The incidence of non-communicable disease such as cancer, diabetes, and cardio vascular disease (CVD) is increasing. Major contributing factors are diets that are poor in minerals, vitamins and antioxidants. Leafy green vegetables (LGV) are an important source of these nutrients. In addition, indigenous LGV are drought resistant, robust and easy to cultivate than more exotic LGV such as spinach and cabbage. Little is known about the effects of cooking and digestion on the polyphenol, flavonoid and ß-carotene content of African LGV as well as the associated antioxidant activity. The aims of this study are firstly to evaluate the effect of pH, the digestive enzymes and the phase of digestion on the polyphenols, flavonoids and β-carotene content and associated antioxidant activity of LGVs, spinach, pumpkin, cowpea, amaranth and jute mallow. Then to determine whether following digestion the antioxidant properties of the African LGVs is better than spinach which is commercially and widely cultivated.
Spinach, pumpkin, cowpea, amaranth and jute mallow leaves were cooked for 30 minutes, the water was discarded and the remaining solid cooked LGV which is usually consumed was subjected to two methods of digestion. The first referred to as gastrointestinal model of digestion (GI) of digestion consisted only of the stomach and intestinal phase of digestion, while the second model which was more complex included the oral, stomach and intestinal (OGI) phases of digestion. Raw, cooked, pH adjusted and digested samples at each phase of digestion were collected. For each sample the total polyphenol content (TPC), the total flavonoid content (TFC) was determined with the Folin-Ciocalteu method and aluminum chloride methods respectively. The ß-carotene content was determined with HPLC. Antioxidant activity was determined with the Trolox equivalent antioxidant capacity (TEAC), the 1, 1-diphenyl–2-picrylhydrazyl (DPPH) and the oxygen radical absorbency capacity (ORAC) assays.
Cooking decreased significantly the TPC, TFC, TEAC and DPPH of spinach, pumpkin and cowpea, while the ß-carotene content of these LGV remained unchanged. For amaranth and jute mallow, cooking increased the TFC, ß-carotene and DPPH values, while their TPC and TEAC remained unchanged. All the LGVs decreased their antioxidant activity when measured by ORAC assay, except for jute mallow which was unchanged. During digestion, the pH controls had varying effects with different assays, as it did not change the TPC of spinach, pumpkin and cowpeas, but decreased that of amaranth while the TPC of jute mallow was higher. Overall, the gastric and intestinal digestion increased the antioxidant activity of all the LGVs as measured by the ORAC assay. TPC and TEAC increased significantly for spinach, pumpkin and cowpea, while their TFC remained unchanged and antioxidant activity measured with the DPPH assay was lower. For the amaranth and jute mallow the TFC was decreased, DPPH increased, while TEAC was unchanged. Except for pumpkin the ß-carotene content was reduced following digestion. In general, the effect of pH was minimal while digestive breakdown of the LGV matrix resulted in a significant release of molecules with antioxidant activity.
In the OGI model of digestion, the oral phase of digestion contributed significantly to the levels of polyphenols and flavonoids released from the matrix. The effect on ß-carotene content was variable. Antioxidant activity for all LGV was increased (TEAC assay), variable (DPPH assay) and reduced (ORAC) assay. When considering the effect determined by the ORAC assay which is considered to be the physiologically the most relevant assay, using models that only simulate gastric and intestinal digestion may over estimate antioxidant activity.
Compared to spinach the antioxidant properties of the LGVs, although in some instances statistically different, were similar to spinach. In conclusion, LGVs evaluated in this study retained to various degrees the antioxidant activity following digestion. The biggest contributing factor to these antioxidant properties was not pH but the ability of the digestive enzymes to break down the food matrix resulting the release of antioxidant molecules. Based on the findings of this study are that African LGV are an ideal source of bioactive molecules that can prevent diseases of the GIT and if bio- available can effectively reduce the risk of diabetes, cancer and CVD.