At present, the standard of fish meal implemented in China is SC/T3501-1996 [1]. The specific indicators are three sensory indicators: color, texture and smell, and the physical and chemical indicators are seven items: crushed particle size, crude protein, crude fat, moisture, salt, ash and sand. The standard also stipulates fish meal. The hygienic index of fish meal should conform to the provisions of GBL 3078-2001Feed Hygienic Standard [2].
Specific indicators include: arsenic, lead, fluorine, mold, mercury, cadmium, nitrite, DDT, total number of bacteria, etc. SC/T350L-1996 standard also stipulates that fish meal should not be parasitized by insects, and separately lists that the allowable amount of metallic chromium (calculated as hexavalent chromium) in fish meal is less than10 mg/kg. The acid value of fish meal is taken as a mandatory index, and it is only tested when it is put forward by users or inspection institutions. The acid value index is clearly stipulated, and the acid value of the lowest grade fish meal is required to be less than (equal to) 7 mg KOH/g. The above is the specific technical index requirements of fish meal stipulated by national standards. Relevant data [3 ~ 6] introduce that the nutritional index for judging the quality of fish meal should also determine the true protein and the stomach of animal protein.
Protease digestibility, health indicators should also determine ethoxyquin, formaldehyde and other indicators.
There are many items in the adulteration test of fish meal, including feather powder, bone powder, lignin, tanned leather powder, calcium carbonate powder, shell powder, eggshell powder, starch, crude fiber, blood powder, miscellaneous cake powder, urea nitrogen, ammonia nitrogen and so on. The adulteration test is qualitative, and it is stipulated in SC/T350L-1996 standard. There is no exact data to explain the quality of fish meal. The author thinks it is a very troublesome thing to finish the above inspection. In daily production inspection, enterprises can accurately reflect the quality of fish meal as long as they master the following quantitative analysis indicators. Now these analysis items and operation methods are summarized as follows:
How to use quantitative chemical analysis method to test the quality of fish meal in daily production quality control
How to test and judge the quality of fish meal by quantitative chemical analysis method is a crucial problem for feed manufacturers and distributors. The author summed up the quantitative chemical analysis to test the quality of fish meal from many years of production practice, mainly focusing on the following inspection items.
2.1test of crude protein
The content of crude protein is the main item to test the quality of fish meal. The national industry standard takes the content of crude protein as the main index to measure the quality and grading of fish meal, and it is also the primary test item. According to the provisions of GB/T 6432 [7], the determination of crude protein has high accuracy and precision, which is a classic test method, but this method consumes reagents and takes a long time to digest. The data introduced are the rapid determination method of direct distillation with strong alkali, the rapid determination method of increasing digestion temperature, and the rapid determination method of H2SO4-H2O2. In production practice, the author often uses the rapid digestion method of H2S 04-H2O2 to determine crude protein, which is a rapid and easy-to-master routine analysis method, and the results are basically consistent with the national standard. As for the application of distillation device, the accuracy and precision of total distillation device are high, but the distillation time is long; The speed of semi-micro distillation unit is faster, but the operation must be mastered to meet the requirements of accuracy. The rapid nitrogen determination device [8] designed by the author is a heating, pumping, absorption and titration device, which integrates heating, distillation and absorption, and can continuously determine samples without cooling water (that is, without changing utensils), and the whole determination process is completed in about 10min. The determination accuracy and precision are also high, and the application effect in production practice is good (see figure 1).
2.2 the test of real protein
It has been proved in production practice that the quality grade of fish meal can not be determined by measuring only one index of crude protein, because the crude protein content is obtained by measuring the total nitrogen content and multiplying it by the coefficient of 6.25, which can not exclude the interference of non-protein. Many materials have introduced the overview and detection methods of non-protein nitrogen. From the nutritional point of view of zoology, it is more important to measure the content of true protein. At present, the method for determining the content of true protein has not been included in the national standard, but many published materials and regular textbooks of the national education department have been introduced for a long time. The basic principle and operation steps of the determination method introduced by them are basically the same. The principle is that the sample is alkaline in copper sulfate solution, and sodium hydroxide solution is added, so that protein is precipitated and insoluble in non-protein.
Nitrogen-containing substances are separated, and the nitrogen content in the sediment is determined and converted into the content of real protein. According to the data, the percentage of true protein in fish meal in crude protein is: imported fish meal is more than 80%, and domestic fish meal is more than 75%. The national standard has not yet stipulated this index, but the author thinks it is very important to test this item in production practice. The author has tested the fish meal produced by a manufacturer, and its crude protein is 55% ~ 60%, while the true protein is only about10%. This kind of fish meal is absolutely inferior. For some fishmeal with poor quality in the market, the proportion of true protein in coarse protein is often less than 75%.
2.3 Determination of pepsin digestibility
Another important quantitative index of fish meal quality is pepsin digestibility, which indicates the ratio between protein and crude protein which can be decomposed by pepsin. The national standard does not stipulate the pepsin digestibility of fish meal, and the data introduction requires that the imported fish meal be ≥90%. Domestic fish meal is ≥ 85%. Relevant data and national standard GB/T1781-1999 all introduce the method of determining the digestibility of protein feed for animals-pepsin method. Determining this index can distinguish whether fish meal is mixed with other high-protein and difficult-to-absorb raw materials, such as feather powder and so on. Fish meal mixed with these raw materials has high contents of crude protein and true protein, but its pepsin digestibility is often low, generally only 50% ~ 60%, and even 30% ~ 40%. It is worth noting that the quality of fish meal in protein should be measured by comprehensive analysis of crude protein, true protein and pepsin digestibility, so as to determine its quality. The author has tested some fishmeal in production practice, and its crude protein is 5%-60%, and the pepsin digestibility is above 85%, but its true protein is only about10%, which shows that these fishmeal are inferior.
2.4 Determination of other quantitative analysis items
The author thinks that the quality analysis items of fish meal can determine the total fiber, crude fiber, phosphorus, calcium and other indicators in addition to the above three nutritional components. The standard SC/T3501-1996 stipulates that the total chromium content is less than10 mg/kg. If it exceeds this index, it means that the fish meal is mixed with tanned leather powder. In the daily inspection work, the author found that the total chromium content of fish meal sold in the local market is as high as 400-500 mg/kg. The national standard of acid value index has also been stipulated, and the acid value of some rotten fish meal is relatively high. Generally, it will exceed the standard requirement of 7 mg KOH/g. The national standard does not specify the index requirements of crude fiber. According to the data, the crude fiber content of fishmeal is 0.2% ~1.5%. If fishmeal is mixed with other high protein plant raw materials, the crude fiber content will exceed this range. Regarding the determination of phosphorus and calcium, experts believe that the contents of phosphorus and calcium in fish meal from a certain place are in a certain range and in line with a certain proportion, and it is abnormal to be too high or too low.
The above is the practical experience summarized by the author from years of inspection work. Of course, there are many feasible qualitative inspection methods for adulteration of fishmeal, but as a testing department, qualitative analysis often can't accurately explain the specific quality problems of fishmeal. Only by mastering the above quantitative analysis methods and listing the real data in the analysis and inspection report can we make a definite conclusion and judgment for the production and sales units.