Drying of spinach (Spinacia oleracea L.) and kale (Brassica oleracea L. var. acephala D.C.) is required to determine percentage of dry matter (%DM) and pigment concentration of fresh leaves. 'Melody' spinach and 'Winterbor' kale were greenhouse-grown in hydroponic nutrient solutions containing 13 or 105 mg·L-1 N. Using vacuum freeze dryers and convection ovens, plant tissues were dried for 120 h at five different temperature treatments: 1) freeze drying at -25°C; 2) freeze drying at 0°C; 3) vacuum drying at +25°C; 4) oven drying at +50°C; and 5) oven drying at +75°C. Spinach leaf tissue %DM was affected, but kale %DM was unaffected by drying temperature. Spinach and kale leaf tissue %DM were both affected by N level. The high N spinach decreased from 7.3 to 6.4%DM when drying temperature increased from +25 to +75°C. The low N spinach decreased from 12.7 to 9.6%DM as the drying temperature increased from -25 to +50°C. Kale averaged from 14.8%DM for the high N treatment and from 21.8%DM for the low N treatment. However, drying temperature did not have a significant impact on measured %DM in kale. Lutein, β-carotene, and chlorophyll levels for both spinach and kale leaf tissue were affected by drying temperature. Measured concentrations of all pigments decreased over 70% as the drying temperature increased from -25 to 75°C. The largest pigment fresh and dry weight concentrations for spinach and kale were measured at drying temperatures below +25°C. The spinach and kale samples dried between -25 and +25°C were not significantly different from each other in %DM or pigment concentration measured on a dry or fresh weight basis. Thus, drying leaf tissue for accurate pigment analysis requires temperatures below +25°C using vacuum or freeze drying technology.
All Science Journal Classification (ASJC) codes
- Crop production
- Dry matter
- Functional foods