How to Enhance the Aroma of Natural Liquid Tea? A Detailed Explanation of the Key Technology

Tea fresh leaves contain large amounts of glycosidic substances with monoterpene alcohols and aromatic alcohols as aglycones. During tea processing, these substances are catalytically hydrolyzed by endogenous glycosidases to release volatile aglycones, which are an important source of the floral and fruity aromas of various teas. It has been confirmed that in fresh tea leaves there exist β-glucosides, β-primeverosides and a small amount of other types of diglycosides, with cis-3-hexenol, linalool, methyl salicylate, geraniol, benzyl alcohol and 2-phenylethanol as the main aglycones. After fresh tea leaves are picked, when being processed into various finished teas, they all undergo spreading to varying degrees, combined with different processes such as fermentation, fine manipulation, rolling and drying, to form the aroma quality of the tea.

Using dried tea as raw material to produce various liquid teas is one of the important aspects of tea deep processing, and the aroma enhancement technology for liquid tea has always been a difficult problem troubling the liquid tea processing industry. In liquid tea produced by ordinary processes, because the raw tea undergoes high-temperature thermal extraction during processing, the temperature of the product is high and the duration is long, and the free aroma components have already volatilized away during processing, so the aroma is extremely faint, or even nonexistent, and synthetic flavorings must be added for aroma blending. On the other hand, during the tea extraction process, most of the bound-form aroma remains in the tea residue, and a portion enters the tea infusion but exists in bound form and cannot be utilized.

After the tea residue remaining from tea extraction is mechanically crushed, acid-adjusted, and heated under sealed conditions for hydrolysis, the hydrolysate is then recovered by distillation to obtain aroma-containing steam, which is returned to the liquid tea, thereby producing highly aromatic liquid tea.

(1) Processing method

Using commercially available grade-3 pan-fired Mee tea (produced in Huangshan City, Anhui) as raw material, the following process is used to process highly aromatic liquid green tea:

To the tea residue remaining after extraction of the raw tea, add 50% water; after mechanical crushing, add food-grade citric acid to adjust the acid until the pH value is less than 3; hydrolyze in a sealed reaction vessel at 80°C for 3 hours; the volatiles produced by hydrolysis are then recovered by distillation to obtain aroma-containing steam; after the aroma-containing steam is cooled through a condenser tube, it is returned to the liquid tea to obtain natural highly aromatic liquid tea.

(2) Results and analysis

Sensory evaluation was conducted on ordinary liquid tea and highly aromatic liquid tea. The results showed that, compared with ordinary liquid tea, the highly aromatic liquid tea changed little in liquor color and taste, while its aroma changed from a remnant hint of roasted aroma to a rich, high, brisk tea aroma with a floral and fruity note, achieving a natural highly aromatic flavor quality.

The results of gas chromatography analysis showed that the natural highly aromatic liquid tea was far higher than ordinary liquid tea in both the types and amounts of aroma substances. The natural highly aromatic liquid tea had 20 aroma types detected, with an essential oil content of 35.519 micrograms per 500 milliliters; while ordinary liquid tea had only 7 aroma components detected, with an essential oil content of only 4.853 micrograms per 500 milliliters.

(3) Discussion

Various teas (green tea, black tea and oolong tea) are all rich in bound-form aroma existing in glycosidic form. Although these bound-form aromas are water-soluble substances, their extraction rate in hot water is very low, which may be related to the state in which the bound-form aroma substances exist in tea cells. During the tea extraction process, most of these bound-form aromas still remain in the tea residue, and a portion enters the tea infusion but exists in bound form and cannot be utilized.

Research shows that the bound-form aroma of tea is mainly glycosides with cis-3-hexenol, linalool and its oxides, geraniol, methyl salicylate, nerol, benzaldehyde, benzyl alcohol and phenylethanol as aglycones. After these glycosides are hydrolyzed and released, the aglycones become free-form tea aroma components, which are an important material basis for the floral and fruity aroma presented by high-grade tea and can significantly improve the aroma quality of liquid tea.

The bound-form aroma in tea is mainly glycosidic substances. The glycosidic bond belongs to an acetal structure and is easily catalytically hydrolyzed by dilute acid to generate the corresponding sugar and aglycone (free-form tea aroma). During liquid tea processing, after the tea residue remaining from tea extraction is mechanically crushed, it is acid-adjusted with citric acid, hydrolyzed at high temperature in a sealed reaction vessel, and the hydrolysate is then recovered by distillation to obtain tea aroma-containing steam. After the tea aroma-containing steam is cooled, it is returned to the liquid tea to obtain natural highly aromatic liquid tea. Since citric acid is not volatile, it will not enter the tea aroma-containing steam along with the volatiles during distillation, and therefore will not cause a change in the acidity of the highly aromatic liquid tea.

By adopting this technology, the production cost is low, there is no harmful residue, the operation is simple, and it is suitable for industrial production. The liquid tea product produced has a natural tea flavor, does not require artificial aroma blending or flavoring, and its aroma quality is significantly better than that of ordinary liquid tea.