Effects of Acid and Alkali Cooking on Edible Quality of Lotus Rhizome

Acetic acid (AA) and sodium bicarbonate (SB) are commonly used as additives for regulating the texture of fruits and vegetables. However, the effects on the nutritional quality of lotus rhizome during texture modification are not well-understood. In this study, with distilled water as control group, the effects of cooking with 0.1% acetic acid (v/v) and 0.1% sodium bicarbonate (w/v) on the hardness, color, vitamin C, total phenol and digestion of lotus rhizome were investigated. The flavor components of lotus rhizome treated with sodium bicarbonate were also analyzed. The results showed that AA treatment could significantly improved the cooking hardness of lotus rhizome while keeping its white color, but SB-cooking significantly reduced the hardness of lotus rhizome and possessed a reddish color. The highest total phenol (about 1300 μg/g) and V_C content (about 0.3 mg/g) were found in fresh sample. Cooking resulted in a significant decrease in the total phenol and V_C content, with no significant difference observed between distilled water and sodium bicarbonate treatment groups except for the higher total phenol content of AA-treated group (about 900 μg/g). SB treatment altered the digestion and absorption pattern of lotus rhizome, resulting in a lower level of total carbohydrate in the gastric digestive juices (about 300 g/L). Less differences were found between the distilled water and AA treatment, while in the simulated intestinal digestion, the total carbohydrate levels of both remained high. The results of electron nose and solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) showed that the volatile components of lotus rhizome flavor mainly included dimethyl sulfide, n-octanal, nonanal, and sunflower aldehyde, etc. The dimethyl sulfide as well as esters of lotus rhizome were increased after SB treatment. Acid and alkaline cooking had opposite effects on the hardness and color of lotus rhizome, but did not significantly alter the total phenol and V_C contents. Instead of distilled water, cooking with acetic acid did not significantly alter the in vitro gastrointestinal digestion pattern, but alkaline cooking concentrated the carbohydrate digestion utilization in the intestinal digestion phase. The organic acids and esters produced by sodium bicarbonate cooking and the elevated dimethyl sulfide content formed the special flavor of alkali-cooked lotus rhizome.