X-ray diffraction study of kanwa used as active ingredient in achu soup in Cameroon

In this study, x-ray powder diffractometry (XRPD) technique was used to identify the mineral constituents of kanwa; an earthy material widely used as active ingredient in achu soup and other vegetable soups in Cameroon and several other West African countries. Results depicted trona (Na3H (CO3)2.2H2O) to be the main mineral constituent. It reacts with palmitic and oleic acids in palm oil and both sodium palmitate and sodium oleate are produced which are responsible for the unique, characteristic and cherished taste of achu soup.


INTRODUCTION
Kanwa is a local earthy material used widely in West Africa and particularly in Cameroon, Ghana and Nigeria, as a cooking ingredient in vegetable soups.In the Northwest Province of Cameroon, it is used to prepare the most cherished achu soup; the mainstay soup made from water, palm oil and dried beef.Palm oil consists mainly of 40 -50% oleic acid and 35 -45% palmitic acid.Other compounds in palm oil in small to minute quantities are linoleic acid, stearic acid, myristic acid, arachidic acid, lauric acid and linolenic acid.It also contains > 500 g Kg -1 of carotene; which is its main colouring agent.The kanwa is dropped into the boiling water and palm oil.As it boils, air bubbles are released.The soup is eaten with pounded cocoyam.Although kanwa is widely used in Cameroon and several other West African countries, no documented study has been reported of its mineralogy.In this work, Xray diffraction technique was used to identify the mineral constituents of kanwa and an attempt was also made to elucidate on essential chemical reactions of it and palm oil.

MATERIALS AND METHODS
Five samples of kanwa purchased from vendors in five different towns (Bamenda, Limbe, Kumba, Mutengene and Tiko) in Cameroon were obtained for the identification of minerals contained in them.The samples were gently crushed into powder and the minerals identified with the aid of a Philips PW 3710 XRPD X-ray diffractometer system, operated at 40 kV and 45 mA, having a Cu-K radiation and a graphite monochromator.A PW 1877 Automated Powder Diffraction, X'PERT Data Collector software package was employed to capture raw data and a Philips X'PERT Graphics and Identify software package was used for qualitative identification and semi quantitative analyses of the minerals from both the data and patterns obtained by scanning at a speed of 1°2 / min.Samples were scanned from 2°2 to 70°2 .The interpreted results were compared with data and patterns available in the Mineral Powder Diffraction File, data book (International Centre for Diffraction Data, 2001).

RESULTS AND DISCUSSION
The main mineral constituent identified is trona, Na 3 H (CO 3 ) 2 .2H 2 O (Figure 1), which is basically sodium bicarbonate with water of crystallisation.Traces of halite (NaCl) and quartz (SiO 2 ) were also found to be present.Trona is sodium bicarbonate formed as a non marine evaporate.
Palmitic and oleic acids are carboxylic acids.More specifically, palmitic acid ((CH 3 (CH 2 ) 14 COOH or simply C 15 H 31 COOH) is a C 16 saturated carboxylic and oleic acid ((CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH or simply C 17 H 33 COOH) is a C 18 mono-unsaturated carboxylic acid denoted as: (C18:1), with one double bond in the carboxylic acid.These acids are generally represented as RCOOH (carboxylic acids) and they dissociate to give carboxylate ion and a proton as shown in Equation 1: In this vein, NaHCO 3 (sodium bicarbonate) which constitutes trona reacts with carboxylic acids contained in the palm oil as follows: Where RCOONa is sodium carboxylate.Considering palmitic acid, its reaction with NaHCO 3 is shown in Equation 3: Then the H 2 CO 3 decomposes to H 2 O and CO 2 is given off.

Conclusion
Trona which is basically sodium bicarbonate with water of crystallisation has been identified to be the main constituent of kanwa.It reacts with the weak carboxylic acids in palm oil producing CH 3 (CH 2 ) 14 COONa and CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COONa.Being weak acids (RCOOH) reacting with a weak base (NaHCO 3 ), the reaction may be slow and is therefore quickened by the hot boiling water as CO 2 is given off as air bubbles.
Both the palmitate and oleate of Na give the unique, characteristic and cherished taste of achu soup.For West Africans and particularly Cameroonians in diaspora who would like to continue to enjoy their traditional dishes and particularly achu soup but cannot readily obtain kanwa, pharmaceutical grade NaHCO 3 can possibly be used.