Food Sampling & Analysis - VIII

Preparation of food Samples for Laboratory Analysis

Across the world, food safety testing has become a growing concern as more and more contamination scares arise. Melamine in milk products, carbendazim in orange juice, fish tainted with PCBs, mercury-tainted milk powder and food supplements containing unauthorized food ingredients are just a few of the most recent alerts and scandals. In response to the increase of foodborne illness, regulatory authorities continue to prescribe methods for food testing, making it even more important for laboratories to apply appropriate quality control procedures to ensure safe products from “farm to fork.” Because of the increased regulation for food safety, laboratories must be diligent when selecting their means of analyzing for contaminants or other harmful substances. Sample preparation is perhaps the most important step in the analysis because it can affect the analyte concentration and the cleanliness of the sample prior to further analysis. The proper sample preparation technique must be selected to ensure a reliable downstream analysis. 

In order to achieve the most reliable analysis of chemical hazards, the analyst must first consider how the sample preparation step will affect the instrumental analysis. Food matrices are notoriously complicated because they contain components such as carbohydrates, lipids and proteins. It has been estimated that about 30 percent of analytical errors originate from the sample preparation step, making it even more important that the technique chosen is reliable and repeatable. Whenever possible, official analytical methods provided by international organizations should be followed. However, it is sometimes necessary for a laboratory to create its own documented in-house method. In this case, the analyst will need to determine the most appropriate preparation technique for the samples at hand.

Once we have selected a sample that represents the properties of the whole population, we must prepare it for analysis in the laboratory. The preparation of a sample for analysis must be done very carefully in order to make accurate and precise measurements. Once a sample preparation technique is chosen, the laboratory must validate the procedure to prove that the chosen method is reliable and fit-for-purpose, which can be done by participating in proficiency testing/inter-laboratory comparison programmes. Participation in at least one proficiency test/inter-laboratory comparison is required in order to achieve and/or maintain accreditation according to ISO/IEC 17025:2005. Traditionally in food safety testing laboratories, liquid-liquid extraction (LLE) has been the most popular sample preparation choice. LLE relies upon the partitioning of analytes and interfering compounds into immiscible organic and aqueous layers. During the separation of organic and aqueous solvents, it is sometimes impossible to avoid the formation of emulsions that make it difficult to collect the layer that contains the target analytes, and the repeatability and accuracy of the method can be compromised. Two of the more rapidly growing sample preparation techniques are QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) and solid phase extraction (SPE).

Preparation of Homogeneous Samples

The food material within the sample selected from the population is usually heterogeneous, i.e., its properties vary from one location to another. Sample heterogeneity may either be caused by variations in the properties of different units within the sample (inter-unit variation) and/or it may be caused by variations within the individual units in the sample (intra-unit variation). The units in the sample could be apples, potatoes, bottles of ketchup, containers of milk etc. An example of inter-unit variation would be a box of oranges, some of good quality and some of bad quality. An example of intra-unit variation would be an individual orange, whose skin has different properties than its flesh. For this reason it is usually necessary to make samples homogeneous before they are analyzed, otherwise it would be difficult to select a representative laboratory sample from the sample. A number of mechanical devices have been developed for homogenizing foods, and the type used depends on the properties of the food being analyzed (e.g., solid, semi-solid, liquid). Homogenization can be achieved using mechanical devices (e.g., grinders, mixers, slicers and blenders), enzymatic methods (e.g., proteases, cellulases, lipases) or chemical methods (e.g., strong acids, strong bases and detergents).

Reducing Sample Size

Once the sample has been made homogeneous, a small more manageable portion is selected for analysis. This is usually referred to as a laboratory sample, and ideally it will have properties which are representative of the population from which it was originally selected. Sampling plans often define the method for reducing the size of a sample in order to obtain reliable and repeatable results.

Preventing Changes in Sample

Once we have selected our sample we have to ensure that it does not undergo any significant changes in its properties from the moment of sampling to the time when the actual analysis is carried out, e.g., enzymatic, chemical, microbial or physical changes. There are a number of ways these changes can be prevented.

Enzymatic Inactivation – Many foods contain active enzymes they can cause changes in the properties of the food prior to analysis, e.g., proteases, cellulases, lipases, etc. If the action of one of these enzymes alters the characteristics of the compound being analyzed then it will lead to erroneous data and it should therefore be inactivated or eliminated. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control enzyme activity, with the method used depending on the type of food being analyzed and the purpose of the analysis.

Lipid Protection – Unsaturated lipids may be altered by various oxidation reactions. Exposure to light, elevated temperatures, oxygen or pro-oxidants can increase the rate at which these reactions proceed. Consequently, it is usually necessary to store samples that have high unsaturated lipid contents under nitrogen or some other inert gas, in dark rooms or covered bottles and in refrigerated temperatures. Providing that they do not interfere with the analysis antioxidants may be added to retard oxidation.

Microbial Growth and Contamination – Microorganisms are present naturally in many foods and if they are not controlled they can alter the composition of the sample to be analyzed. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control the growth of microbes in foods.

Physical Changes – A number of physical changes may occur in a sample, e.g., water may be lost due to evaporation or gained due to condensation; fat or ice may melt or crystallize; structural properties may be disturbed. Physical changes can be minimized by controlling the temperature of the sample, and the forces that it experiences.

Sample Identification

Laboratory samples should always be labeled carefully so that if any problem develops its origin can easily be identified. The information used to identify a sample includes:

1. Sample description;
2. Time sample was taken;
3. Location sample was taken from;
4. Person who took the sample, and;
5. Method used to select the sample; 

The analyst should always keep a detailed notebook clearly documenting the sample selection and preparation procedures performed and recording the results of any analytical procedures carried out on each sample. Each sample should be marked with a code on its label that can be correlated to the notebook. Thus if any problem arises, it can easily be identified.

Reference

http://www.sepscience.com/Sectors/Food/Articles/415-/The-Importance-of-Sample-Preparation-in-Food-Analysis

http://people.umass.edu/~mcclemen/581Sampling.html