be exposed to moisture, volatilization, air-drying, deterioration, etc.
, to ensure the accuracy of the measurement results
.

The prepared average sample should be packed in a clean, air-tight container (glass bottles are best, containers with rubber pads should not be used), and stored in a dark place if necessary
.
Perishable samples should be stored in a refrigerator at 0～5℃; samples that are easy to lose water should be tested for moisture first
.
Generally, samples should be kept for one month after the inspection for re-examination when needed
.

In addition, the sample storage environment should be clean and dry, and the stored samples should be placed according to the date, batch number, and serial number for easy search
.

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