Processing of soluble tea solids

This invention concerns a method of altering the colour characteristics of a tea composition, including treating a tea infusion that contains cold water soluble tea solids but is substantially free of cold water insoluble tea solids, with an oxidising agent in a reaction vessel at a temperature and pressure in excess of the ambient temperature and pressure.

FIELD OF THE INVENTION 
The invention relates to a method of treating cold water soluble solids 
derived from tea leaves. 
BACKGROUND OF THE INVENTION 
When hot aqueous infusions of black leaf tea are prepared, it is found that 
the infusion comprises substances which are insoluble in cold water, which 
substances therefore tend to precipitate as the infusion cools. These cold 
water insoluble substances comprise tannin complexes (known as tea 
"cream"), and typically comprise 15-35% of the total tea solids present in 
the infusion. 
Black leaf tea infusions may be used to produce "instant" teas and other 
products which are preferably soluble in cold water. For this reason, it 
is known to separate the insoluble tea cream from the "decreamed" fraction 
(which is the term given to the cold water soluble materials after removal 
of the cold water insoluble cream). This is typically accomplished by 
centrifugation of the chilled (3.degree.-10.degree. C.) extract. The 
insoluble cream fraction represents a significant proportion of the tea 
solids in the infusion. Accordingly, to prevent the cream fraction (which 
contains desirable flavour components) going to waste, it is known to 
treat the cream fraction, in one of a number of ways, so as to render it 
soluble in cold water and then to recombine the solubilised cream with the 
decreamed fraction. Various treatments of the cream fraction of tea 
infusions are described, for example, in GB 1,311,255, GB 1,461,726, U.S. 
Pat. No. 3,787,590 and U.S. Pat. No. 4,156,024. In particular, U.S. Pat. 
No. 3,787,590 discloses a method of solubilising tea cream, the method 
comprising oxidation of the cream using hydrogen peroxide (H.sub.2 
O.sub.2) in an autoclave at 240.degree. F. (115.6.degree. C.) at 50 pounds 
per square inch (0.35 MPa), at a pH of 3.2. 
Further discussion is provided by N. D. Pintauro in "Tea and Soluble Tea 
Products Manufacture" (1977 Noyes Data Corporation, USA). For example, at 
page 96 is disclosed a "batch reactor method" for solubilising cold water 
insoluble tea tannins, the method comprising treatment with pressurised 
oxygen (2.1 to 14 kg/cm.sup.2, equivalent to 0.2-1.35 MPa) at a 
temperature of 71.degree. to 107.degree. C. and at a pH in the range 5.5 
to 7.5. Again, these conditions are not sufficient to significantly 
increase the solubility of oxygen in the aqueous mixture, and the use of 
higher pressures is generally to be avoided because of the cost of 
creating and maintaining the necessary pressure. Additionally, in the 
method disclosed by Pintauro, bleaching with hydrogen peroxide is taught 
to reduce the amount of colour formation. 
In contrast to the foregoing, very little is known about oxidation of the 
decreamed fraction of tea infusions, which contains the cold water soluble 
components. As the constituents of the decreamed tea are already soluble, 
there has been no motivation whatsoever to apply to decreamed tea those 
processes applied to the cream fraction for the purpose of solubilising 
cold water insoluble materials. 
It is an object of the present invention to provide a process, applicable 
to the decreamed fraction of a tea infusion, which serves to improve the 
colour characteristics of the mixture. 
SUMMARY OF THE INVENTION 
In a first aspect the invention provides a method of altering the colour 
characteristics of a tea composition, comprising treating an aqueous 
mixture comprising cold water soluble tea solids and substantially free of 
cold water insoluble tea solids, with an oxidising agent in a reaction 
vessel at a temperature and pressure in excess of the ambient temperature 
and pressure. 
The term "cold water soluble tea solids" as used herein is intended to 
refer to substances present in a hot water infusion prepared from tea 
leaves, which substances remain soluble when the infusion is chilled to 
3.degree.-10.degree. C. In contrast, "cold water insoluble tea solids" is 
intended to refer to substances present in a hot water infusion prepared 
from tea leaves, which generally precipitate when the infusion is chilled 
to 3.degree.-10.degree. C. 
Typically the aqueous mixture comprising cold water soluble tea solids will 
be derived or formed from the decreamed fraction of an aqueous tea 
infusion. 
Preferably the method of the invention will be applied in such conditions 
of temperature and pressure that distilled water would have a maximum 
capacity for dissolved oxygen, at equilibrium, of at least 0.5 
grams/liter. The maximum obtainable solubility of dissolved oxygen in 
distilled water under given conditions of temperature and pressure can be 
readily determined by reference to standard texts. Thus Perry's Chemical 
Engineering Handbook (Perry & Green 1984, Sixth Edition p.3 103, McGraw 
Hill) gives the values for maximum solubility of oxygen in water at 
elevated pressure for temperatures up to 100.degree. C. For temperautures 
in excess of 100.degree. C., reference may be made to the paper by Pray et 
al., (1952 Industrial and Engineering Chemistry 44, 1146-1151). The 
maximum dissolved oxygen concentration obtainable at equilibrium under the 
same conditions in aqueous mixtures comprising cold water soluble tea 
solids (such as are the subject of the process of the present invention) 
may vary somewhat from those values obtainable in distilled water. In 
particular the presence of other solutes in the aqueous phase, competing 
with oxygen molecules for hydration by water molecules, will tend to 
decrease the solubility of oxygen in the aqueous phase. However this is 
unlikely to cause a large reduction in oxygen solubility in the conditions 
of interest. The actual concentration of dissolved oxygen in the aqueous 
mixture cannot readily be determined--under the typical conditions of the 
process, standard methods of determining oxygen concentration (e.g. by the 
use of an oxygen electrode) are not feasible. 
The conditions employed in the method defined above are generally more 
extreme than those conventionally applied in the prior art to the 
processing of aqueous mixtures of cold water insoluble tea solids and 
result in a far greater maximum obtainable dissolved oxygen concentration. 
In general those skillled in the art would be motivated to avoid such 
conditions because of the energy costs in achieving the same. 
However, the present inventors have found that such processing has a highly 
desirable effect on the colour of mixture, which becomes much darker (i.e. 
less luminous) and more highly red coloured, than mixtures which have not 
undergone the process of the invention. The invention cannot be considered 
obvious because, as stated above, one skilled in the art would have no 
motivation to apply such a process to cold water soluble solids. In 
addition, the chemical composition of cold water soluble solids is very 
different to that of cold water insoluble solids, such that similar 
treatment of the two materials would not necessarily be expected to 
produce the same results. 
The preferred oxidant is oxygen. Use of high partial pressures of oxygen 
serves to increase the maximum capacity of the aqueous mixture for 
dissolved oxygen. Preferably conditions are such as to create a maximum 
capacity for dissolved oxygen (in distilled water) at equilibrium in the 
range of 0.5 to 5 grams per liter, more preferably 0.5 to 1.5 grams per 
liter, and most preferably 0.7 to 1.0 grams per liter. 
Preferably conditions are arranged (e.g. by the use of high partial 
pressures of oxygen and by the use of agitation) such that the actual 
concentration of dissolved oxygen in the aqueous mixture approaches the 
maximum obtainable at equilibrium under the selected conditions. However, 
it is quite possible that the system never attains equilibrium (e.g. 
because dissolved oxygen is consumed in oxidation reactions), such that 
the maximum obtainable equilibrium concentration of dissolved oxygen in 
the aqueous mixture is not reached. 
Those skilled in the art will appreciate that other oxygen-containing or 
generating substances may be used to give an equivalent oxygen solubility 
in the aqueous mixture. For example, a higher partial pressure of air or 
oxygen-enriched air may be used, or (less preferably) aqueous solutions of 
hydrogen peroxide may be added. Alternatively ozone, or other oxidising 
gas, may be used so as to give an "oxidising power" in the aqueous mixture 
equivalent to that generated by a maximum oxygen solubility of at least 
0.5 grams per liter. 
The temperature at which the method is performed is generally in the range 
60.degree.-160.degree. C. and preferably above 100.degree. C., 
conveniently in the range 100.degree. to 140.degree. C., preferably in the 
range 100.degree. to 120.degree. C., typically 116.degree. to 120.degree. 
C. 
It will be apparent from the foregoing, and those skilled in the art will 
appreciate, that increased temperature in a closed reaction system will 
increase pressure, and so tend to increase the amount of oxygen dissolving 
in the aqueous mixture. Under certain circumstances, it may be preferred 
to use an "open" system, whereby the concentration of a gaseous oxidising 
agent is held constant, whilst being passed through the reaction vessel at 
a given flow rate. Alternatively, the gaseous oxidising agent may be 
advantageously introduced in pulses. 
The reaction may be performed as a batch process (where the reaction vessel 
may be, for example, a stirred tank) or may be a continuous process 
(performed, for example, in a stirred tank or a conduit, such as a pipe). 
The method of the invention may successfully be performed on aqueous 
mixtures comprising suspensions of cold water soluble tea solids in the 
range 0.3-20.0% (w/v). Conveniently a concentration in the range 3-10% 
(w/v) may be selected, which gives a reasonable amount of tea solids 
without making the solution unmanageably viscous. 
Conveniently the process is performed at a pressure in the range 0.11 to 
4.0 MPa gauge, preferably 0.2 to 3.5 MPa gauge, and more preferably 0.3 to 
3.0 MPa gauge. 
Generally, the prior art teaches that oxidation of tea cream is performed 
at alkaline pH. In contrast, the process of the present invention may be 
performed at the natural (acidic) pH of the cold water soluble tea solids 
composition, although the pH of the composition may be varied, if desired, 
without adverse effect. 
The time taken to complete the reaction will of course depend in part on 
the reaction conditions used. Typically, the reaction will take between 10 
minutes and 1 hour, more normally 10-30 minutes. The reaction time may be 
shortened by the incorporation of other oxidising agents (e.g. ozone, 
H.sub.2 O.sub.2) into the aqueous mixture, either in a single batch or 
incrementally. 
Conveniently the cold water soluble solids treated in accordance with the 
present invention may be combined with solubilised tea cream. The 
resulting solution may optionally be concentrated and dried, typically by 
spray drying, to give a cold water soluble powder, which may be the basis 
of an instant tea powder. 
In the examples that follow, a 3% (w/v) suspension of tea solids was 
prepared in deionised water, starting from a freeze-dried powder prepared 
from an aqueous tea extract from which the cold water insoluble tea solids 
had been removed. This arrangement allowed for optimum reproducibility of 
experimental conditions and had the advantage of simplicity. In practice, 
on an industrial scale, it is envisaged that the aqueous mixture used in 
the process of the invention will be an aqueous decreamed tea extract, 
without having gone through an initial freeze-drying stage. The aqueous 
decreamed extract may conveniently be concentrated prior to processing 
according to the method of the invention. The examples below further 
illustrate the nature of the present invention.

EXAMPLES 
Method: 
A decreamed tea extract was prepared from a black tea in the following 
manner. Deionised water and black tea, at a water to leaf ratio of 10:1, 
were contacted in a 7 stage countercurrent continuous extractor, wherein 
the black tea had a residence time of approximately 10 minutes and the 
deionised water had a residence time of approximately 15 minutes. The 
extraction was carried out at 85.degree. C. (The deionised water extract 
of tea solids is referred to as an infusion of tea solids.) The infusion 
was then chilled to 5.degree. C. to precipitate the cold water insoluble 
tea solids, which were removed by centrifugation. The supernatant was 
freeze dried, to give a powder which can be used as a source of cold water 
soluble tea solids. 
The powder described above was used to prepare an aqueous solution 
containing 3% (w/v) of decreamed black tea solids. The solution was added 
to a Parr bench top mini reactor model number 4562, which is capable of 
operating safely under high pressures and of maintaining a desired 
temperature. The solution of tea solids was then placed into the Parr 
reactor, the reactor sealed and the vessel was pressurized to between 1.9 
and 2.2MPa gauge with oxygen. The vessel was then heated to the required 
temperature between 70.degree. C. and 120.degree. C. using an electric 
mantle heater. As a result of heating, the oxygen partial pressure within 
the reactor increased to between 2.1 and 2.7 MPa gauge at the reaction 
temperature, so as to obtain a constant maximum oxygen solubility of 0.7 
grams/liter at the different reaction temperatures. The reaction was 
allowed to proceed for 15 to 30 minutes, after which time the reactor was 
cooled to between 80.degree. C. and 90.degree. C., the pressure within the 
reactor was released and the solution of tea solids collected. 
The resulting treated cold water soluble tea solids were then dried, giving 
a powder which was instantly soluble in water and was found to have the 
organoleptic properties desirable for an instant tea powder for use in 
beverages with an acidic pH. 
The colour assessment of the supernatant from the centrifugation process 
was made using a Minolta CT-310 instrument using illuminant C, a 2.degree. 
observer, a 1 cm pathlength transmission cell and the results are based on 
the CIE 1976 L*a*b colour space (see International Standards Organisation 
ISO! standards 7724-1, 7724-2 and 7724-3). All samples for colour 
analysis were measured at pH 3.7 and a solids concentration of 0.32% 
(w/v). The results for reactions performed at three different temperatures 
are shown in Table 1. 
Those skilled in the art will appreciate that the absolute pressure used in 
a given system will depend on the oxidising power of the gaseous oxidising 
agent used. Where the oxidising agent is used as a source of oxygen, this 
will depend on the partial pressure of oxygen in the gas. For example, to 
achieve a maximum oxygen solubility of 0.7 g/l in the system detailed here 
requires: oxygen gas at partial pressures of 1.9 to 2.8 MPa gauge, whilst 
use of air would require partial pressures of 9.5 to 14.0 MPa gauge. 
TABLE 1 
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The effect of heating a black tea infusion of 3% 
(w/v) solids in the presence or absence of an increased 
maximum oxygen solubility at the native pH, on the 
measured colour properties at pH 3.7 and 0.32% (w/v). 
Maximum 
Dissolved 
oxygen 
Temp concentration 
(.degree.C.) 
(g .multidot. 1.sup.-1)* 
L a b 
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70 0.7 76.7 7.5 65.7 
100 0.7 70.4 11.9 66.0 
120 0.7 61.7 20.2 68.9 
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*Theoretical determination based on data from Perry's Chemical Engineerin 
Handbook (R. H. Perry & D. Green, p 3. 103, 1984 sixth edition, McGraw 
Hill).