Glass, stainless steel, ceramic, porcelain, cast iron, terracotta, the choice of teapots is wider than ever today. Some tea lovers prefer certain materials to optimize their infusions. But what would be the best materials to use, scientifically speaking?
Different materials for even more designs, not easy to find your way around! In general, we buy our teapots according to the visual aspect. People who are really specialized in the tasting of natural teas use different materials, shapes, capacities often less than 20cl. Among the possible materials, Chinese terracotta or Yixing clay is particularly well known for tea preparation.
Yixing clay, a material as complex as it is fascinating.
Yixing clay is a sandstone clay that was originally produced in Yixing, China. This clay is said to be porous because it retains the fragrances of the infused tea leaves. In a clay teapot, only one family or even one kind of tea is brewed at a time. By dint of brewing the terracotta tea, a coloured film forms on the surface of the clay: the teapot is said to be brewed.
From the moment this film begins to form, tea lovers notice an intensification of the aromas of the infused tea, resulting in a more aromatic and finer tea. This is even more true when comparing a brewed Yixing teapot with porcelain or glazed ceramics, glass or stainless steel.
But scientifically, how does it work with terracotta?
In 2017, Liao and his collaborators took microphotographs of the surface of Yixing clay teapots using a scanning electron microscope to check the porosity of these teapots. This type of microscope makes it possible to zoom in on a microscopic scale (µm) and to carry out a 3D reconstruction of the surface of the visualized samples. They were thus able to observe that the Yixing earth was composed of microcavities.
But do these microcavities retain the molecules of the infused tea leaves? A publication dating from 2015 gives this answer. For this, they analyzed the molecular composition on the surface of earthen teapots (Zisha and Zhuni, two types of Yixing earth) brewed for more than 3 years with oolong teas. Each molecule has a "signature". It corresponds to the proportion of each component of the molecule. It is possible to detect each component of a molecule and its proportion in order to identify which molecule we are dealing with. The aim: to detect which types of molecules are located on the surface and therefore in the cavities of the clay. The authors of this work showed that the molecules retained on the surface of the clay were the lipids present on the surface of the leaves. These lipids are known to bind to other molecules present in tea leaves such as the molecules that give the fragrance of a tea or the tea polyphenols, which are true antioxidant molecules.
But is it still necessary to prove that the Yixing soil would retain more polyphenols and odorous molecules? The Liao team's publication answers this question for oolong tea. They compared the composition of the oolong tea infusion in different teapots: two made of Yixing clay (Zisha and Zhuni), one made of ceramic, one made of glass, one made of stainless metal and one made of plastic. Each teapot had a volume of 18cl, an equivalent diameter and an infusion temperature maintained at 95°C for 5min. To have strictly the same infusion conditions, they first infused 20g of oolong in a litre of purified water for 5min at 95°. They then transferred 10cl of this infusion into each teapot for 5min before removing it for analysis.
Using analysis techniques similar to those used to analyse the film formed on the surface of the clay, the authors found several differences between the materials:
- The total amount of catechins: Zisha > Zhuni > stainless metal >> ceramic = glass > plastic
- The amount of EGCG (most powerful catechin): Zisha > Zhuni > stainless metal >> ceramic = glass > plastic
- The amount of caffeine: Plastic > glass >> ceramic = stainless metal > Zhuni > Zisha
- Volatile compounds giving the smell of steam to the tea infusion (e.g. coumarin gives a vanilla smell): Despite the heterogeneity of some results, it seems that Yinxing clay releases less volatile molecules than other materials.
- Amino acids released by the leaves such as theanine: There is no difference between the materials.
- The potassium (gives a more bitter taste to the tea) released: Yxing clay releases less potassium than plastic and stainless metal which releases more.
Several conclusions can be drawn from these results. First of all, Yixing clay allows for an optimal infusion of oolong tea. The catechins would bind more to the surface of the Yixing clay, which gives a more concentrated infusion of antioxidants. The volatile molecules are absorbed by this clay which would make the tea's aromas less aggressive in the infusion. The resulting infusion would also be less bitter and less exciting due to the reduced presence of potassium and caffeine in the infusion.
Enamelled, glass and stainless steel materials: why are they used?
Unlike terracotta, these materials have a smooth surface: there are no pores and therefore no odour memorization. They can be used for all tea families. The difference between these materials will come from thermal conduction, which will dictate the speed of passage from hot (the liquid) to cold (the walls of the mug and the air around the mug). So how long will we keep the heat. If we take a mug made of glass, porcelain and steel, the liquid in the glass will cool faster than the porcelain and even faster than the metal because glass causes faster heat conduction.
This is why tea lovers prefer to use glass for heat-sensitive teas such as white or green tea or for a tea they want slightly less full-bodied like blends of black teas.
Enamelled porcelain is very versatile and can be used for all tea families as it does not cause too much heat loss.
Enamelled cast iron is a good alternative to porcelain as it is a good heat retaining material. However, be careful with the wear of the enamel because otherwise water will infiltrate the cast iron which will rust and give a "red water", toxic for the organism.
Stainless steel is perfect to maintain a tea for long infusions or for teas requiring high heat such as Moroccan green tea. However, the metal is decried in some scientific publications for releasing toxic heavy metals in the infusion such as lead. The authors of a 2013 publication showed that for a "Moroccan-style" infusion of Gunpowder green tea in a stainless steel teapot, the cadmium level was higher than the limits set by public health agencies for both moderate (30cl/day) and high (80cl/day) consumption. As for traditional brass teapots, the levels of lead and nickel were higher than the limits set.
Yixing clay would therefore be the best material to use to optimize each infusion made. It could be the same for other terracotta such as Tokoname clay or St Amand clay. Glass and enamelled materials could be used according to specific tea families and your personal taste. Metal and plastic would not be recommended for your daily infusions.
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