Production process of instant tea: concentration and dissolution
Concentration
In instant tea processing, concentration is a crucial step that impacts quality and is essential before drying. Its purpose is to remove moisture from the extract, producing a highly concentrated tea concentrate, which is then dried to produce solid instant tea.
Currently, the main methods for concentrating instant tea in industrial production are vacuum concentration and reverse osmosis membrane concentration. Vacuum concentration can increase the solids concentration of the extract to 30%-40%. However, due to the phase transition involved, it consumes significant energy. Furthermore, the relatively high temperature and long concentration process can easily lead to oxidative polymerization of heat-sensitive compounds in the tea extract, resulting in loss or changes in the product’s cold solubility. In particular, during vacuum concentration, the combined effects of high temperature and vacuum can significantly reduce the loss of volatile aroma components, resulting in a weak aroma in the instant tea.
To achieve excellent product quality and efficiency, vacuum concentration requires the highest possible vacuum and the lowest possible concentration temperature. Typical operating conditions are a temperature of 50-60°C and a vacuum of approximately 0.003 MPa. Vacuum concentration can be categorized as vacuum scraper concentration or centrifugal membrane concentration, depending on the type of concentration equipment.
With the widespread application of membrane technology, reverse osmosis membrane concentration has become widely used in instant tea processing, significantly outperforming vacuum evaporation in terms of product yield, cold solubility, clarity, and flavor quality. This is because reverse osmosis membrane concentration exhibits advantages such as no phase change, can be performed at room temperature, and requires a short processing time, minimizing the loss and change of heat-sensitive and volatile substances in the feed-liquid system.
However, the solids concentration of the concentrate using reverse osmosis membranes is generally around 20%. The efficiency and quality of reverse osmosis membrane concentration are related to the operating temperature and pressure. Generally, higher concentration temperatures and pressures increase concentration efficiency. However, excessively high temperatures can reduce the aroma and flavor quality of the concentrate, and excessively high operating pressures can shorten membrane life. Therefore, in industrial production, the suitable operating conditions for reverse osmosis membrane concentration are: concentration temperature 35~40℃, pressure 3.0 MPa.
Dissolution
The first step in instant tea processing is generally to extract the tea leaves with hot water. As the extract cools, it gradually becomes turbid, eventually forming a milky substance known as tea cream. This is a complex and colloidal substance formed by the interaction of tea polyphenols, their oxidation products, caffeine, and other tea components, accounting for approximately one-third of the tea extract. These substances are readily soluble in hot water but poorly soluble in cold water, and are a major factor affecting instant tea’s cold solubility, tea liquor clarity, flavor, and other qualities, as well as processing yield.
The purpose of transsolubilization is to convert tea curd into a soluble form or remove tea curd during the instant tea production process. While not undergoing transsolubilization has little impact on hot-soluble instant tea, it can affect the solubility and appearance of cold-soluble instant tea, limiting its application in tea beverage processing. Transsolubilization involves converting insoluble compounds into soluble forms based on the composition and physical and chemical properties of these complexes and colloidal substances, using methods such as enzymes, acids, or bases. This improves the instant tea’s solubility, especially cold solubility.
The formation of tea curd is primarily influenced by factors such as the concentration of tea polyphenols, their oxides, and caffeine, as well as temperature and pH. Therefore, transsolubilization methods are primarily categorized into concentration inhibition, alkaline transsolubilization, and separation agent transsolubilization.
1. Concentration inhibition method
The concentration inhibition method is to remove part of the tea cream or caffeine or tea polyphenols and their oxides, thereby reducing the appearance of “cloudiness after cooling”. The main methods include enzyme dissolution method, polyphenol removal method, non-polyphenol high molecular compound removal method, caffeine removal method, low temperature precipitation method, ethanol precipitation method, etc.
(1) Enzyme dissolution method: Tannin or papain is mainly added in the production of instant tea, because tannin can cut the ester bond of gallic acid on catechins, releasing free gallic acid, which can compete with theaflavins and thearubigins for caffeine, forming a water-soluble substance with a relatively small molecular weight; while papain can form a precipitate with tea polyphenols and remove part of the tea polyphenols, thereby achieving the purpose of dissolution. The enzymatic dissolution conditions are mild, the tea soup after dissolution is clear, and the dissolution efficiency is high, but the dissolution cost of enzyme preparations is relatively high.
(2) Polyphenol removal method: Using edible organic acid aqueous solution of polyamide, polyvinylpyrrolidone, and chitosan to remove tea polyphenols can inhibit the formation of tea curd, but this method will reduce the taste quality of instant tea.
(3) Non-polyphenol high molecular compound removal method: Using ultrafiltration, microfiltration, cross-linked dextran column, polyacrylamide column, anhydrous ethanol and other methods to remove protein and polysaccharide substances in tea soup, thereby solving the problem of low-temperature turbidity and precipitation of tea soup caused by substances such as protein, starch, and pectin.
(4) Caffeine removal method: Removing some caffeine can significantly improve the cold dissolution characteristics of instant tea. There are many methods for decaffeination, such as dichloromethane and chloroform extraction, column chromatography adsorption separation, membrane separation, supercritical carbon dioxide extraction, etc. Among them, the decaffeination method using organic solvents such as dichloromethane and chloroform reduces product safety, increases operation difficulty, and is costly, making it unsuitable for industrial production. At present, membrane separation and column chromatography are mainly used to remove caffeine in industrial production.
(5) Low-temperature precipitation method: The tea extract is cooled below 10°C and centrifuged to remove some tea curd, which can effectively improve the cold solubility of instant tea. However, some tea curd and its precursors still remain in the tea soup, and the solubility effect is not complete.
(6) Ethanol precipitation method: A certain amount of anhydrous ethanol is added to the concentrated tea juice to cause the macromolecular substances such as starch, pectin, polysaccharides, and proteins in the tea juice to condense and precipitate. After centrifugation to remove impurities, the product is distilled under reduced pressure and the ethanol is recovered. The flavor of the obtained product does not change significantly, but the solubility is greatly improved.
2. Alkali Dissolution Method
Hydrogen bonds are relatively weak covalent bonds. The hydroxyl groups released by alkaline dissociation can break the hydrogen bonds between tea polyphenols and caffeine, severing the tea curd molecules. Furthermore, the alkaline can compete with caffeine for the tea polyphenols and their oxides to form water-soluble salts, thereby reducing tea curd flocculation and achieving the purpose of dissolution. Commonly used alkalines include NaOH, KOH, and NH OH. In actual production, to minimize the side effects of alkaline dissolution and improve product yield, alkaline dissolution is often performed on the tea curd removed by centrifugation after cooling. During the alkaline dissolution process, an appropriate amount of oxygen is introduced, and acid is added to adjust the pH to improve the flavor quality of the instant tea.
3. Separation agent transfer method
(1) Polyphosphate method: Adding polyphosphates such as metaphosphate, tetrasodium pyrophosphate, and sodium hexametaphosphate to the tea soup can change the charge characteristics of turbid particles and prevent the formation of flocculent precipitation in the tea soup. In terms of clarity, sodium hexametaphosphate has the best effect.
(2) Stabilizer method: Adding xanthan gum, carrageenan, sodium carboxymethyl cellulose (CMC-Na), emulsifiers, maltotriose and other stabilizers to the tea soup can prevent the formation of precipitation in the tea soup.
4. Other Dissolution Methods
Adding 8-cyclodextrin to tea can effectively inhibit the formation of “cloudiness” after cooling, but its large molecular weight and poor solubility. Electrodialysis of black tea extract using an asymmetric alumina ceramic membrane can also improve the stability of the system and the quality of instant black tea, potentially enabling the production of pure, sediment-free black tea beverages. Furthermore, citric acid can compete with polyphenols for caffeine, improving product solubility. However, its strong sourness limits its usage.