FOOD CHEMISTRY

Module 13. Beverages

Lesson 29 A
CHEMICAL CHANGES DURING PROCESSING – TEA

29.A.1 Introduction

Tea is second only to water in worldwide consumption. The scientific interest in tea is due in part to the unusual chemical composition of its leaf and the complex series of reactions that occur when these components are converted to those found in commercially available dry tea. “Tea and tea brew” refers only to the plant Camellia sinensis. Tea/tea blends are considered to be the tender shoots of tea shrubs consisting of young leaves and the bud. Tea is processed in a way traditional to country of origin.

Two most widely cultivated & important varieties of the species of tea are:

1. Camellia sinensis var. sinensis - with small leaf size (Chinese variety)
2. Camellia sinensis var. assamica - with bigger leaf size

29.A.2 Types of Tea

There are three types of tea depending on the processing of the tea leaves.

1. Black tea – Fermented type of tea in which flavanol oxidation is desirable.
2. Green tea –a dry product exhibiting the desirable twisted leaf appearance, but without flavanol oxidation. Because of the presence of unoxidized catechins, green tea beverage is yellow-green in color and more astringent than black tea.
3. Oolong tea - only partially oxidized, so that its appearance and chemical composition is somewhat intermediate between that of green and black tea.

29.A.3 Black Tea Manufacturing

The processing of tea flush to black tea comprises the following major steps:

1. Withering
2. Preconditioning
3. Rolling
4. Fermentation
5. Firing

29.A.3.1 Withering

All the steps in black tea manufacturing are designed to accelerate the oxidation of the tea flavanols and effectively control the reactions to produce the end products with optimized flavor and leaf & beverage appearance. Fresh leaf is brought to the factory after harvesting. It is then subjected to a withering step to reduce leaf moisture from 75-80% to 55-65%. Withered leaf is flaccid and can be worked further without excessive fracture. Leaf is spread in 8 to 10 cm layers on nylon netting occupying a high proportion of total factory space. Warm air from the tea-drying ovens is usually circulated across the beds to facilitate evaporation. Depending on the system used and the prevailing weather conditions, the withering process takes 6 to 18 h during which time, moisture content of leaf drops either to 60% called soft withering or up to 50% called hard withering. In this stage, the leaf acquires a “kid glove feel” (i.e. that of a goat baby). Significant chemical changes begin at this step. Cell membranes become more permeable and level of caffeine, amino acid, and organic acid increases. The end point of withering is usually determined by experienced observation of leaf texture or sometimes by checking the weight loss of an isolated portion of leaf.

29.A.3.2 Preconditioning

Preconditioning of the withered tea leaf consists of rolling the leaf 10-15 min. without application of any pressure. The purpose of tea conditioning is to impart the desired twist and compactness to the leaf and also to make available maximum amount of the polyphenol oxidase. Care is to be taken that destructing of tea leaves do not take place.

29.A.3.3 Rolling

The “rolling” or leaf maceration step is carried out in order to disrupt cell structure and allow contact between tea flavanols and tea polyphenol oxidase. The physical condition of the leaf mass must also facilitate oxygen availability. Orthodox rolling takes place on a rotating circular table 1 to 1.3 m in diameter that is equipped with battens. Rolled leaf has a coating of leaf juices on the surface and a moist, fluffy texture. It is desirable to prevent leaf temperature from rising above 35^oC during the maceration process to preserve quality. Additionally new reactions that are the important for black tea characteristics are themselves initiated by more intimate mixing of the leaf constituents. These reactions continue during rolling and are allowed to proceed to the desired extent in the fermentation stage of tea processing.

29.A.3.4 Fermentation

The oxidative process actually starts with the onset of maceration of withered leaf. At the end of the rolling process leaf is allowed to oxidize in 5 to 8 cm beds on trays in another fermentation room. It is desirable to keep temperatures below 30^oC. Oxidation at 15 to 20°C is said to improve flavor. Oxidation time depends on the temperature, degree of maceration, degree of wither, and the type of tea to be produced. It ranges from 45 min to 3 h. During this period tea leaves change colour from green to coppery red and aroma changes from grassy to sweet and flowery – an indication of end of fermentation. A slight loss of extractable caffeine at the level of 5-7 % is observed. Some of the reactions occurring during fermentation are enzyme catalyzed and of these the most important are:

1. Oxidation of tea flavanols by polyphenol oxidase - leads to development of colour, strength and quality of tea, brews.

2. Occurrence of reactions – leads to characteristic aroma of black tea

Chemistry of Tea Oxidation

Tea oxidation is generally referred to as “fermentation” because of the erroneous early conception of black tea production as a microbial process. Not until 1901 was there recognition of the process as one dependent on an enzyme catalyzed oxidation. This step and further reactions result in the conversion of the colorless flavanols to a complex mixture of orange-yellow to red-brown substances and an increase in the amount and variety of volatile compounds. Extract of oxidized leaf is amber-colored and less astringent than the light yellow-green extract of fresh leaf and the flavor profile is considerably more complex.

Flavanol Oxidation

The initial oxidation of the flavanol components of fresh leaf to quinone structures through the mediation of tea polyphenol oxidase is the essential driving force in the production of black tea.

Theaflavin formation

The oxygen-consuming reaction between a quinone derived from a simple catechin and a quinone derived from a gallocatechin results in the formation of a theaflavin. Theaflavins are orange-red substances that contribute significantly to the desirable appearance of black tea beverage. Although theaflavin content is considered to be an important criterion of black tea quality, it does not exceed 2% of final product (dry leaf) weight and therefore only accounts for 10%, at the most, of the original catechin content of the leaf. Theaflavin content increases initially as the oxidation process proceeds but falls off rapidly on prolonged oxidation. (Fig. 29.A.1: Structure of major catechins)

Bisflavanol formation

Bisflavanols are the compounds formed by the coupling of the quinines produced by the oxidation of epigallocatechin and epigallocatechin gallate. The three predicted bisflavanols have been found and characterized in black tea. They occur only in very small quantities in black tea, presumably because of high reactivity. (Fig. 29.A.2: The tea flavanols)

Thearubigen formation

Approximately 15% of the original fresh-leaf flavanol fraction is recoverable from black tea, and about 10% is identifiable in the form of theaflavins and bisflavanols. While some flavanol material may become tightly bound to the insoluble portion of the leaf, the major amount is found in a complex mixture of only partially resolved substances known as thearubigens because of their red-brown color. Theaflavin levels decrease during prolonged oxidation of tea leaf. In addition, it is possible that chlorogenic acid, theogallin, and the flavonol glycosides are also oxidized by the quinones and become included in the thearubigen fraction. Thearubigen should be considered a sensory parameter, useful in qualitative assesment of the progress of tea fermentation.

Fig. 29.A.3 Calculate oxidation

The enzymatic oxidation of flavanols via corresponding to o-quinones gives theaflavin (bright red colour and good solubility), bis-flavanols (colourless) and epitheaflavin acid (bright red colour & excellent solubility). Thus the theaflavins and epitheaflavins acid are the important derivatives that impart colour to the black tea. Apart from these, the theaflavins also impart the properties of quality & brightness of colour to tea brew. Another heterogeneous groups of compounds found in tea after enzymatic oxidation of flavanols is the thearubigins. This group of compounds is responsible for the characteristic reddish yellow colour and astringent taste of black tea extract. Thus, the thearubigins make an important contribution to colour, strength and mouthfeel of tea liquors.

29.A.3.5 Firing

It is the final stage of tea processing, when the rolled and fermented leaf having moisture of 45-50% is dried to produce black tea containing 3% moisture. It is usually accomplished by passing trays of fermented leaf through a hot-air dryer (inlet temperature of 87-90 °C and outlet temperature of 56-57 °C) in a countercurrent mode. The drying process takes about 20 min. Control over time –temperature during firing is crucial to final product quality. A noticeable effect of firing is the change of color brought by the transformation of chlorophyll to pheophytin, which imparts the desired black color to the dried product. Reduction of astringency of fermented leaf due to reaction of poly phenol with tea leaves proteins at elevated temperature. This leads to mellowing of flavour during firing. Much of the characteristic black tea aroma is generated during firing. Some low-boiling fresh leaf volatiles are lost but many new components are generated. Firing results in the loss of small amounts of caffeine through sublimation.