Abstract:

Three variables (operating parameters) affecting the extraction efficiency of free amino acids from milk powder,
solid-to-solvent ratio, zinc sulfate concentration (for protein precipitation), and centrifugation time, were optimized by
response surface analysis (RSA) based on a Box-Behnken design. Simultaneous determination of mixed free amino acids
and free amino acids in milk powder samples was performed by visible spectrophotometry combined with partial least
square (PLS). The results showed that under the optimal conditions: solid-to-solvent ratio, 1:3.02 (g/mL); zinc sulfate
concentration, 10.23%; and centrifugation time, 6.79 min, the extraction yield was 170 μg/g. The complexation of copper
sulfate with glycine, glutamic acid, valine and phenylalanine resulted in highly overlapping absorption spectra when mixed
together, which made it difficult to simultaneously determine the individual components. The simultaneous determination of
mixed free amino acids using PLS demonstrated that the recovery rate was 98.2%–99.2% and the relative prediction error
(RPE) was 1.7%–2.9%.

Key words: milk power, free amino acids, response surface analysis, extraction, partial least squares, simultaneous determination