Internal standards and maltitol were added at the following levels: 200 ll 10lg/ml D3-acrylamide, 100 ll 880 lg/ml 15N2-asparagine, 400 ll 0.2% sorbitol and 200 ll 0.2% maltitol. The sample was extracted by an Ultra Turrax T25 homogenizer (Janke & Kunkel, Germany) at 10,000–12,000 rpm for 2 min.