Studies on the Effect of Coffee Creamer with Different Emulsion Properties on Instant Coffee Polyphenols

Abstract

Coffee is among the most widely consumed beverages worldwide and a good source of polyphenols, which prevent oxidative-stress related diseases. This health benefit was identified in epidemiological investigations and has been evaluated in terms of the biological activity of phenolic compounds interacting with milk proteins in dairy matrices (due to frequent consumption of coffee simultaneously with phenolic-rich foods and dairy products). The objectives of this study are to evaluate effects of creamers containing milk proteins on the bioaccessibility of chlorogenic acids, and the antioxidant activity of instant coffee, considering the consumption pattern of instant coffee in Korea. Creamer is an oil-in-water emulsion that contains milk proteins as emulsifiers, which impart long-term stability. The effects of the types and contents of milk proteins on antioxidant activity were investigated. Firstly, creamer A and creamer B, differing in contents and types of milk proteins, were prepared and their emulsion characteristics were compared. Creamer A, which had a low protein content, had high emulsion stability due to the high emulsifying ability of acid casein. Creamer B, which had a high protein content, being mainly composed of aggregated milk proteins, showed low emulsion stability. In creamer emulsion systems, creamer A was more stable than creamer B to the droplet aggregation caused by pH changes in coffee, but there was no significant difference in the mean particle size or distribution associated with emulsification. Creamer A was more stable than creamer B because the rates of aggregation and creaming (following clarification) in creamer B were faster. Creamer A had a higher proportion of adsorbed to non-adsorbed protein than creamer B. Therefore, the adsorption ability of acid casein in creamer A was higher than that of skim milk and milk protein concentrate in creamer B, although creamer B had a higher total protein content than creamer A. Second, complexation between the polyphenols in instant coffee and milk proteins in the creamers could vary depending on changes during in vitro digestion, which affect the bioavailability of polyphenol. Therefore, changes in the distribution of particles in instant coffee alone (IC), instant coffee with creamer A (CRA), instant coffee with creamer B (CRB), and instant coffee with skim milk (SM), were evaluated by Turbiscan analysis. In addition, the bioaccessible contents and bioaccessibility of chlorogenic acids were compared at each step of in vitro digestion. Changes in delta-backscattering (Δ-BS) profiles during gastric digestion indicated that flocculation, coalescence, and creaming at the top occurred in CRA and CRB due to low pH and protein hydrolysis by digestive enzymes. In SM, which had the highest protein content, sedimentation of protein aggregates and complexes involving structured clots occurred at the gastric phase. At the pancreatic phase, changes in Δ-BS profiles indicated sedimentation at the bottom and clarification at the top by lipid and protein hydrolysis, and milk proteins were fully hydrolyzed after pancreatic digestion. There were no significant differences (p > 0.05) in chlorogenic acid contents among IC, CRA, and CRB, but the total chlorogenic acid content in SM was significantly lower than that of the other samples (p < 0.05) at the gastric phase. Pancreatic digestion caused a significant decrease (p < 0.05) in the total chlorogenic acid content in all samples, except SM. The bioaccessible content of CRA and CRB was not significantly changed (p > 0.05) during gastric digestion, suggesting that chlorogenic acids in CRA and CRB retained their structures in the gastric phase. After the pancreatic phase, the bioaccessibility of CGAs in IC, CRA, CRB, and SM was 26.4%, 40.5%, 51.7%, and 83.6%, respectively, indicating that bioaccessibility increased significantly with increasing protein concentration. Moreover, CRA and CRB increased the bioaccessibility of chlorogenic acids as a result of prevention of degradation by the milk proteins in creamers. Finally, the effects of creamer addition on the total polyphenols and antioxidant activity of instant coffee were evaluated during in vitro digestion. The total phenolic (TP) contents of CRA and CRB were significantly higher (p < 0.05) than that of IC, at all digestion stages. The TP content of IC decreased after pancreatic digestion compared to that before digestion and after gastric digestion. These results were attributed to the low stability of phenolic compounds in coffee at neutral pH. In addition, the changes in 5-caffeoylquinic acid (5-CQA), the most abundant chlorogenic acid in instant coffee, were analyzed during digestion. The results after pancreatic digestion were similar to those of IC, suggesting that coffee polyphenols were degraded at the pancreatic phase. The antioxidant activity of IC, CRA, CRB, and SM during in vitro digestion was measured by ABTS+ and ferric reducing antioxidant power (FRAP) assay. The antioxidant activities of CRA, CRB, and SM were higher than those of IC after pancreatic digestion and showed smaller decreases than IC during pancreatic digestion. IC showed the greatest decrease (25.6%) in FRAP compared to that before digestion; smaller decreases were observed for CRA (12.7%), CRB (16.2%), and SM (13.8%). These results suggest that milk proteins preserved the antioxidant activity of phenolic compounds during digestion, particularly at the pancreatic phase. As it was not elucidated how the protein in creamers had effects on enhancing the antioxidant activity of instant coffee, it is necessary to clarify the exact mechanism in terms of the complexation between proteins and polyphenols. Furthermore, the correlation of in vitro result with in vivo effects in human seems to be a further work to provide more exact and practical results. This study demonstrated that the addition of creamers to instant coffee can significantly affect the bioaccessibility of chlorogenic acids and antioxidant activity, possibly attributable to complexation of milk proteins and coffee phenolics. Therefore, the consumption of instant coffee with creamer enhances the biological activity of coffee phenolics after gastro-pancreatic digestion, compared to the consumption of instant coffee alone.