Section 1: Overview

GUM ARABIC'S (GUM ACACIA'S) JOURNEY FROM TREE TO END USER

O.H.M. Idris and G.M. HADDAD

1 INTRODUCTION

Gum acacia, also known as gum arabic, is exuded from Acacia trees - mainly from Acacia senegal and Acacia seyal. The main gum acacia producing countries are Sudan, Nigeria, Chad and Senegal. Sudan is considered to be the world's largest producer of gum acacia. Gum acacia is the oldest and best known of all the polysaccharide plant exudates. It was used by the Ancient Egyptians in their paintings as an adhesive for mineral pigments and as an adhering agent to make flaxen wrappings for embalming mummies. It has been reported that the gum acquired the name "gum arabic" after its place of origin or port of export to Europe. A different explanation has been given by a Sudanese researcher who concludes that the name originates from a Sudanese tribe called Beja who live near the Red Sea and ports of export. In the Beja language, arabic means "transparent" and gum arabic refers to "excellent quality of gum that is transparent and free from bark".

Gum acacia (INS No. 414) is defined by FAO/WHO Joint Expert Committee on Food Additives (JECFA) in FNP 52 Add 7 (1999) as: "Gum arabic is a dried exudate obtained from the stems and branches of Acacia senegal (L.) Willdenow or Acacia seyal (fam. Leguminosae)". Acacia gum is defined by the European Pharmacopoeia 6.8 as: "Air- hardened, gummy exudates flowing naturally from or obtained by incision of the trunk and branches of Acacia senegal L. Willdenow, other species of Acacia of African origin and Acacia seyal Del.". United States Pharmacopoeia Official Monograph for NF26 (USP 31) defines gum acacia as: "Acacia is the dried gummy exudates from the stems and branches of Acacia senegal (Linné) Willdenow or of other related African species of Acacia (Fam. Leguminosae)". The Japanese Official monograph for part II / Powdered Acacia (JP XIV) defines gum acacia as: "Acacia is the secretions obtained from the stems and branches of Acacia senegal Willdenow or other species of the same genus (Leguminosae)".

Gum acacia is a highly heterogeneous complex polysaccharide consisting of galactose, arabinose, rhamnose, glucuronic acid and 4-O-methylglucuronic acid. The carbohydrate composition can vary, depending on the location, the age of the tree and site of tapping, and from season to season. Also, gum acacia consists of a small amount of protein (~0.8% with A. seyal ~2.0% with A. senegal gums), which forms an integral part of the gum structure. Gum acacia has been fractionated by hydrophobic chromatography into three principle fractions, which are Arabinogalactan (AG), Arabinogalactan-protein (AGP) and glycoprotein (GP).

The physicochemical and nutritional data for A. senegal and A. seyal gums are shown in Table 1. The data shows that A. senegal gum has a higher potassium content and emulsion capacity compared to A. seyal gum, while A. seyal shows a slightly higher calcium content and slightly lower viscosity and ash content. Specific optical rotation is one of the most important quality control tools used by the gum acacia suppliers and manufacturers to differentiate between both gums.

2 TAPPING

The weather (temperature) has a considerable influence as to when the gum season starts. If the weather is hot enough directly after autumn (October / November), the leaves will start falling, which is a signal to start tapping the A. senegal trees. The traditional tapping tool was an axe, but modern practise in the Sudan is to use a sharp spear (Sonke) to pierce through an upper branch (or stem) just beneath the bark (not to damage the cambial zone). The spear is then moved up and down along the length of the branch to remove part of the bark and expose an area of the cambial layer about 30 cm long and 5 cm wide. Expert tappers always ensure that no peeled bark is left on the tree, which could stick to the gum nodules. A. seyal trees normally exude the gum naturally although recently farmers have started to tap these trees also.

3 GUMMOSIS

The gum oozes from the stems and branches of the tree within a zone between the inner bark and the cambial zone, when the tree is subjected to stress conditions such as drought (A. seyal) and wounding. The trees must be 5 years or older for gum production to occur, suggesting therefore that gum production is in direct competition with tree growth. Gum exudes in nodule form up to 60 mm in diameter, which then dries in the sun. A. senegal forms hard, glass like lumps which are transparent to amber in colour. A. seyal forms fragile fragments which are amber to brown in colour. A number of hypotheses have been suggested for the biosynthesis of the gum, which is referred to as "gummosis". It was proposed that gummosis is a pathological process resulting from a bacterial or fungal infection of the injured tree, and efforts were made to isolate the bacteria and moulds that may be involved in the formation of the gum. Others believe that gummosis is part of the normal plant metabolism or directly related to starch metabolism and the gum acacia is produced in response to physiological disturbances induced by stress.

4 COLLECTION

The exudate gum hardens in the sun to form nodules, which are manually collected after 4-6 weeks from tapping, followed by 3-5 collections every 2 weeks, depending on the weather conditions and the health of the tree.

5 DRYING

Drying the gum after collection is a very important process to prevent fermentation and forming of agglomerated gum nodules (jammed gum). The gum is spread evenly to dry for a few days. It is recommended to avoid direct sun drying as the gum nodules break into small fragments and siftings. Drying the gum under 45% shade will help in drying the gum nodules without compromising the hand picked selected (HPS) and lump quality gums.

6 MANUAL CLEANING

The crude gum is pre-cleaned manually from sand and bark using sieves and trays. This is traditionally done by women, who manually sort the gum according to the size of the lumps and remove foreign matter.

The processing of gum acacia can be split into three categories:

• Mechanical processing

• Spray-Drying

• Agglomeration

7.1 Mechanical processing

Figure 1 shows the mechanical processing, which involves cleaning, grading and generating a range of different particle sizes from the original lump gum. The mechanical processing also includes the Kibbling stage - a grinding process which breaks up the gum nodules into various specific particle sizes of approximately 0.5 mm to 6.0 mm. The Kibbling process increases the surface area of the gum particles and this allows the gum to dissolve faster in water compared with the lump gum. Mechanical powder can also be produced by further milling the kibbled gum, using mostly pin mills. Although the...