Comparison of Six Sample Preparation Methods for Analysis of Food Additives in Milk Powder

Milk powder matrix is highly complex with significant amounts of lipids and proteins; thus, the determination of trace residues and contaminants in it often requires extensive sample extraction and preparation prior to instrumental analysis. An ideal preparation method is simple, less time consuming, and relatively inexpensive. It enhances the recovery and involves low consumption of toxic organic solvents. In this study, six sample preparation methods for milk powder were established and compared, including liquid-liquid extraction, organic precipitation, heavy precipitation, and three different solid-phase extraction (SPE) methods. The extraction recovery was evaluated by determination of six preservatives and sweeteners (benzoic acid, sorbic acid, natamycin, lysozyme, saccharin sodium, and aspartame) by HPLC-DAD in whole and skimmed milk powder samples. The results showed that the reproducibility of liquid-liquid extraction was not as high as the SPE method although the recovery was beyond 80 %; the organic precipitation and heavy precipitation gave poor recovery (below 20 %) of lysozyme; the highest recovery (> 85 %) was achieved using the HLB cartridge compared to C18 and MAX cartridges. Therefore, the HLB cartridge was selected as the most appropriate sample preparation material for the determination of the six preservatives and sweeteners in milk powder samples by HPLC-DAD.