Reports on findings of hazardous substances in food products are nothing unusual these days: pesticides in fruit, arsenic in rice or toxic plant parts in tea, to name but a few. Therefore, permanent quality control is an indispensable tool of consumer protection. Apart from the actual food testing, sample preparation by homogenization is essential to obtain reliable results.
Most sample materials can be ground to the required analytical fineness at room temperature. However, there are limits, for example when even a small temperature increase affects the sample in a negative way; or when the material is very elastic and will only be deformed. Moreover, food samples which are fatty or sticky may block the mill. Therefore, cryogenic grinding is the best way to pulverize food samples like cheese, raisins, wine gum or marzipan which simply clump together when ground at room temperature.
In the book "Ideas and Applications Toward Sample Preparation for Food and Beverage Analysis," (ISBN 978-953-51-3686-6), edited by Mark T. Stauffer, RETSCH has published a chapter on reliable sample preparation of food samples. This chapter describes the process of turning a laboratory sample into a representative part sample with homogeneous analytical fineness by choosing the most suitable mill. Important aspects of size reduction and homogenization are explained, a variety of application examples is given, and specific applications such as cryogenic grinding are discussed in detail.
How to turn a laboratory sample into a representative part sample with homogeneous analytical fineness -
Food occurs in a great variety of consistencies and is often inhomogeneous. Food testing labs require representative samples to produce meaningful and reproducible analysis results. Therefore, food samples must be homogenized and pulverized to the required analytical fineness, ideally with as little time and effort as possible. Furthermore, reliable analytical results can only be obtained if the entire sample preparation process is carried out reproducibly.
Plastic is an inherent part of our everyday lives; it is used in a huge variety of things such as, for example, packaging, furniture, clothing or electronic devices. Though the utility of the material is undoubted, consumers are increasingly unsettled by recurring news about hazardous substances detected in plastics. Substances such as plasticizers, which are not firmly bound in the material, are absorbed via the skin and can influence the hormonal balance. Plasticizers contained in food packaging, for example, penetrate into the food and thus into the human body when the food is eaten. Plasticizers in toys are a particularly serious problem; children tend to take toys into their mouths thus absorbing the dangerous chemicals. Equally hazardous are polycyclic aromatic hydrocarbons (PAH). The family of PAH comprises more than 100 compounds most of which have been found to be carcinogenic.
The most common types of coal (lignite, bituminous and anthracite) are distinguished by their different chemical and physical properties. The calorific value of coal can be determined by analyzing its carbon content, for example with combustion analyzers. In addition, efficient management of the desulfurization plant requires control of the sulfur content. Compared to the large amount of coal a typical laboratory sample consists of - a 10 liter bucket or even more - the sample volume required for analysis is fairly low, only approx. 1 g. The standard DIN 51701 defines the sample amount to be tested as this relates to the particle size: The bigger the particles are, the more sample material is required.
Like all foodstuff, confectionery is subjected to strict quality controls. Parameters of interest are, for example, nutritional value, moisture or fat content, or the quantification of particular ingredients, such as vitamins or alkaloids. Typically, chromatographic methods like High Performance Liquid Chromatography (HPLC) are used to analyze food samples. Most analytical methods only require a few milligram or gram of sample; the previous size reduction/homogenization process ensures that the small analysis sample is representative of the entire laboratory sample, thus allowing for reproducible results. Moreover, homogenized samples show a much better extraction behavior.
Reliable and accurate analysis results can only be guaranteed by reproducible sample preparation. This consists of transforming a laboratory sample into a representative part sample with homogeneous analytical fineness. Retsch offers a comprehensive range of the most modern mills and crushers for coarse, fine and ultra-fine size reduction of almost any material. The product range also comprises a wide choice of grinding tools and accessories which helps to ensure contamination-free preparation of a great variety of sample materials. The selection of the correct grinding tool depends on the sample material and the subsequent method of analysis. Different grinding tools have different characteristics, such as required energy input, hardness or wear-resistance.
Considerable care must be taken when analyzing a sample like rice in order to achieve an accurate result. The major source of error when analyzing a bulk material comes not from the analytical measurement itself, but from the sample handling, i.e., sampling, sample division, grinding, digestion, etc.
Sample preparation of vegetable-based raw materials with laboratory mills
Thanks to the increasing usage of biomass as a source of energy, the analysis of these materials in the context of R&D and quality control gains importance, too. Due to the complex properties of plant materials, adequate sample preparation can be rather a challenge.
Use of laboratory grinders for size reduction of human bones and bioceramics
Bone implants and substances for bone regeneration are used in surgery to replace degenerated bone material by implants or to “re-build” it with specific substances. The material used in implants varies from autogeneic (supplied by the patient) through allogeneic (supplied by a donor) bones to replacement materials such as hydroxylapatite (HA) and tricalcium phosphate (TCP). Bovine bones and corals are used in conjunction with synthetically produced foamed materials to form a basis for the regeneration of bone substance. Various RETSCH mills are suitable for the preliminary and fine grinding of human bones as well as bioceramic materials.
In the analysis of solid material, the popular adage that “bigger is better” certainly does not apply. The goal is to produce particles that are sufficiently small to satisfy the requirements of the analysis while ensuring that the final sample accurately represents the original material. The “particles” of interest to the analyst generally range from 10 µm to 2mm. Additionally there are many application, where even finer sizes are needed. One example are active ingredients, where it is necessary to grind in the submicron range. Finally for DNA or RNA extraction mechanical cell lysis is well-established.
Materials differ widely in their composition and physical properties. Hence, there are many different grinding principles that can be applied, and this, together with other variables such as initial feed or “lump” size, fineness needed and amount of sample available, results in a wide range of models available to the researcher.
A variety of methods can be used to analyze solid materials. What they all have in common is the necessity to use a representative, homogeneous analysis sample which needs to have a particular fineness, depending on the analytical method used. The size reduction and homogenization of solids is usually carried out with laboratory crushers and grinders.
A faultless and comparable analysis is closely linked to an accurate sample handling. Only a sample representative of the initial material can provide meaningful analysis results. Rotating dividers and rotary tube dividers are an important means to ensure the representativeness of a sample and thus the reproducibility of the analysis. Correct sample handling consequently minimizes the probability of a production stop due to incorrect analysis results. Thus correct sample handling is the key to effective quality control.
With ever increasing socio-economic and geo-political demands, the use of secondary fuels in cement manufacture is a well-established practice for reducing both costs and CO2 emissions. The depletion of resources, combined with an increasing demand for primary fuels such as oil, gas or coal, make it paramount for energy consuming industries to search for alternative energy sources.
Why Size Reduction? A reliable and accurate analysis can only be guaranteed by reproducible sample preparation. The "Art of Milling" describes the process of turning a laboratory sample into a representative part sample with homogeneous analytical fineness.