Comprehensive Compositional Analysis of Fish Feed by Time Domain NMR

Abstract

The quality of fish feed and the effectiveness of fish feed production process are important issues for both fish feed suppliers and fish farmers. A correct combination of protein, carbohydrate, fat, and moisture contents in fish feed is crucial for achieving a desirable growth rate and other key characteristics of farmed fish. Ability of fast at-line control of the composition of fish feed would give fish feed producers an advantage of more flexible control over the production, resulting in a more effective consumption of energy and raw ingredients, increasing the production speed, and improving the quality of the final product. Ability to do rapid corrections in the ongoing production, based on the at-line analysis results would substantially reduce the amount of rework and low-grade production. At the initial stage of fish feed production, a mixture of raw ingredients is prepared to produce the semi-solid matrix of the pellet. This mixture of raw ingredients mainly contains protein, carbohydrate, moisture, and fat. During the extrusion process, this mixture is compacted under high temperature and pressure to form a pellet with a porous, semi-solid wet matrix. The pellet matrix is then dried and saturated with fish or vegetable oil. Wide variety of well-known analytical procedures currently adopted by feed producers for compositional analysis of fish feed has a number of common disadvantages. These standard chemical–physical tests are usually time consuming, demanding highly trained staff, costly, most of the standard methods are destructive for the sample, often require thorough calibration, many of them utilize dangerous toxic solvents and cannot be performed at the production line. To the contrary modern time domain nuclear magnetic resonance (TD NMR) analyzers offer a wide spectrum of quick, non-destructive, and precise applications. Thanks to the high automation of modern NMR instruments, most routine tests can be performed by the ground-floor personal. A TD NMR instrument can perform a number of experiments, providing with various types of information about the studied material. This is accomplished by programming and running specific NMR pulse sequences, such as, for example, “free induction decay” (FID), “Hahn echo” [1], CPMG [2], “solid echo” [3], and others. Choice of the NMR pulse sequence depends not only on the type of the information required, but also to a large extent on the physical and chemical properties of the sample. Presence of solid and liquid phases, their mobility, rigidity, and NMR relaxation times are the most important parameters to be taken into consideration. A number of TD NMR applications have recently been developed and adopted for quality control of foods such as rapid determination of fat content in meat [4,5], characterization of fat and water states in cheese [6], prediction of the content of water, oil, and protein in rape and mustard seeds [7], solid fat content analysis [8], monitoring of textural changes in frozen cod [9], and NMR relaxation time studies of intact fish flesh [10] and meat [11].