FOOD CHEMISTRY

Module 13. Beverages

Lesson 30
DETERMINATION OF TOTAL ASH, ALKALINITY OF WATER SOLUBLE ASH AND WATER EXTRACTIVES IN TEA AND DETECTION OF CHICORY IN COFFEE POWDER

30.1 Problem to determine the Water Extractives in Tea Leaves

Instant tea processing begins with extraction of tea (mostly black tea). The extract represents approximately 85% of the soluble solids in the tea leaves. Tea extract contains both caffeine and tannins (polyphenols) which are soluble in hot water, but some tannins form haze in cold water. Some “instant tea” manufacturers add dextrin in the extract prior to drying. Approximately 30% of the dry weight of tea leaves is made up of polyphenols formerly known as “tea tannins”. The sugars found in fresh tea flush are glucose, fructose, sucrose together with traces of raffinose and stachyose. Fresh tea flush contains organic acids namely, oxalic, malic, citric, succinic and isoditric (in decreasing order). Copper is the main important mineral and is found as a constituent of tea catechol oxidase (12 to 15 ppm).

Apparatus

1. Conical flasks
2. Evaporating dish
3. Air condenser
4. Desiccator
5. Hot air oven
6. Water bath
7. Volumetric flasks (250 ml)
8. Analytical balance

Principle

Ground tea sample is suspended in hot water and extraction is carried out by slow boiling using an air condenser for preventing loss of volatiles. This boiled tea suspension is filtered and an aliquot of it is dried in two stages:

1) Partial drying on boiling water bath
2) Complete drying in a hot air oven

The dry solids obtained are expressed as percentage water extractives of tea.

Procedure

1. Take 2 g of ground tea sample (dried in oven at 100°C for 6 h, passing through sieve no. 30 – AOAC) in 500 ml conical flask.
2. Add 150 ml hot water and connect it with 75 cm long tube (as air condenser).
3. Reflux over a low flame for 1 h rotating occasionally (heat very slowly to prevent evaporation losses).
4. Cool and dilute to 250 ml volume. Mix thoroughly and filter it.
5. Transfer 50 ml of the aliquot to a weighed evaporating dish and evaporate to dryness on water bath.
6. Cool in desiccator and weigh accurately.
7. Repeat this process of heating for 30 min., cooling in a desiccator and weighing until the loss in mass between two successive weighings is less than 1 mg.
8. Record the lowest mass and determine the tea extractives.

Observations

1. Weight of tea sample taken = W g
2. Final volume of tea extract = V ml
3. Aliquot taken for drying = X ml
4. Weight of empty dry evaporating dish = W₁ g
5. Weight of dish + dry tea extract = W₂ g

Calculations

Results: The water extractive content in the tea leaves is __________%

30.2 Problem: To Determine the Total Ash and Alkalinity of Water Soluble Ash in Tea

The ash content in many tea ranges between 5 and 6%. In fresh tea the proportion of ash may be as high as 10%. As per BIS standards, A tea must contain 4 to 8% (by mass) total ash and minimum 40% of which must be (water soluble ash in spent tea, however, the ash drops) below 3% and the ash itself is high in calcium. In genuine tea about 50% of the ash is water soluble ash and is found in the range of 3 to 3.5% whereas in exhausted teas (spent) it drops to about 0.5%. The major element in tea minerals is potassium which is half the total mineral content. Thus, the soluble ash obtained from spent tea will have lower alkalinity value.

Apparatus

1. Burette
2. Funnel
3. Conical flasks
4. Beakers
5. Silica dish/crucible
6. Desiccator
7. Muffle furnace
8. Analytical balance

Reagents

1. HCl (0.1 N)
2. Methyl orange indicator (0.1%)

Principle

Total ash is obtained by igniting tea sample in muffle furnace until it is free from carbon. The ash obtained is mixed with water and heated to boiling and filtered through ash less filter paper. The filtrate and washings are combined together and the alkalinity of soluble ash is determined by titrating against 0.l N HCl using methyl orange as an indicator. The alkalinity of soluble ash is expressed as number of ml of 1 N acid per 100 g sample.

Procedure

A. For total ash

1. Take 5 g of sample in a tarred silica dish/crucible.
2. Heat it at 100°C in an oven until moisture is expelled.
3. Incinerate it at as low a temperature as possible.
4. Place the dish in furnace at 525 + 20°C and leave until white ash is obtained (30 min).
5. Transfer the dish in a desiccator for cooling and weigh the dish.
6. Express the results as % total ash

B. For alkalinity of water soluble ash

1. Add 10 ml of water to the ash obtained as above and heat almost to boiling.
2. Filter through ash less filter paper and wash the residue with hot water until the combined filtrate and washings measure about 60 ml.
3. Cool the filtrate and washings and titrate with 0.1 N HCl using methyl orange indicator.
4. Express alkalinity of soluble ash as ml of 0.1 N acid required for 100 g of sample.

Observations
A. For total ash

1. Weight of silica dish = W₁ g
2. Weight of silica dish + sample = W₂ g
3. Weight of silica dish + ash = W₃ g

B. For alkalinity of water soluble ash

1. Normality of HCl = N
2. Titre value = V ml

Calculations

Results

The ash content in the given tea sample is ________%
The alkalinity of its water soluble ash is__________ ml of 1N HCl per 100 g sample.

30.3 Problem: To Determine the Presence of Chicory in Coffee Powder

Chicory is blended in coffee mainly for reducing the cost. As per BIS standards the roasted coffee – chicory mixture shall contain caffeine not less than 0.6 % and the aqueous extract shall be 35 to 50 % (on dry matter basis). Under PFA rules the instant coffee powder containing chicory are required to be labelled as “mixed with chicory” or “blended with chicory”.

Principle: There are a few methods used for detecting the presence of chicory in coffee powder which includes:

1. Microscopic method
Under the microscope the chicory shows numerous thin walled parenchymatous cells, lactiferous vessels and sieve tubes with transverse plates. There are also present large vessels with huge well defined pits. The coffee grain has an altogether different characteristic structure, enabling therefore, an easy distinction between the two.
2. Chemical method
Roasted coffee contains 1.9 to 2.6% reducing sugars as compared to 25 to 27% in chicory. Thus the cupric reducing power of the coffee extract could be used for concluding the presence of chicory. The strained coffee extract is treated with excess of lead acetate and the precipitates are allowed to settle. A colourless supernatant indicates the absence of chicory.

Procedure

1. Boil 10 g of the coffee powder with 250 ml of water.
2. Strain and add an excess of basic lead acetate.
3. Allow the precipitates to settle.
4. Note the presence of colour in the supernatant.
5. A coloured supernatant indicates the presence of chicory.