The chemistry of flavonoids in model beverages and human milk.

摘要

Interest in the chemistry and stability of flavonoids in foods has been generated by countless epidemiological, in vitro, and in vivo studies that suggest a variety of potential health promoting effects of diets rich in flavonoids. Flavonoids have been shown to interact with multiple components in beverages including macromolecules, minerals, certain vitamins, and other flavonoids. Furthermore, environmental conditions including light exposure, elevated temperatures, and relative humidity have been shown to modify flavonoid stability. However, specific mechanisms, relative kinetics of degradation reactions and the impact of macromolecules in model beverages and biological fluids on flavonoid stability remain largely unknown. Recently, interest in the application of natural pigments to replace synthetic dyes in beverages has grown. However, there is limited information concerning the impact of prominent beverage ingredients, photo and thermal stress, and the potential mechanisms involved in anthocyanin degradation. Chapter 2 of this dissertation investigates the stability of anthocyanin rich grape and purple sweet potato (PSP) extracts to photo and thermal stresses in ready-to-drink beverage models including hot-fill beverages with varying concentrations of ascorbic acid, a preserved beverage, and a vitamin enriched water beverage. Thermal and photo stress were induced at 40, 60, and 80 oC and 250, 500, and 750 W/m2, respectively. Increasing concentration of ascorbic acid resulted in more rapid degradation of anthocyanins through thermal stress, but had a protective effect through photo stress. Additionally, PSP was significantly less stable than grape extract in the vitamin enriched water model beverage through photo stress. Furthermore, a potential degradation mechanism involving the formation of monoacylated peonidins from diacylated peonidins under photo, but not thermal, stress was identified. The transfer of dietary flavonoids to human milk and the potential exposure to nursing infants quickly became of interest. Chapters 3 and 4 of this dissertation focuses on the presence of several flavonoids in human milk samples. In our initial screening, samples were collected from 17 women who delivered healthy term babies (37 wk of gestation) at 1, 4, and 13-wk postpartum intervals. Epicatechin (63.7-828.5 nmol/L), epicatechin gallate (55.7-645.6 nmol/L), epigallocatechin gallate (215.1-2364.7 nmol/L), naringenin (64.1-722.0 nmol/L), kaempferol (7.8-71.4 nmol/L),hesperetin (74.8-1603.1 nmol/L), and quercetin (32.5-108.6 nmol/L) were present in human milk samples with high inter/intraindividual variability. With the exception of kaempferol, the mean flavonoid content in human milk was not statistically different among lactation stages. In contrast, carotenoids in human milk significantly decreased from weeks 1 to 13 of lactation. In our follow up study, cohorts from the USA, China, and Mexico each comprised of 20 women in each country. Milk samples were collected at 2, 4, 13, and 26-wk postpartum intervals. Glucuronides of epicatechin, naringenin, kaempferol, hesperetin, and quercetin were the predominant metabolites detected. Despite all efforts, anthocyanins were not detected in any of the 4 cohorts between the 2 studies and their relatively low bioavailability and presence in tissue is likely a contributing factor. The presence of flavonoids in human milk suggested that select flavonoids maintained relative stability in protein rich fluid in order to avoid degradation between emptying through feeding. Previous studies indicated that polyphenols bind to proline rich segments, including those on caseins, and may have a stabilization effect. Chapter 5 investigates the impact of protein rich milk on tea flavan-3-ol degradation during thermal treatment. Single strength milk (36.2 protein per L), quarter strength milk (9.0 g protein per L), and a zero protein control models were incubated with isolated epigallocatechin gallate and green tea extract at 62 or 37oC for 180 minutes. Intact flavan-3-ols as well as auto-oxidation products including theasinesins (THSNs) and P-2 dimer concentrations were quantified by LC-MS. In general, greater polyphenol to protein ratios increased first order degradation rates, which consequently decreased the formation of THSNs and P-2 dimers. The presence of the galloyl and hydroxy moieties increased apparent affinity of flavan-3-ols to proteins, thereby stabilizing monomeric flavan-3-ols to processing conditions with increasing protein concentrations. In contrast, the absence of these moieties led to no observable interactions to proteins. A thorough understanding of protein-polyphenol interactions may provide further insight to their chemistry in protein rich beverages and biological fluids.