A study was conducted to investigate the effect ofspray drying on concentration of sulfamethazine (SMZ) in fluid milk dried to powder «10% moisture). Antibiotic-free skim and whole (homogenized) milk were spiked at 5, 10, 50 and 100 ppb sulfamethazine levels, pasteurized and stored at 4°C till further processed. All samples were spray dried at an inlet temperature of 180 ± 2°C and outlet temperature of 100 ± 2°C and stored at -20°C until analyzed.

Sulfamethazine concentration was determined quantitatively by HPLC, a microbial receptor assay (Charm-II®) and an ELISA assay (LacTek®) and qualitatively by an ELISA method (Cite®) in milk samples before and after spray drying. Dry milk samples were reconstituted (10% w/w) for all analyses. statistical determination of significant differences (p = 0.05) between fluid and dry milk samples and whole and skim milk sample~was completed by paired t-tests. Sulfamethazine concentrations increased 81.4% and 84.1% in skim and whole milk respectively at 100 ppb spiked level but were lower than expected increase of 88-91% based on their total solids for whole and skim milk as obtained by modified FDA HPLC method. At lower levels of 5 and 10 ppb, the HPLC method was not sensitive enough to provide usable data. Increase in sulfamethazine concentration from fluid to dry milk was also determined by· Charm-II® and LacTek® techniques. Poor recoveries and variability in data were evident due to binding of sulfamethazine to undetermined milk components as a result of processing and storage also due to break-down of sulfamethazine (mp = 176°C) at 1S00C during spray drying. Sulfamethazine 163 ppb by LacTek® and 94.6 ppb by Charm-II® (at a spiked level of 10 ppb fluid milk) was successfully removed from dried milk after 120 min using supercritical CO₂ (pressure = 5500 psi, 50 0 C