THE CHEMICAL AND PHYSICAL MODIFICATION OF STARCH

III. THE CHEMICAL AND PHYSICAL MODIFICATION OF STARCH

For decades starch has been and still is a primary ingredient for use in processed foods. It has been used to contribute functional characteristics such as viscosity, shelf-stability, clarity, opacity, flavor, as well as many others. In the native or nonderviatized form starch contributes only partial functionality and to a very limited degree. Through the use of chemical modification and/or physical processing the starch industry now can offer a vast product line of starch-based food ingredients. This section identifies those modifications commonly utilized, their functional attributes, and derivatives of their physical state that further expands their functional characteristics.

A. Chemical Modification

First, we note that chemical modification is an accepted process throughout the world. However, not all chemicals and/or derivatives are allowed either in certain foods or in given combinations around the globe. Differences will be noted as we discuss the various modifications. Refer elsewhere in this book for more details related to world use and labeling for food ingredients. Functional properties will be discussed in detail in Section

IV. Modifications discussed can be performed on all native or common starches as previ- ously outlined. Their ultimate functional properties are still influenced by their origin or source.

1. Hydrolysis Hydrolysis ( Fig. 16 ) of the bonds (acetyl) in starch creates the potential for a vast number

of products. Acid hydrolysis is a very random reaction, thus yielding similar but inconsis- tent finished products. Functional characteristics may be very similar and difficult to mea- sure analytically. Chemical distribution by the constituents are usually quite different. With the inclusion of enzymes for the hydrolysis of starch, we are now offered methodol- ogy that produces very specific products. During hydrolysis you can form simple modified starches, dextrins, and other differentiated sweeteners. These are also referred to as amylo- dextrins, maltodextrins, and pyrodextrins.

a. Amylodextrins. Amylodextrins are also referred to as acid-thinned or thin boiling starches. These have been traditionally produced via acid hydrolysis of starch on a com-

Figure 16 Starch to dextrin.

uct. Because the process causes the cleavage of molecular chains within the starch granule, potential viscosity development as compared to the native starch is less. Amylodextrins are still granular starches and therefore retain characteristic structure and birefringence. They are known for their solubility in water, rather than their swelling. Thin boiling starches produce enhanced gel properties and significantly lower viscosity sols. Molecular weights can be determined for analytical differences. This can usually be accomplished by size exclusion chromotography or chain length distribution by anion-exchange chromo- tography. However, for commercial production a fluidity number is associated with a specific product’s functionality (40). A fluidity number is the volume of slurry (aqueous) that flows through a standard funnel within a fixed time. This time is usually relative to

a fixed volume of water flowing through the funnel. In some cases of hydrolysis, an alka- line system may be used instead of pure water. The same funnel could be used for several products. This is accomplished by utilizing differing starch weights and solution pH as contributing factors. Amylodextrins are commonly used in the food industry for their high gelling characteristic.

b. Maltodextins. Maltodextrins (see Table 5 ) are known throughout the world. How- ever, when attempting to utilize this type of sweetener solid, be aware that there are differ- ences in definition based on country of use. In the United States, maltodextrins are defined by dextrose equivalence (DE). Saccahride polymers consisting of D-glucose units linked primarily by α-1,4 bonds with an average degree of polymerization of 5 or higher are generally recognized as the measurement of reducing sugars, calculated and reported as percent dextrose. The value must be ⬍20 DE ( Fig. 17 ). At this time there is no lower limit in the Unites States. However, a minimum is regulated for other countries around the world. Sweetener products exceeding the 20-DE level move into a category referred to as low DE sweetener solids (see Fig. 18 ). Today they must be identified by starch origin. In years past they were called ‘‘corn syrup solids,’’ primarily because all products

Table 5 Dry Corn Sweeteners/Maltodextrins Feed stock

Starch from common corn, waxy corn, tapioca and potato Process

Starch hydrolysis Acid Enzyme Acid/enzyme

Products Maltodextrins Corn syrup solids Crystalline dextrose Crystalline fructose

Figure 17 Cyclodextrin complex relative weights.

lized in the food industry for their low viscosity, low sweetness, clarity, and bland flavor. More will be said about their functionality in the section on applications.

c. Dextrins (Pyrodextrins). Dextrins fall under the guidelines for starch modification, however they differ in that they must be labeled as dextrins, not ‘‘Food Starch Modified.’’ Dextrins are produced by elevated temperatures, with or without added acid. This roasting of the starch at high temperatures can create yellow (canary) dextrins (Fig. 19). These are water soluble, low in viscosity, and contain a variety of glycosidic linkages. Again, based on the source of starch, commercial products can offer a wide array of functional properties for food application. Dextrins are usually limited in their food use due to the flavor profile generated during production. As this is a dry (nonaqueous) process, several compounds are present in the finished product. Dextrins typically contain high levels of salts (ash) and therefore can contribute to off-flavors.

Cyclodextrins will not be covered in this chapter. They possess properties unique for encapsulation of essential oils, but have received only limited approval in food applications around the world. The technology of forming a cyclic configuration has significant poten- tial ( fig. 20 ).

2. Oxidation Modification There are two primary reactions used to produce commercial quantities of oxidized

starches for food use. They involve the use of sodium and calcium hypochlorite com- pounds ( fig. 21 ). Sodium hypochlorite is more commonly used for large volume produc- tion. Other compounds are permitted for the bleaching of starch, however at significantly

Figure 20 Native starch Visco-Amylo-Graph.

reduced levels as compared to those for oxidation ( Table 6 ). Sodium hypochlorite is an aqueous reaction, while the calcium hypochlorite modification is done via dry blending at typical moisture levels for starch (8–12%). The oxidation reaction constitutes the cleav- age of polymer chains resulting in the oxidation of alcohol groups into carbonyl and car- boxyl groups. As in hydrolysis, the depolymerization of amylose and amylopectin signifi- cantly reduces the granules swelling and paste viscosity. The introduction of the carbonyl and carboxyl groups results in a reduction of the gelatinization temperature, increased solubility, and decreased gelling. With the introduction of some reducing ends to the mo- lecular structure one could expect an effect causing browning. Highly oxidized starches exhibit exceptional dry flow properties as a powder. This can have functional advantages in some food applications.

3. Monosubstitution of Starch Monosubstitution ( Fig. 22 ) is in actuality the esterification or etherification of starch with

monofunctional reactants. The common reagents utilized throughout the world for starch organic and inorganic monoesters for food use areacetic anhydride, succinic anhydride,

Table 6 Regulations for Modification of Food-Grade Starches in the United States 172.892 Food starch-modified .

Food starch-modified as described in this section may be safely used in food. The quantity of any substance employed to effect such modification shall not exceed the amount reasonably required to accomplish the intended physical or technical effect, nor exceed any limitation prescribed. To insure safe use of the food starch-modified, the label of the food additive container shall bear the name of the additive ‘‘food starch-modified’’ in addition to other infor- mation required by the Act. Food starch may be modified by treatment, prescribed as follows:

(a) Food starch may be acid-modified by treatment with hydrochloric acid or sulfuric acid or both. (b) Food starch may be bleached by treatment with one or more of the following:

Use

Limitations

Active oxygen obtained from hydrogen peroxide and/or peracetic acid, not to exceed 0.45% of active oxygen Ammonium persulfate, not to exceed 0.075% and sulfur dioxide, not to ex- ceed 0.05%. Chlorine, as calcium hypochlorite, not to exceed 0.036% of dry starch.

The finished food starch-modified is limited to use only as a component of

batter for commercially processed foods.

Chlorine, as sodium hypochlorite, not to exceed 0.0082 pound of chlorine per pound of dry starch. Potassium permanganate, not to exceed 0.2%.

Residual manganese (calculated as Mn), not to exceed 50 ppm in food

starch-modified.

Sodium chlorite, not to exceed 0.5%.

Table 6 Continued (c) Food starch may be oxidized by treatment with chlorine, as sodium hypochlorite, not to exceed 0.055 pound of chlorine per pound of dry starch.

(d) Food starch may be esterfied by treatment with one of the following: Use

Limitations

Acetic anhydride Acetyl groups in food starch-modified not to exceed 2.5%. Adipic anhydride, not to exceed 0.12%, and acetic anhydride

Do.

Monosodium orthophosphate. Residual phosphate in food starch-modified not to exceed 0.4% calculated

as phosphorus.

1-Octenyl succinic anhydride, not to exceed 3%. 1-Octenyl succinic anhydride, not to exceed 2%, and aluminum sulfate, not

to exceed 2%. 1-Octenyl succinic anhydride, not to exceed 3%, followed by treatment

Limited to use as stabilizer or emulsifier in beverages and beverage base as with a beta-amylase enzyme that is either an approved food additive or is

defined in §170.3(n)(3) of this chapter.

generally recognized as safe. Phosphorus oxychloride, not to exceed 0.1% Phosphorus oxychloride, not to exceed 0.1%, followed by either acetic anhy-

Acetyl groups in food starch-modified not to exceed 2.5%. dride, not to exceed 5%, or vinyl acetate no to exceed 7.5%. Sodium trimetaphosphate

Residual phosphate in food starch-modified not to exceed 0.04%, calculated

as P.

Sodium tripolyphosphate and sodium trimetaphosphate. Residual phosphate in food starch-modified not to exceed 0.4% calculated

as ⬃P.

Succinic anhydride, not to exceed 4%. Vinyl acetate

Acetyl groups in food starch-modified not to exceed 2.5%.

(e) Food starch may be etherified by treatment with one of the following:

Limitations

Acrolein, not to exceed 0.6%. Epichlorohydrin, not to exceed 0.3%. Epichlorohydrin, not to exceed 0.1%, and propylene oxide, not to exceed

Residual propylene chlorohydrin not more than 5 ppm in food starch-modi- 10% added in combination or in any sequence.

fied

Epichlorohydrin, not to exceed 0.1%, followed by propylene oxide, not to

Do.

exceed 25%. Propylene oxide, not to exceed 25%.

Do.

(f) Food starch may be esterified and etherified by treatment with one of the following:

Limitations

Acrolein, not to exceed 0.6% and vinyl acetate, not exceed 7.5% Acetyl groups in food starch-modified not to exceed 2.5% Epichlorohydrin, not to exceed 0.3%, and acetic anhydride.

Acetyl groups in food starch-modified not to exceed 2.5%. Epichlorohydrin, not to exceed 0.3%, and succinic anhydride, not to exceed 4% Phosphorus oxychloride, not to exceed 0.1%, and propylene oxide, not to

Residual propylene chlorodrin not more than 5 ppm in food starch-modi- exceed 10%.

fied.

(g) Food starch may be modified by treatment with one of the following:

Limitations

Chlorine, as sodium hypochlorite, not to exceed 0.055 pound of chlorine Residual propylene chlorohydrin not more than 5 ppm in food starch-modi- per pound of dry starch; 0.45% of active oxygen obtained from hydrogen

fied.

peroxide; and propylene oxide, not to exceed 25%. Sodium hydroxide, not to exceed 1%.

(h) Food starch may be modified by a combination of the treatments prescribed by paragraphs (a), (b), and/or (i) of this section and any one of the treat- ments prescribed by paragraph (c), (d), (e), (f), or (g) of this section, subject to any limitations prescribed by the paragraphs named. (i) Food starch may be modified by treatment with the following enzyme:

Enzyme

Limitations

Alpha-amylase (E.C. 3.2.1.1). The enzyme must be generally recognized as safe or approved as a food ad- ditive for this purpose. The resulting nonsweet nutritive saccharide poly- mer has a dextrose equivalent of less than 20.

Source : Code of Federal Regulations, Title 9.

Figure 22 Granule hydration.

vinyl acetate, sodium tripolyphosphate, 1-octenyl succinic anhydride, and propylene oxide ( Table 6 ). Although still approved, vinyl acetate is rarely used today. We should also note that the succinate esters are anionic polymers. Acetylation and propylation are the two most common forms of reactions utilized for the production of food starches. These modi- fications contribute significantly to the stabilization of food systems for refrigeration and freezing. Differences will be explained in more detail within the applications section. Determination of the level of substitution by hydroxypropylation is by proton NMR (41– 43). Substitution position determination is also possible utilizing the same proton spectra (43). Commercial differentiation of starch products is generally characterized by change

in viscosity. This change is usually measured using the Brabender or RVA (Figs. 23– 25 ). Monosubstitution with propylene oxide only can be done at an addition level up to 25% by weight of starch (Table 6). These higher levels of substitution do yield water-soluble starch products.

All of the modifiers discussed thus far produce hydrophilic products. The introduc- tion of these monoesters or ethers contributes significantly to the functional properties of the starch product. Regardless of starch origin, the effect is the same. Monosubstitution

Figure 24 Effect of modification upon starch viscsoity—crosslinking.

reduces the gelatinization temperature, increases water holding capacity, raises viscosity, and reduces shear tolerance.

The 1-octenyl succinate ester forms food grade starches hydrophobic in nature. They are considered more lipophilic in functional characteristics. This functional property lends itself to unique food application.

4. Crosslinking The crosslinking (Fig. 26) of starch for commercial application is done today via the

introduction of various multifunctional reagents. Those permitted today have been self- regulated by processors in the United States to exclude epichlorohydrin. Others that are still utilized both in the United States and other facilities around the world are phosphorus- oxychloride (POCl3), acetic anhydride, adipic anhydride, and sodium trimetaphosphate with or without sodium tripolyphosphate. ( Table 6 ). Today the anhydrides and POCl3 are the most common reagents used. Phosphates are utilized, but for very specific products requiring given functional parameters. The effect of crosslinking on starch is dramatic. Altered functional properties include an increase in gelatinization temperature, reduction in viscosity, and increase in acid, heat, and shear stability ( fig. 27 ). These later characteris- tics significantly improve the total functional contribution of starch to a food formulation.

B. Physical Modification

All that we have discussed thus far has been the chemical modification of starch. The alteration of the internal structure through the introduction of compounds to replace a hydroxyl group or the degradation of structure via molecular breakdown with acid or

Figure 27 Pregelatinized waxy starch.

enzymes was the methodology used. For many years we have utilized some form of physi- cal modification, however not to its fullest extent. Within the dry milling industry physical modification is and was common practice. The grinding and milling of grain yielded a variety of products. Use of air or screen classification also enhanced the functionality of specific milled products. In addition to particle size, moisture classification was incorpo- rated. The use of drying techniques offered a wide variety of dry milled products con- taining starch for food use.

In the starch industry, those manufacturers that do not dry mill flour but isolate relatively pure starch via wet milling also found physical processing advantageous for producing unique and value added products. These processes went beyond those for physi- cally processing flour; they offered new opportunities for native starches (Table 7).

1. Pregelatinization The first of these is a process called pregelatinization. This process starch is cooked beyond

the gelatinization point and dried, utlimately producing an instant hydrating, starch product

Table 7 Starch Types Cook-up

Instant

Conventional Low temperature

CWS (135–200 °F)

Pregelatinized

(90–130 °F)

Drum dried

Extruded

Aqueous High pH, alcohol

spray-dried

Figure 28 Granular starch versus pregelatinized starch.

(Fig. 28). Depending upon the native starch and whether it has been chemically modified, the functional properties it possesses are determined. Starches produced via pregelatiniza- tion are cold water soluble and hydrate very rapidly. The rate of hydration can be con- trolled either by modification, particle size, the use of dispersing aids, or a combination of all of these (Fig. 29). Today, most commercial pregelatinized starches are prepared using single drum dryers. A small percentage of food grade instant starches are still pre- pared on double drum drying systems. With most starches using either of these drying procedures the granules are greater than 80% fragmented ( Fig. 30 ).

2. Instant Granular Preparation

A second method to produce instant starches differs in that the process forms an instant hydrating product that is not fragmented but retains the granules intact ( Fig. 31 ). This is

Figure 29 Crosslinked waxy starches (pH 6.5; 5% ds. starch; all highly substituted with various

Figure 30 Solution and solubility Demo

accomplished today via two different commercial processes. One is that of spray-drying starch specifically pretreated through a uniquely designed nozzle (44). The other process is that of dispersing starch in a mixture of alcohol and water. Subjecting this mixture to time, temperature, and pressure creates an instant starch. The alcohol retards the granules from becoming soluble, thus retaining birefringence and granule integrity.

Considering the number of native starches we have to select from for commercial food grade products along with the various methods of modification, both chemical and physical, we now have the potential for hundreds of unique and application-specific starch products.

3. Extrusion Extrusion is another process where external heat is transmitted into a starch/water mixture.

This is somewhat similar to drum drying, however a significant amount of mechanical shear is introduced with extrusion. Drum drying does not have this mechanical impact upon the starch matrix. Also, the starch matrix for extrusion is generally considerably lower in moisture content. Drum drying is done with a starch slurry, while extrusion presents more of a dough. Experience with these two systems has proven that similar This is somewhat similar to drum drying, however a significant amount of mechanical shear is introduced with extrusion. Drum drying does not have this mechanical impact upon the starch matrix. Also, the starch matrix for extrusion is generally considerably lower in moisture content. Drum drying is done with a starch slurry, while extrusion presents more of a dough. Experience with these two systems has proven that similar

Extrusion is usually accomplished using one of two types of extruders. A single and a twin-screw system are available. Each yields a different product, even with the same starting material. An important factor to remember when considering extrusion is the physical force or energy implied; this can be a benefit or a drawback. Shear is still an important factor. Granule degradation or fragmentation can and does occur. With it come characteristic and functional changes.

4. Heat Treatment of Starch One last product type which has been introduced in just that past couple of years, is

referred to as resistant starches (Table 8). These starches, have been uniquely processed to contain retrograded amylose resistant to α-amylose digestion. Resistant starch has gen- erated considerable interest as a food additive. The smooth consistency, lower caloric contribution, and low water affinity are desirable characteristics. In addition to these prop- erties it has also exhibited possible health benefits as related to cardiovascular disease, diabetes, and colon cancer (45–48). How do we obtain these unique starches? One process is the heating of an aqueous suspension of starch granules at a temperature just below the gelatinization point for an extended period of time. The lowering of the Tg of the amor- phous phase allows the polymer chains to be mobile or in a rubbery state (49). Annealing results with an increase in gelatinization temperature. This can be done in excess salt solutions for heating starch. Essentially any medium that retains the starch just below the gelatinization temperature offers the potential to generate this type of physically modified starch. It has been shown that some heat and acid tolerance can be introduced into the starch granule via this process. Therefore producing a native starch with similar properties to lightly crosslinked starch via chemical modification is possible.

We have completed the discussion for both chemical and physical modifications as independent processes. Within the scope of starch production, regardless of what part of the world or the origin of the starch, the industry has been granted the clearance to incorpo-

Table 8 Resistant Starch Classifications Category listing

Accepted definitions Type I

Physically inaccessible starch which is locked in the plant material, e.g., milled grains, seeds, legumes Type II

Native granular starch, found in food containing uncooked starch, e.g., ba- nanas Type III

Indigestible starch that forms after heat and moisture treatment; may be present in foods such as cooked potatos Type IV (proposed)

Resistant starch that has been produced by chemical or thermal modifica- tions

Figure 32 Emulsion versus encapsulation.

rate multiple modifications upon or within the starch granule. This agreed upon procedure creates the opportunity for the production of hundreds of modified starches, each possess- ing moderately to significant differing functional characteristics. Most governing bodies of the world that regulate food production and the ingredients utilized within have granted such approval to their respective starch industries. Many of the food grade starches dis- cussed in the applications section offer varying functional characteristics primarily due to the use of two or three modifications within the starch granule (Fig. 32). This does not include the opportunity to bleach the granule for the purpose of providing a whiter dry starch product to the consumer.