Banana Starch & its Modifications- Boon for low glycemic foods and pharmaceutical industry


Starch, a white powder, without out any taste or odor becomes household name and not new to anyone today.  It has embedded its role for utility in divergent spectrum of society and industrial processes. With annual starch demand of around 70 million tons worldwide, it is used for thickening, gelling, imparting consistency, stability and varied applications in food processing industries. Food industries (bakery items, sausages, ice cream, etc.) consume more than 60% of starch whereas other sectors (pharmaceuticals, paper industry) uses 40% of starch. It is commonly extracted from a variety of plant sources, especially cereals (corn, rice, wheat), potatoes and cassava. 

With the growing demand, new sources are being explored for starch extraction and utility. Unripe green bananas are found to be an excellent reserve material, contains up to 80–90% starch (dwb). Bananas and plantains (Musa spp.) are growing in over 120 countries, with the production of 140 million tons from 8 million ha.  More than 20% of banana is getting rejected in the export line and cooking varieties like Monthan (ABB), Bontha (ABB) etc are under-utilized in the process line; opened up new vista of opportunities. Banana starch has a great potential, from its digestion and functional properties, to have application in processed foods and become a commercially viable starch product with high resistant starch content. 

Banana- A viable product for starch extraction
Banana is the second largest produced fruit after citrus, contributing nearly 16% of the world’s total fruit yield.  It accumulates a large amount of starch during fruit development which is converted into soluble sugars during ripening. Being a climacteric fruit, it is harvested when unripe but matured. Small and big bananas along with the damaged and spoiled fingers are culled in the pack house before shipping. 

Resistant starch (RS) which resists digestion in the human small intestine and they slow down the release of energy. The slowly digestible RS when consumed, it results in low glycemic index (GI). RS can also assist in blood glucose level and body weight regulation. The vital one among its properties is the ability of RS to act like a prebiotic, as it supplement the colonic microbiota in the large intestine. Fermentation of RS also promotes the growth of beneficial bacteria. RS is generally classified into four types. RS I is physically indigestible resistant starch, found in seeds or legumes and unprocessed whole grains. RS2 is inaccessible to enzymes due to starch conformation, as in green bananas. RS3 is formed when starch-present foods are cooked and cooled. RS4 have been chemically modified to resist digestion. 

Commercially available starches are usually modified to fulfill the needs of various applications. Native starch obtained from banana have restrictions, such as low thermal resistance, low shear resistance, low solubility, high viscosity; and tendency towards retrogradation leading to staling and syneresis, which may adversely affect the quality of food products. In these conditions, the importance of starch modification demands attention; and these limitations may overcome by modification of starches. For this the reactive sites (hydroxyl groups) of the amylopectin polymers have to become accessible to the reactants. 

Banana starch modification – Methods and Advantages
The two major composition of starch is mainly two different component likes amylose and amylopectin, both comprising of glucose units joined together by glycosidic linkage. Among them amylopectin is more soluble in water and is easier to digest than amylose. To perform a particular role, starch need to have certain specific requirements. Modified starch is widely used in a pharmaceutical company especially as binder.

Modified starches with expected desirable properties can be prepared by critically selecting a suitable modifying agent and method. Modified food starches can show better paste clarity, stability, increased resistance to retrogradation, and freeze–thaw stability, higher water binding capacity, improved swelling power and low solubility. Therefore, these starches can act as tailor made ingredient for its specified role. In general starches can be modified by physical, chemical and enzymatic methods, hence the same can also be adapted for banana starch modification.

ICAR- National Research Centre for Banana has developed simple methodologies for starch modification depending upon the purpose and need. Physical modification of starch is mainly applied to change its granular structure and convert native starch into cold water-soluble or small-crystallite form. These set of techniques are generally given more preference as these do not involve any chemical treatment that can be harmful for human use. The physical method of starch modification usually involves heat thermal modification, heat-moisture treatment, retrogradation, microwave irradiation and gamma irradiation. Heat thermal modification and heat-moisture treatment are widely adopted method for physically modifying starch for food purposes, followed by retrogradation. Microwaves and gamma radiation method of starch modification is least explored by food industries. However, starch modified by the above-mentioned methods were prove to have improved granular crystallinity, surface morphology, water holding capacity, emulsifying activity and gelatinization properties.

Chemical modification process involves the substitution of functional component of the starch molecule. The resulting modifications changes the physiochemical properties as well as alter their gelatinization, cooking characters, improve emulsion stability and retrogradation behaviour. The substitution of functional components depends on reactant concentration, time, pH, the presence of catalyst, types of substitution etc. Modification is generally achieved through derivation such as etherification, esterification, cross-linking and hyroxyphosporylation of starch and decomposition (acid hydrolysis and oxidization of starch). Acid-modified starches have vast applications in food, pharmaceutical, textile, paper while, oxidized starch can be used for biodegradable film production filming. Starch modified by esterification, can results in more fluid paste and better long term clarity. Interestingly, owing to cold water swelling, and yields reduced gelatinization temperature,, hydroxy propylated starches are  gaining interest in medicine.

A ground-breaking method of starch modification is by using enzymes, which involves the exposure of starch suspensions to a number of enzymes primarily including hydrolyzing enzymes that tend to produce highly functional derivatives. Origin of this technique can be dated back to the times when glucose syrup or high fructose corn syrup was produced. The enzymes aminomutases (a-1,4?a-1,4 glucosyl transferases) found in eukarya, bacteria and archea representatives, break an a-1,4 bond between two glucose units to subsequently make a novel a-1,4 bond producing a modified starch that can be used in foodstuffs, cosmetics, pharmaceutics, detergents, adhesives and drilling fluids. ICAR- NRCB came out with the two-step process of using amyloglucosidase and pullulanase to modify the starch with better functional characteristics.  

Banana starch, being a non-conventional source, has numerous uses as an ingredient in food systems and other industrial applications. It is unique enough both nutritionally and functionally, to warrant investigation into its potential commercial uses in processed food and non-food application and become a commercially viable starch product. With the advantageous features for unripe green banana as a source of starches and its further modification can definitely be considered as a fortunate thing to the starch starving sectors which looking for an alternate biological starch source for a long period. Monthan, Popoulu and Saba starches could be adapted for industrial applications where withstanding higher temperature is essential like ready to cook food products, canned foods, and as a thickening agent in soup and pie fillings. The retrogradation properties of Grand Naine, Popoulu and Nendran indicated their ability to be used in Ice-cream, dessert and freeze thawed products. We opine that with the better structural and functional characteristics, the banana starches could be used as a supplement to the existing commercial starches for being used in the food and non-food applications based on their inherent properties.

Authors are from ICAR-National Research Centre for Banana, Tiruchirappalli. Tamil Nadu