"Tea catechins can bind irreversibly to protein on storage, and the effect in an oil-in-water emulsion is that a synergistic increase in antioxidant activity occurs if both protein and catechin are present," wrote the authors in the journal Food Chemistry.
Interest in green tea extracts has mostly focussed on the potential health benefits, with scientists reporting a wide range of effects, including a lower risk of certain cancers, improved heart health, weight loss, and protection against Alzheimer's.
Interest in the extracts beyond the health benefits, as flavours and antioxidants in foods, is beginning to garner interest amongst food formulators.
"Green tea, together with extracts of blueberry, curcuma and particularly grape seeds, were identified, from a wide range of plant extracts, as being interesting antioxidants for use as natural additives for foods," explained lead author Pilar Almajano from Technical University of Catalonia.
Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate (ECG), and epicatechin (EC).
The new research, by Almajano and Eugenia Delgado from the Technical University of Catalonia in collaboration with Michael Gordon from the University of Reading, reports that the shelf-life and nutritional potential of an emulsion may be enhanced by addition of the proteins albumin and ovalbumin.
The researchers prepared model oil-in-water emulsions by dissolving Tween-20 (1 per cent) in an acetate buffer, with or without the bovine serum albumin (BSA) or ovalbumin proteins (0.2 per cent), and tea catechins (EC, ECG or EGCG), and then adding the oil (methyl linoleate or sunflower oil) drop wise to the water phase.
Almajano and co-workers report that the stability after 45 days of the emulsions prepared using the albumin and EC and EGCG was increased synergistically, as determined by measuring the peroxide and hexanal contents.
"Although BSA had very little antioxidant activity in the absence of phenolic antioxidants the combination of BSA with each of the catechins showed strong antioxidant activity," said Almajano.
She reports that the emulsions were more stable when they contained BSA and EC and EGCG, but not ECG. The ovalbumin-ECGC emulsions were also reported to be more stable that the ovalbumi-EC.
The reason for the relatively poor activity of BSA-ECG in emulsion is not known, said the researchers.
"The catechins studied are polar water-soluble compounds, and it is known that oil-in-water emulsions are more poorly stabilized by polar water-soluble antioxidants than are less polar antioxidants, such as tocopherols, which are present at the oilwater interface, where they are more effective," they said.
"Since BSA is known to be surface active, as shown by its ability to stabilize emulsions, the probable mechanism leading to the increase in antioxidant activity of emulsions containing BSA and catechins is that BSA binds the antioxidant and transports it to the oilwater interface, where it is highly effective at reducing the rate of oxidation."
Only recently the potential of green tea as a luxury flavour was reported by FoodNavigator.com. Japanese firm, Aiya, was at HIE in Frankfurt recently to promote its range of macha tea ingredients, which it believes has huge potential in a number of food categories.
The Japanese firm, which has its European head office in Vienna, was promoting macha primarily as a luxury tea flavour, or in other words a premium food and beverage ingredient.
Macha, which is used in Japanese tea ceremony and has been drunk for centuries, is different from traditional tea in that it is not infused but ground. It is also grown differently, and is noticeably more expensive.
Source: Food Chemistry Published on-lime ahead of print; doi: 10.1016/j.foodchem.2006.06.067 "Albumin causes a synergistic increase in the antioxidant activity of green tea catechins in oil-in-water emulsions" Authors: M.P. Almajano, M.E. Delgado and M.H. Gordon