Solid food matrices, drying

The thermal desorption must take place at a temperature below the decomposition point of other materials in the sample matrix. Solid materials should have a high surface area (e.g., powders, granules, flbers). Bulk materials are ground with a coolant such as solid carbon dioxide prior to weighing. This technique simplifies sample preparation and avoids the necessity of dissolving samples or solvent extraction. Thermal desorption is well suited for dry or homogeneous samples such as polymers, waxes, powders, pharmaceutical preparations, solid foods, cosmetics, ointments, and creams. There is essentially no sample preparation required. 

Delivery systems can be solid or liquid, depending on the food matrix where they are introduced. Some examples of solid systems are spray-dried and gel microparticles, whereas liquid systems include liposomal and emulsified systems. Each of these delivery systems has its own specific advantages and disadvantages for encapsulation, protection, and delivery of food ingredients. These aspects are briefly discussed in the following, together with a description of the basic principles of each technique, its physicochemical characteristics, and the current challenges for its application in foods. 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

Foods are supplemented with vitamin A in the form of standardized preparations of synthetic fatty acyl esters, nowadays chiefly retinyl palmitate. The preparations are available commercially as either dilutions in high-quality vegetable oils containing added vitamin E as an antioxidant or as dry, stabilized beadlets in which the vitamin A is dispersed in a solid matrix of gelatin and sucrose or gum acacia and sucrose. The oily preparations are used to supplement fat-based foods such as margarines the dry preparations are used in dried food products such as milk powder, infant formulas, and dietetic foods (24). 

Steam-solvent distillation using diethyl ether has been used to remove and analyse for odour and taint from additives in food packaging films. Another technique that has been used is vacuum/thermal extraction. This procedure has been applied to polyamides and fluorocarbon polymers. The procedure is used for the direct isolation or release of volatile components from a polymeric matrix and may involve the combined use of vacuum and heat, as for example in the mass spectrometer direct insertion probe or during dry vacuum distillation. Alternatively, the volatiles may be swept from the heated sample by a flow of inert gas for concentration by freeze trapping and/or collection on to a solid adsorbent prior to thermal or solvent desorption for GC or mass spectrometric (MS) examination. 

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