Modified Food Starch

In the native form, starches do not have the properties desired by food processors. For example, when aqueous slurries of native starches are heated, the starch granules swell, then rupture—causing the viscosity to increase rapidly, then fall. Cooling of the cook produces weak-bodied, cohesive pastes or gels. Modification can correct such a defect. According to the U.S. Code of Federal Regulations (21 CFR 172.892), modified food starch (1,9) may be prepared using the following treatments: acid modification with hydrochloric and/or sulfuric acids; bleaching with hydrogen peroxide, peracetic acid, potassium permanganate, or sodium hypochlorite; oxidation with sodium hypochlorite; es-terification with acetic anhydride, adipic-acetic mixed anhydride, monosodium orthophosphate, 1-octenylsuccinic anhydride, phosphoryl chloride, sodium tripolyphosphate and/or sodium trimetaphosphate, or succinic anhydride; etherification with propylene oxide; or various combinations of the preceding. Also allowed under the regulations are bleaching with ammonium persulfate or sodium chlorite, acetylation using vinyl acetate, and etherification with acrolein or epichlorohydrin (crosslinking); but these reactions are not currently used to produce food starches. Limitations of modifications are given in the regulations. Most U.S. modified food starch is prepared from waxy maize, common corn, or potato starch. In the United States, the ingredient label of the product containing one of these food starches must designate "modified food starch" or "food starch modified" as one of the ingredients but does not need to indicate the source of the starch or the modification(s).

In general, derivatization increases solution and gel clarity, reduces the tendency to gel and/or crystallize, improves water binding, increases freeze-thaw stability, reduces the gelatinization temperature, and increases the peak viscosity. Combinations of derivatizations and other modifications are used to obtain desired properties for specific applications. The useful properties of starches that can be modified and controlled by various treatments are their property of forming a suspension of insoluble granules, without thickening, until the slurry is heated; their ability to thicken aqueous systems and to form a paste on heating; their ability to form semisolid gels on cooling of pastes; their ability to form strong, adhesive films and coatings and to act as a binder; the ability of their pastes to disperse and suspend fats, oils, and particulate matter; and the ability of their pastes and gels to provide important textural qualities to prepared foods.

Crosslinked Starch

Crosslinking is the most important modificaton applied to food starches. It is used to control texture and provide heat, acid, and shear tolerance and, thus, to provide better control over end-product quality and greater flexibility in dealing with formulations and processing and storage conditions. Diphosphate esters, which can be introduced by reaction of starch with either phosphoryl chloride or sodium trimetaphosphate (Fig. 3), are the most common crosslinks. Crosslinking of the polymer chains of starch granules is also done by making an adipic acid diester.

Crosslinking strengthens the granule. A small amount of crosslinking (eg, treatment with only 0.0025% sodium trimetaphosphate) greatly reduces both the rate and the

Starch molecule

Na3P309

Catalyst

P0C13

Figure 3. Crosslinking of molecular chains of starch granules with phosphate diester linkages.

degree of granule swelling and the sensitivity of starch pastes to processing conditions (high temperature; extended cooking times; high shear during mixing, milling, homogenization, or pumping; low pH). Pastes of cross-linked starches are more viscous, more stable, shorter-textured (more pseudoplastic), and generally heavier-bodied than those of native starch from which they were prepared. Crosslinked waxy maize starch products are especially popular in the food industry. Starches with a range of properties are prepared by varying the kind and degree of crosslinking (Table 3). In general, when choosing a starch for a particular application, one should select a starch that is crosslinked sufficiently to enable it to withstand the processing conditions to which it will be subjected and still give optimum viscosity in the fined product. Crosslinked starches can be pregelatinized.

Stabilized Starch

Pastes of ordinary starch will gel, and the gels will generally be cohesive, long-textured, and prone to syneresis— all undesirable characteristics. Pastes of underivatized waxy maize starch, which has no amylose, are less inclined to retrograde or gel; however, in the refrigerator or freezer, they will eventually increase in opacity and become chunky. This is especially true if processing or cooking conditions effect cleavage of chains, producing linear fragments.

lb improve textural aesthetics and/or other properties, other starch derivatives are prepared (Table 4). Few of the hydroxyl groups of modified food starches are derivatized, in other words, ether or ester groups are present in small amounts. Most modified food starches contain, on average,

Table 3. General Reasons for Crosslinking Starches

Changes in granule properties

Modification of cooking characteristics Inhibition of swelling

Changes in paste properties

Reduction of cohesiveness Inhibition of gel formation Improvement of acid stability Improvement of heat stability Improvement of shear tolerance one substituent group per every 5 to 10 a-D-glucopyranosyl units, a DS (degree of substitution, the average number of hydroxyl groups derivatized per monomelic unit of the polymer, of 0.1-0.2). These small degrees of derivatization change dramatically the properties of the starch and increase its range of application.

The derivatives most commonly used to prepare a stabilized starch are the hydroxypropyl ether, the acetate ester, and the monophosphate ester. Addition of these groups reduces the ability of starch molecules to form junction zones (intermolecular associations).

Crosslinked Stabilized Starches

Often, it is desired to improve both processing and cooking tolerances and shelf and storage stability, so many food starches are derivatized with two reagents to introduce both crosslinking and non-crosslinking substituent groups.

Addition of Hydrophobic Groups

Other derivatives are made to add specific functional properties. For example, nonwetting hydrophobic starches are used as release agents for dusting on dough sheets or as processing aids. Other starch products with hydrophobic groups are used as emulsifiers and emulsion stabilizers.

Acid-Modified Starches

Acid-modified (acid-thinned, thin-boiling) starches are prepared by treating a suspension of native or derivatized

Table 4. General Reasons for Making Stabilized Starches

Changes in granule properties Reduction in energy required to cook

Changes in paste properties

Increased stability Increased freeze-thaw stability Enhancement of clarity Increased sheen Inhibition of gel formation Reduction of gel syneresis Increased viscosity

Improved interactions with other substances Improvement in stabilizing properties starch with dilute mineral acid at temperatures below the gelatinization temperature for varying periods. When it is determined that a product that gives the desired viscosity after cooking has been produced, the acid is neutralized and the starch is recovered by centrifugation or filtration, washed, and dried. In this process, a small amount of glycoside bond hydrolysis occurs, resulting in products that produce much less viscosity. A concurrent weakening of the granule structure occurs. The result is that there is more granule disintegration when acid-modified starches are heated in water; and although they have reduced viscosity-imparting power, they form gels with increased strength and improved clarity.

These so-called thin-boiling starches are used when strong gel strength is desired, such as in gum candies (eg, jelly beans, orange slices, and spearmint leaves) and in processed cheese loaves. Where especially strong or fast-setting gels are desired, high-amylose starches are used. Other of these low-viscosity products, which allow higher-concentration pastes to be formed, are used as film formers and adhesives in products such as pan-coated nuts and candies.

Bleaching

Absolute whiteness, particularly of corn starch, requires bleaching. The bleach most commonly used is sodium hypochlorite. During the bleaching operation, the starch is oxidized. Small amounts of carboxyl and carbonyl groups are introduced and some glycosidic bonds are cleaved. The result is decreased pasting temperature, thickening power, and tendency to retrograde.

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