Lactose was purchased from Shanghai Second
Reagents Factory (China) and used without further purification. Three milk powder samples were Nestle soluble
milk powder, Campina Birch Tree full creammilk powder
and Anlene Hi-Calcium fat-free milk powder, respectively. Each milk powder sample was carefully weighed
and dissolved with doubly distilled water, then filtered
with 0.22-mm teflon ®lter paper (Shanghai Bandao Purification Material Factory, China).