Statistical Evaluation of Moisture and Time Effects on the pH of Zingiber zerumbet Extracts by Subcritical Water Extraction

Zingiber zerumbet is a medicinally important rhizome, with subcritical water extraction (SWE) offering a sustainable approach to recover its bioactive compounds. However, the influence of raw material moisture and process parameters on extract quality remains unclear. This study investigates two factors which affect the pH of Z. zerumbet extracts produced via SWE; (i) its initial rhizome moisture content (MC), (ii) the extraction time. Fresh rhizomes were extracted at subcritical condition for 5 - 25 min. MC and pH were measured, and statistical analyses were performed using General Linear Model (GLM) and Multiple Linear Regression (MLR). All extracts were slightly acidic (pH 5.85 ± 0.03 – 6.24 ± 0.02). GLM showed that extraction time significantly impacted pH (F(4,9) = 55.92, p < 0.001, partial η² = 0.961). The final multivariate model explained 98.3% of the pH variance (Adjusted R2=0.973), but the MC which shown explanatory bivariate association with pH was non-significant after controlling extraction time (p=0.129). Visual observations indicated stable, homogeneous extracts, with only darkening at longer extraction times. Overall, extraction time is the dominant factor influencing acidity of Z. zerumbet extracts under SWE, while initial moisture exerts only minor and inconsistent effects. These findings underscore the importance of controlling extraction duration for process optimization and product reproducibility, with moisture monitoring serving as a secondary safeguard. Future studies should expand on other SWE parameters and predictive validation to strengthen industrial applicability.