STARCH SOURCES, STRUCTURE, CHARACTERISTICS, AND PROPERTIES

II. STARCH SOURCES, STRUCTURE, CHARACTERISTICS, AND PROPERTIES

A. Sources

As previously noted, there are several sources for starch. However, for the quantity neces- sary and the quality needed to make it industrially economical, we are somewhat restricted today to the following sources: maize, potato, rice, wheat, and tapioca. These dominate the world as primary raw material sources ( Fig. 3 ). There are a few, such as sorghum, arrowroot, and pea starches, that have some commercial use. However, they represent very selective regions of the world or are in very limited supply. The aforementioned primary sources also contain hybrid derivatives. These hybrids are referred to as the waxy or high-amylose varieties. Maize contributes both. Rice and potato starches, commercially, supply only the waxy variety. Wheat, however has not evolved commercially with hybrid varieties as of this writing ( Table 2 ).

B. Components of Starch

Let us now consider the amylose and amylopectin role in starch (see Fig. 4 ), the two major polymeric components, which contribute significantly to the structure, characteristics, and properties of the different starch sources. As shown in Table 3 amylose content and struc-

Figure 3 Progressive enlargements of a typical starch granule.

Table 2 Starch Sources Commercial

Common corn Wheat Pea Waxy maize

Potato Waxy rice High-amylose corn

Rice Tapioca Intermediate-amylose corn New hybrids Waxy potato Waxy wheat

Table 3 Starch Characteristics

Texture Flavor Common corn

Percent amylose

28 Firm gel Cereal Waxy maize

Slight cereal Tapioca

Paste

Neutral Potato

22 Soft gel

Earthy Wheat

18 Sauve

30 Firm gel Slight grain High amylose corn

Rigid gel Cereal Rice

Slight grain Waxy rice

24 Soft gel

the hybrids of maize. These can be less than 1% to greater than 70% present in varieties today.

Research by Colona and Mercier, revealed the presence of a third component (inter- mediate fraction). This varied in percentage, but was considered to be approximately 5– 10% in most starches (19).

1. Amylose Amylose (Fig. 5) is known as a linear polymer, but is not defined as just a straight chain

molecule. It frequently forms a helix and is thought to inter-twin, even through the several layers of amylopectin (20). Not only does amylose have this unique shaping, but it has been shown that it also consists of limited but distinctly measurable branch points. Amy- lose consists predominately of α-1,4-D-glucose, bonds (21,22). Amylose also forms a very strong complex with iodine. This complex yields a very intense blue color producing

a λ max of 620 nm. Because of this characteristic one can use a light microscope to assay qualitatively for amylose in starch.

2. Amylopectin Differing significantly from amylose, amylopectin ( Fig. 6 ) is a highly branched polymer.

These branch points are α-1,6-D-glucose bonds. Due to the vast number of chains in the

Figure 6 Growth ring of starch granule.

structure of amylopectin, we have an infinite number of potential branch points. Because of this it has been very difficult to determine the exact structure representation of amylo- pectin. Most carbohydrate scientists have accepted structural models depicting clusters,

double helices, and irregular chain lengths (see Figs. 7– 9 ) (23–27). It has yet to be deter- mined as to how amylose and amylopectin are distributed within a starch granule. Amylose was first considered to exist in the amorphous region. Recent research has indicated that amylose is located in the granule as bundles between amylopectin clusters and is randomly dispersed. Thus they could be located among the amorphous and crystalline regions of the amylopectin clusters (27).

3. Minor Constituents As with most ingredients supplied to industry and the food chain, starch does not exist

as a pure entity. Due to the vast regions of the world where starch sources are grown,

Figure 8 Model of a starch crystallite showing the possible positioning and interactions of various components.

Table 4 Composition of Starch Granules Waxy

Starch Components

Wheat Tapioca maize Amylose content (% on dry stubtance)

Potato

Maize

21 28 28 17 0 Amylopectin content

79 72 72 83 100 Lipids (% on dry substance)

0.05 0.7 0.8 0.1 0.15 Proteins (% on dry substance)

0.06 0.35 0.4 0.1 0.25 Ash (% on dry substance)

0.4 0.1 0.2 0.2 0.1 Phosphorus (% on dry substance)

minor constituents differ somewhat and vary in amount. Typically in starch the most common minor constituents are moisture (water), lipids (fats), nitrogen (protein), phospho- rus, and trace elements (minerals). See Table 4.

a. Lipids. Commercial starches supplied to the food industry usually contain less than 1% fat. Levels greater than this are removed, typically via extraction or hydrolysis. A small percentage of lipids are thought to be bound within the matrix of the starch amylose/ amylopectin configuration. This would account for the small percentage usually found through acid hydrolysis assay. It is also postulated that some lipids are in association with internal proteins (28).

b. Protein. Protein content varies based on the source of the starch. All protein analysis is reported as percent nitrogen. For those starches isolated from high protein containing flours (wheat, barley, etc.) the starchy phase is isolated from the protein. Other starches commercially sold within the food industry contain less than 1% protein. As with lipids it is also postualted that the protein present may be structurally bound within the matrix of the starch granule, thus making it unavailable for simple extraction (28).

c. Phosphorus. The root (tuber) and legume sources of starch contain esterified phos- phorus (phosphate monoesters). It is present as phosphate linked to the C-6 and C-3 hy- droxyl groups of the glucose units. Most cereal starches contain very small amounts of phosphorus. If present, it is typically analyzed as phospholipids (29).

d. Trace Elements. In addition to the forementioned compounds present in starch, starch can and does contain very small amounts of minerals and inorganic salts. During the isolation of starch products for commercial application, these compounds are assayed for and reported collectively as ‘‘ash.’’ Ash content does vary on the native starches. This variance is primarily dependent upon source or origin and the regions of the world from which they have been produced. The ash content for most commercial starches is reported to be ⬍0.5% based on a dry starch basis (dsb).

e. Moisture. We have considered starch an almost pure carbohydrate with typically very small amounts of trace materials (contaminants). However, we usually overlook one of the most common materials in nature, water. Water or moisture content of starch varies significantly in its native state. Therefore, as a refined product for the food industry, we expect and receive a more consistent moisture level than what is found in nature. Starch as prepared commercially contains on the average approximately 12.0% moisture. There are exceptions, ranging from as low as 3.0% to as high as 18% for some commercial

C. Characteristics and Properties

1. Gelatinization First we need to be certain that we do not confuse one event or definition with another.

When considering what happens to starch granules during heating in excess water, several events take place. As heat is introduced and the starch granules begin to hydrate (swell), the crystallites within the granule disappear over a range of temperatures. A loss of bire- fringence and X-ray diffraction occur. With this change in crystallites, referred to as ‘‘melting,’’ is thought to occur the process known as gelatinization. This term has been attempted to be defined for all researchers, however to date only one definition has been somewhat collectively generated:

Starch gelatinization is the collapse (disruption) of molecular orders within the starch granule manifested in irreversible changes in properties such as granular swelling, native crystalline melting, loss of birefringence and starch solubilization. The point of initial gelatinization and the range over which it occurs is governed by starch concentration, method of observation, granular type and heterogenities within the granule population under observation (30).

Today we also have available the differential scanning calorimeter (DSC) for inter- preting gelatinization. Utilization of this technique has significantly improved the measure- ment of water-heat effect on starch. It not only confirmed and refined past data pertaining to the gelatinization temperatures of known starches, but it simplified the data generation and time involvement (31–36). However important this is for basic research, for the com- mercial use of starch it plays only a minor role in the formation of many food products. Some baked items (cookies, cakes, etc.) are formulated in limited water systems. This affects the hydration and functionality of the selected starch(es). Thus, knowledge of gela- tinization can be a key indicator as related to functionality.

2. Swelling It is now appropriate to discuss what happens when incorporating starches into a food

system with excess water. This phenomenon is what generates swelling, pasting, and even- system with excess water. This phenomenon is what generates swelling, pasting, and even-

Studies have shown that tuber (root) starches have a tendency to swell more rapidly as compared to cereal grain starches. This faster swelling usually corresponds to a rapid increase in viscosity. This is particularly true of potato starch. There can be a drawback to producing more highly swollen granules. They can be more readily disintegrated by excessive shear or continue to introduce heat. Disintegration is typically measurable and depicted by a loss of viscosity (see Fig. 12 ) (37,38). Examples such as monoglycerides, sodium stearoyl 2-lactylate, salts, and sucrose esters have shown the ability to inhibit the swelling of starch and lower the viscosity of the final pastes.

3. Pasting Pasting is more of a sequence of events rather than a fixed point or defined region. There

has been a proposal made to differentiate pasting from gelatinization. ‘‘Pasting is the

Figure 12 Syneresis example.

phenomenon following gelatinization in the dissolution of starch. It involves granular swelling, exudation of molecular components from the granule and evenutally, total dis- ruption of the granules’’ (30). It should be noted again that pasting is a sequence. This then creates the potential for gelatinization to occur during pasting and, therefore, pasting to occur prior to and after gelatinization. In turn, this explains why to best utilize starch as a source of viscosity and/or water-binding matrix, one needs to fully hydrate or com- plete pasting during heating of starch granules. Doing so then offers the greatest potential for maximum use of the starch for developing viscosity, clarity, and textural characteristics

(see Figs. 13 and 14 ).

4. Retrogradation It was mentioned earlier that as viscosity is increased during cooling we generate a phe-

nomenon referred to as set-back. This sometimes is misinterpreted as retrogradation. As with pasting and gelatinization an attempt has been made to offer a definition for consensus agreement: ‘‘Starch retrogradation is a process which occurs when starch chains begin to re-associate in an ordered structure. In its initial phases, two or more starch chains may form a single juncture point which then may develop into more extensively ordered re- gions. Ultimately, under favorable conditions, a crystalline order appears’’ (30). This definition offers a general guideline for consideration. However, the event of retrograda- tion is more detailed and deserves some additional discussion. Starch in its cooked form is a viscous or semisolid starch paste, which upon cooling forms or sets to a gel. This gel system is generally considered to be a three-dimensional mass formed from the amylose containing starches. The mass is generated by a mechanism known as ‘‘entanglement.’’ The entirety of the structure is not exclusively due to the amylose fraction of starch. Because some short-chain branches can be sheared from the amylopectin structure, they too can contribute to the gel. As cooling continues these entangled molecules lose their translational motion, thus entrapping water within the matrix. As crystallites form, the gel slowly increases in rigidity. At a point in formation the entrapped water is slowly released or squeezed out of the gel. This process of freeing moisture is referred to as ‘‘syneresis.’’ Syneresis is very common with native or nonchemically derived starches ( Fig. 15 ). The

Figure 13 Hydrolysis

Figure 15 Average chain length versus dextrose equivalents.