INTRODUCTION   





     Feed microscopy is a flexible, fast and inexpensive feed quality control technique. It is the ideal complement for the classical chemical analyses (Weende, van Soest,), and provides faster general information on ingredient quality. It is especially useful for the identification of tainting or adulterating materials, or for quantitative determinations when specific physical or chemical methods do not exist. It can become a part of any quality control system, because the required equipment is simple and affordable for any feed manufacturer.

     Feed Microscopy can be classified in two main groups: qualitative and quantitative.

     Both types can be performed using stereo microscopes and/or compound microscopes. In practical terms, the stereo microscope is more widely used, because it is easier to apply to this field. This is the reason why the images on this CD have been taken using stereo microscopes, to help the feed microscopist with the correct identification of ingredients based on their external morphology. This Handbook does not cover quantitative microscopy


FEED MICROSCOPY ON THE FEED MILL

     Quality control on the feed mill starts with ingredient reception, before they are unloaded. Simply observing a homogeneous and representative sample of the raw material and comparing it with a standard, allows determining whether there is dirt or soil, tainting materials, or alterations due to the climate (rain, heat, moisture) and/or deficient preservation (moulds or insects).

     Identifying the contaminant or tainting materials during routine quality control is probably the most important role of Feed Microscopy. Based on “quality standards” or “variation margins” set by the feed company in their purchase contracts, the feed microscopist can take the decision, in real time, of accepting or rejecting the shipment, reducing the problems posed by stocking materials in the reception areas. If there is no feed microscopist immediately available, a trained supervisor can make a quick check with a 6x-8x handheld magnifying glass.

     Feed Microscopy can also be applied to any segment of the feed manufacturing process. It helps to solve potential operational problems, and can be especially important during the mixing stage. Mix quality (formula compliance), and the presence or absence of some micro-ingredients (minerals, vitamins) can be established by the observation of specific microtests. The microscopic analysis of mixtures can constitute an important probe of the manufacturing process’ adequacy, especially when it includes further processing that may alter the initial characteristics of the ingredients. An example of this is starch gelling due to extrusion or expansion. On the same token, charred particles, fragments of bone, metal, rubber or insects can be identified faster than with any other physical or chemical procedure.

     Final product evaluation against its theoretical formulation is the second most important application of feed microscopy. The detection of main ingredients and some minority components, with obvious features, via microscopy can achieve 99.5% accuracy in meal or pelleted feed. This is not true in extruded or expanded feed, as it is virtually impossible to determine relative amounts of starch coming from different raw materials due to the high gelatinisation occurring during feed processing. Therefore, the evaluation and quantisation on complete feed or food is much more difficult than on single ingredients.


EQUIPMENT

     The basic equipment for Feed Microscopy usually contains:

SAMPLE PREPARATION

     The accuracy of results requires a representative sample of the material to observe to be available, and of graphic reference materials, such as those included in this Handbook. If the material is pelleted, it has to be disgregated without reducing the particle size, by using water or a mortar and pestle. Milling the sample is not convenient, as this reduces particle size and causes loss of identification features from the materials.

     The material as a meal, will be mixed and fractionated into four parts. The final sample will be 10 to 15 g. A fraction of this sample can be sieved to obtain four separated samples, which will make the microscopy analysis easier. The technician can skip this step, but it is verify useful for both the novel and the experienced microscopist.

INSPECTION WITH STEREO MICROSCOPE

     The sample, whole or sieved must be spread over a piece of light cardboard and examined with a binocular magnifying glass or a stereo microscope at 10x and 20x, starting with the coarser particles and ending with the finer ones.

     The inspection path goes from one side of the sample to the other, separating and sorting the material with tweezers or dissecting needles. In both cases it is advisabled that the instruments have tapered or needle tips, to facilitate the isolation of individual fragments and testing features as hardness or texture.

     The inspection should be initially performed on majority organic ingredients. The collection of ingredients or the use of the Handbook is very useful here, to solve doubts or check aspects. Throughout the inspection, the minoritary ingredients and tainting and adulterating materials noticed will also be recorded. For increased accuracy the inspection will have to include the finer particles or dust, as sometimes the adulterating materials added are extra finely milled, to avoid detection. Sieving the sample and separated examination of each fraction is so recommended.

     After identification of the organic ingredients, the most common inorganic ingredients (minerals) are checked. This relates especially to all those that are used in significant amounts in feed and are not milled to dust, like salt and calcium and phosphorus supplements.

     The use of the Flotation Technique facilitates their identification. This way, the mineral fraction can be separated from the organic one. This technique is also very useful as a treatment for high fat samples where the observation of isolated particles is very difficult. To perform the Flotation Technique, a small amount of the sample is placed in a beaker or test tube and an excess of solvent (Trichloroethylene) is added until covering the material completely. Upon thorough shaking, the beaker is left standing to allow separation of the organic material towards the surface, while the mineral fraction sinks to the bottom. The floating fraction is decanted, taking care of preserving the heavy fraction in the beaker. Evaporate the solvent and the fractions are ready for examination.

     The Stain Tests for chemical identification allow detecting very minute amounts of mineral aggregates. It is better performing the tests under the microscope, as the minerals usually appear as dust and this difficults observing the expected reaction with the naked eye.

Index