Asian Food Science Journal

Aims: To optimise steaming time (1 – 3 minutes) and drying temperature (60 – 80 °C) using Response Surface Methodology (RSM) to maximise the retention of calcium and total dietary fibre in kale (Brassica oleracea) powder for functional food applications.

Study Design: A central composite design (CCD) with full-factorial response surface methodology comprising two independent variables (steaming time and drying temperature) generated 10 experimental combinations.

Place and Duration of Study: Food Science & Technology Research Centre and Horticulture Research Centre, MARDI Headquarters, Serdang, Selangor, Malaysia, between 2023 and 2024.

Methodology: Freshly harvested curly kale leaves were washed, coarsely chopped, and pretreated by steaming for 1 to 3 min before rapid cooling in ice water. Steamed kale was dried in a forced-air convection oven at specified temperatures (60 – 80 °C) for 12 to 13 hours until the moisture content was <10%, and was then ground into powder. Calcium content was determined using dry ashing and Atomic Absorption Spectrophotometry (AAS, AOAC 968.08). Total dietary fibre (TDF) was measured via enzymatic-gravimetric analysis (AOAC 993.21). RSM modelling with ANOVA, regression coefficient analysis, and desirability function optimisation was conducted using Minitab version 19.

Results: Calcium content in pretreated kale powder ranged from 1997 to 2494 mg/100 g dry weight, significantly higher (P = .004) than untreated controls (1805–1923 mg/100 g). Total dietary fibre ranged from 27.8 to 34.9 g/100 g. Calcium increased significantly with both steaming time (P = .004) and drying temperature, showing strong interaction effects (P = .019). Dietary fibre increased with temperature (P = .032) but decreased with prolonged steaming (P = .050). The quadratic RSM models demonstrated an excellent fit (R² = 0.9339 for calcium; R² = 0.8526 for TDF) with non-significant lack-of-fit (P > .05). Optimal conditions were established at 3 min of steaming and a drying temperature of 80 °C (desirability = 91%), yielding experimental values of 2441 mg/100 g calcium and 33 g/100 g TDF, closely matching the predicted model values (2453 mg/100 g and 34 g/100 g, respectively; T-test: P > .05).

Conclusion: Hot-air drying combined with a 3-minute steam pretreatment at 80 °C optimises the retention of calcium and dietary fibre in kale powder. The resulting powder meets Malaysian Food Regulations 1985 standards for high-calcium and high-fibre foods, offering a shelf-stable (up to 2 years at room temperature without additives), nutrient-dense plant-based ingredient for functional food formulation.