Functional Properties of Starches

 Morton Satin

Chief, Agro-Industries and Post-Harvest Management Service

 Introduction

 Few can deny that the indigenous starch crops of the tropics are true wonders of nature. With sun and rain, and little or no artificial inputs, they are able to grow in great abundance. Whether it be cassava, arrowroot, sago, taro, sweet potato or yam, for centuries tropical starches have served as staple foods for millions of people, throughout the hot and humid regions of the world. Indeed, these starch crops are so proficient at supplying essential calories to even the very poorest peoples of the world that they are considered to be the quintessential subsistence crop.

But, what is considered to be a blessing in one situation, can turn out to be a burden under another set of circumstances. In the majority of tropical developing countries, the only foreseeable route to economic development is through agricultural development. The irony is that the very crops which have proven to be most suited for tropical agro-climatic conditions and upon which economic development will depend, have been relegated to the role of subsistence crops. Although these crops have been the subject of much investigation in the area of basic production, they have not benefitted from the kind of value-added research required for economic competitiveness on an international scale.

 It is extremely difficult to break out of this subsistence crop mode and compete with mainstream starch products such as corn, wheat or potato starches, particularly when it is not the commodities themselves that are the competition, but rather the functional characteristics of the value-added products. Consequently, for many indigenous tropical starch crops, the lack of competitive market access has become the major obstacle to their contribution to agricultural development. Efforts to improve production and yields often result in excess supplies of basic commodities for the existing market demand which, in turn, discourages future production. On the other hand, modern value-added products are generally very application-specific and are thus far less susceptible to the sort of market fluctuations that cause chaos to developing countries whose economies are built upon basic commodities.

Until recently, the starch markets of the world were virtually closed to foreign countries. Import duties were so high that it was practically impossible to sell anything but the most basic commodities, at a price dictated by the buyer. All talk of value-addition to starches of developing countries was considered absurd. However, on April 12, 1994 the GATT Uruguay Round was signed in Marakesh, paving the way for new trade opportunities.

 As far as starch is concerned, what are some of the possible consequences of the Uruguay Round? There is tremendous potential for the profitable commercial use of tropical starches, but considerable research and product development of a new type is necessary to properly exploit these materials. The model for product quality and reliability has already been set by the international starch industry. That is who the competition is. If locally-produced tropical starches cannot reach an equivalent level of quality, functionality or reliability, then these products will never survive in the competitive market. There is only so much that a more equitable trade environment can offer.

A review of the sort of research that has been done by both national and international institutes, shows that extensive work has been carried out on agronomic and phenotypic properties for most tropical crops, but relatively little study has been carried out on the sort of functional properties which are of direct technical and economic interest to competitive food and non-food industry. There is little purpose increasing the yield potential of crops that are unsuitable for processors or of limited acceptability to consumers. Far more work must be carried out on those characteristics which will result in products which are more convenient to distribute, easier to process, and have the physical, chemical and organoleptic properties required by the target markets. For those starches that do not have the native functional characteristics that are desired by the target market, an additional effort must be made to value-add or modify them so that they can compete internationally.

 The single most important influence to guide practical research is the marketplace. Just as it takes years of study and continuous inquiry to master the disciplines of science, the same can be said for the profession of marketing - it is not a job for amateurs. Ideally, scientists should be working in a team with professional marketers. If scientists are not able to have access to the services of a professional marketer, and wish to be directly involved in some aspects of marketing, then they must be prepared to spend the time and trouble to be as professional at this task as they are in their own scientific discipline.

 Most marketers will tell you that, aside from everything else, large markets require a consistent supply and a reliable price and quality. They do not like to be pioneers and it is extremely difficult to interest markets in new products unless these criteria can be assured with some confidence. Another factor which large markets require is time - time to test and re-test new products until they are absolutely certain that they are suitable. You can imagine the problems a large paper company would have if 5000 MT of white paper turns yellow after one year on the shelf because a new starch additive proves to be unstable.

 Once these basic factors are accounted for, the next most critical consideration is product performance which, in turn, depends upon the functional characteristics. In fact, that. s how starch should be viewed - as a set of functional characteristics suited to a particular application. The functional characteristics we are after are the same ones that have firmly established other starches and natural polymers in specific markets. These functional characteristics follow on from the basic physico-chemical properties of the starch granules and can often be enhanced through value-addition of one type or another.

 The most basic of the physical properties of starch granules are their size as exemplified in Table 1.

Table 1 - Granule Size Distribution of Various Starches

Starch Species	Granule Size Range        (µm) (Coulter Counter)	Average size µm
Waxy Rice	2 - 13	5.5
High Amylose Corn	4 - 22	9.8
Corn	5 - 25	14.3
Cassava	3 - 28	14
Sorghum	3 - 27	16
Wheat	3 - 34	6.5, 19.5
Sweet Potato	4 - 40	18.5
Arrowroot	9 - 40	23
Sago	15 - 50	33
Potato	10 - 70	36
Canna (Aust. Arrowroot)	22 - 85	53

 The size of and distribution of starch granules can be very important for specific applications and even this very basic physical characteristic can be value-added. For example, the small granule size of rice starch makes it very suitable for applications laundry sizing of fine fabrics and for skin cosmetics. Carbonless paper requires the use of starch as a stilt material to protect ink capsule from premature rupturing, as can be seen in Figure 1.

Figure 1- Carbonless Paper

Figure 2-Wheat Starch Granule Distribution µm

This application requires a starch that is of a particular size and uniformity and arrowroot was the product of choice for many years. A starch such as wheat could not be used because its bimodal distribution of starch granules made it unsuitable (Figure 2 and Figure 3).

 Figure 3- Air-Classified Wheat Starch

However, the variation in supply and cost of arrowroot prompted one company to develop a process of separating the population of small granules from the large ones through centrifugation which resulted in an immediate take over of this market from arrowroot. It was a case of value addition to the very basic physical characteristic of the starch.

Here is another interesting example of unique size, with all granules in the 1µm range.

 Figure 4- Cow Cockle Starch

Other simple physical characteristics which have an impact on functionality are starch granule shape and surface. This is often a critical factor for applications requiring starch to be a surface carrier of materials such as colours, flavours, seasonings and even pesticides.

Starch has two major components: amylose and amylopectin. These polymers are very different structurally, amylose being linear and amylopectin highly branched - each structure playing a critical role in the ultimate functionality of the native starch and its derivatives. The amylose/amylopectin ratios of starches can be genetically manipulated and offer a significant opportunity for the researcher with certain crops. Viscosity, shear resistance, gelatinization, textures, solubility, tackiness, gel stability, cold swelling and retrogradation are all functions of their amylose/amylopectin ratio.

                          Table 2 - Amylose Content of Various Starches

 When aiming at functional properties in starch, most commercial companies examine the characteristics of competitive starches in particular applications. This sets the target to shoot for. For those characteristics which are unattainable with native starches, the only alternative is to look towards some form of value-addition to achieve the desired results. Value-addition can be as simple as sterilizing products required for the pharmaceutical industry to highly complex chemical modification to confer properties totally different from the native starch.

Simple value-addition is represented by washing, air classification, centrifugation and pre-gelatinization. The latter process can be done in many from boiling in crude pots to drum dryers to modern multi-screw extruders, each method having its particular advantages and disadvantages. Complex value-addition is represented by the wide range of chemically modified starches found in the food, paper and textile industries.

 The most common non-food applications for native and value added starches are as follows:

 Non-Food Applications of Starches

Adhesives hot-melt glues stamps, bookbinding, envelopes labels (regular and waterproof) wood adhesives, laminations automotive, engineering pressure sensitive adhesives corrugation paper sacks Explosives Industry wide range binding agent match-head binder Paper Industry internal sizing filler retention surface sizing paper coating (regular and colour) carbonless paper stilt material disposable diapers, feminine products Construction Industry concrete block binder asbestos, clay/limestone binder fire-resistant wallboard plywood/chipboard adhesive gypsum board binder paint filler

Metals Industry foundry core binder sintered metal additive sand casting binder Textiles Industry warp sizing fabric finishing printing Cosmetic and Pharmaceutical Industry dusting powder make-up soap filler/extender face creams pill coating, dusting agent tablet binder/dispersing agent Mining Industry ore flotation ore sedimentation oil well drilling muds Miscellaneous biodegradable plastic film dry cell batteries printed circuit boards leather finishing

As can be seen, there is a great variety of value-added applications for starch in the non-food area, and each application requires very particular functional characteristics. Even in the most basic non-food applications of starch, a great deal of value-addition is employed. Adhesives starches are acid or alkali treated, they are modified with oxidizing agents, salts and different alcohols. Textile starches are esterified, oxidized and are subject to various cross-linking agents.

The use of sophisticated, value added starches in paper products is even more noticeable, when one considers the wide range of applications in that industry. Starches are used to provide greater strength to tissues and paper towels, and they allow a greater use of recycled paper in liner board and cardboard. The growing demand for biodegradability promises to provide additional volumes as starch is used in plastic films and sheets as well as in natural fibre formulations that will eventually replace plastic foams The volume of starch going into non-food uses is enormous and it is all based upon the functional characteristics of the individual products.

The non-food uses of starch are a prime indicator of a country. s economy. During recessions, the volume of starch going into non-food use drops considerably. On the other hand, an active economy needs construction materials for buildings, industrial plants and housing; it needs paper for the bureaucracy, for packaging and wrapping various products, for corrugated boxes and it need adhesives to stick all this economic activity together. As the economy booms, so does the volume of starches going into non-food uses. As countries develop, so does their demand for high quality, highly functional, value-added starches.

Of course, functionality is the key to marketing starches in the wide range of food applications. No other ingredient provides texture to as many foods as starch does. Whether it is a soup, stew, gravy, pie filling, sauce or custard, starch provides a consistent shelf-stable product that consumers rely upon. The extent of specific functional properties of starches required by the food industry is almost unlimited and includes the following:

It is of interest that a growing proportion of these characteristics are being sourced from genetically-modified native starches as a result of the growing demand for natural foods. This may provide an interesting opportunity for new starches from developing countries.

The common applications for modified and native starches in the food industry are varied and numerous. The following Table demonstrates just a few:

 

Canning filling viscosity aid suspension aid for particulates opacity agent body or texture agent for soups, sauces, puddings and gravies aseptically canned products beverages such as coffee, teas or chocolate Cereals and Snacks hot extruded snacks chips, pretzels, etc. extruded and fried foods ready-to-eat cereals Bakery pies, tarts fillings, glazes custards and icings cakes, donuts, danish icing sugar Batters and Breadings coated fried foods frozen battered vegetables, fish and meat dry mix coatings Dressings, Soups and Sauces mayonnaise-type pourable salad dressings (high shear) spoonable dressings instant dry salad dressing mixes low-fat dressing canned gravies and sauces frozen gravies and sauces soups and chowders Cooked Meat Binder water binder for formed meat smoked meats, low-fat meats pet foods (dried and canned)

Frozen Foods fruit fillings meat pies Oriental foods soups, sauces entrees cream-based products Flavours and Beverage Clouds encapsulation of flavours, fats, oils vitamins, spices, clouding agents spray dried flavours for dry beverage mixes, bartender mixes, beverage emulsions liquid and powdered non-dairy creamers Confectionery dusting powder licorice jelly gums hard gums panned candies confectioners sugar Dairy Products yoghurt cheese and imitation cheese chilled desserts UHT Puddings low-fat products Microwavable Products cheese sauces entrees

The range of food products employing starch in one form or another is almost without limit. But the utility of these starches is almost entirely based upon the natural or synthesized functional characteristics. There can be little doubt that the particular physical and chemical properties of individual starches are the keys to their commercial success. I recently carried out a simple search on the Food Science and Technology Abstracts and the Foods Intelligence databases to determine what has been published on the physical properties of various starches. The results of this short survey follows:

It is clear that the information available on the physical properties of cassava, sorghum, millet and yams cannot compare with that from the four major starches. The situation appears even more pronounced on the next pie chart indicating that the combined starches from yam, millet, sorghum and cassava account for less than 12% of all publications :

 This is also reflected in the work on chemical properties as well as on the modification of starches. It is therefore clear that a significant amount of work remains to be done on the functional characteristics of native as well as modified tropical starches if they are ever to become competitive with corn, wheat or potato. While I do not wish to minimize the importance of agricultural research work on productive output, including yield enhancement and disease resistance, I do believe that a shift in emphasis is warranted if the goal of entering competitive world markets is to be realized. Work directed at product characteristics which make processing easier or more efficient, such as uniform-shaped roots or thin, easy to peel skins, make life much easier for the manufacturer. Slight changes in amylose/amylopectin ratios have tremendous effects on a wide range of functional characteristics. These are properties that the end-user requires and would be willing to pay for. Work on modifying starches to improve or broaden their functionality must continue aggressively, because the work on competitive starches never stops.

Tropical countries have a tremendous capacity to produce agricultural products. Unfortunately, this wealth of resources has proven to be a double-edged sword. Historically, it has been relatively easy to export basic raw commodities to the developed world markets. Because so many tropical countries were involved in this trade, it was always a buyers market. Whole economies rose and fell with fluctuations in the basic commodities market. No strong tradition of value-added research and development was established. A majority of value-addition to tropical starches is not being carried out in the countries of origin, it is carried out in developed countries.

This seems to be a fundamental defect in foreign trade strategies and it still continues today. When we examine the research work and the make-up of the professional staff of most national and international agricultural research institutions, this focus on production continues to be the typical pattern. This pattern must change if there is to be any hope of establishing a significant presence of value-added starches from developing countries on world markets.

  AGSI homepage - AGS homepage - AG21 homepage