Tea film formation in artificial tap water

Abstract

On the surface of tea infusions, the formation of a transparent, shiny film which cracks upon disturbance can often be observed. This study aims to determine how water composition, tea varieties, and tea additives impact the formation and properties of tea film, often also called tea scum. The strength of the surface film, composed of polyphenols complexed with various ions from tap water, was investigated by interfacial rheology. Microscopy and ellipsometry were used to investigate structure and thickness of the adsorption layer, respectively. We find that green tea forms more visible layers than black tea in soft and moderate artificial tap water, but in these same waters, black tea demonstrated greater surface strength. In hard artificial tap water, green tea demonstrated greater surface strength than black. No visible layer nor surface strengthening was observed on rooibos tea. Brews in hard artificial tap water formed brittle films for green tea, fracturing at strains one order of magnitude lower than in soft or moderate. Despite large variations in film strength, black tea at all water hardness levels tested formed a film with 20 nm thickness. In black tea an increased resilience to deformation was found when adding β-casein, a protein found in milk.

Fig. 1. (a) Time sweeps showing storage and loss moduli, closed and open circles respectively, for 3 g of black tea brewed in 150 mL of hard, moderate, and soft ATW at 60 °C for the 6 minute brew. Photographs of film fragments after geometry removal in black tea brewed with (b) soft ATW and (c) hard ATW.

Fig. 2. (a) Time sweeps showing storage and loss moduli, closed and open circles respectively, for 3 g of green tea brewed in 150 mL of hard, moderate, and soft ATW at 60 °C for the 6 minute brew. Visible tea surface film broken into pieces on green tea brewed with soft ATW (b), moderate ATW (c), and hard ATW (d).

Fig. 3. Amplitude sweeps showing storage and loss moduli, closed and open circles respectively, for 3 g of tea brewed in 150 mL of hard, moderate, and soft ATW at 60 °C for the 6 minute brew: (a) black tea and (b) green tea.

Fig. 4. Plateau values of storage and loss moduli for black (black symbols) and green (green symbols) tea films formed in soft, moderate, and hard ATW plotted against CaCO₃ equivalent concentration. Storage modulus is represented with solid symbols and loss with open symbols.

Fig. 5. Microscopy for black (left image) and green tea (right image) films from hard ATW brews. Scale bars: 100 μm.

Fig. 6. (a) Time and (b) amplitude sweep for hard and moderate ATW black tea brews when citric acid is added. Circles represent the ATW brews, and diamonds indicate added citric acid. Solid symbols represent storage modulus and open symbols represent loss modulus. All teas brewed with 150 mL ATW and 3 g tea leaves. Citric acid, 4.5 mL, added to the trials indicated.

Fig. 7. (a) Time and (b) amplitude sweeps for hard, moderate, and soft ATW black tea brews when NaCl is added. Solid symbols represent storage modulus and open symbols represent loss modulus. Brews of 150 mL with 3 g black tea leaves have 1.2 g NaCl added and dissolved before the 6 minute brew time.

Fig. 8. (a) Time and (b) amplitude sweeps for hard ATW black tea brews when β-casein or β-lactoglobulin is added. Solid symbols represent storage modulus and open symbols represent loss modulus. Brews of 150 mL with 3 g black tea leaves have proteins added before the 6 minute brew time. (c) Small pieces of visible black tea film on a matte film on black tea brewed with hard ATW containing β-casein.

Fig. 9. Films observed after second rheological protocol run for hard ATW brews for black (a) and green (b) tea. Brews are made with 3 g leaves and 150 mL of hard ATW, a time sweep and amplitude sweep are run, a 1 minute stir is performed at 300 rpm, and the rheological sweeps are repeated. Photos were taken after bicone is removed the second time.

Fig. 10. pH of (a) black tea, (b) green tea, and (c) rooibos tea brewed with hard, moderate, and soft ATW and Milli-Q water. The pH of black tea with addition of (d) NaCl and (e) citric acid are shown with hard, moderate, and soft ATW. Plots (a)–(e) are measured at the beginning and end of the experiment and after the 6 minute brew.

Fig. 11. Refractive indices over the wavelength range 370–1690 nm are shown for Milli-Q, Hard ATW, and tea brews prepared with hard ATW. The tea brews were measured at 1 and 2 hours and substrate measurements come from wiping the film off after the 2 hour measurement and the cleared surface being measured.