Modified Food Starches:Why, What, Where and How1
Joseph M. LightNational Starch and Chemical Co.Bridgewater, NJ 08807
1 Adapted from a presentation at the symposium on Modified Food Starches at AACCs74th Annual Meeting in Washington, DC, October 29-November 2, 1989.
Since their development in the 1940s, modified food starches have become a vital part ofthe food industry. Practically every category of food utilizes the functional properties ofstarch to impart some important aspect of the final product. In this article, the why, what,where and how of the use of modified food starches will be reviewed.
Why Modified Food Starches Are NeededA pudding mix that could be found in any grocery store has food starch modified as amajor item in the ingredient list. It will be found on many product labels besides pudding,but, why is it in there?
Reasons for Modifying StarchesIn general, modified food starches are used for three reasons. First, they providefunctional attributes in food applications that native starches normally cannot provide. Inthe pudding mix, the starch provides thickening power, a creamy short texture, andconvenience if it is an instant system. In other applications, modified starch can provide awide range of functions, from binding to disintegrating; imbibing or inhibiting moisture;producing a short, stringy, or cuttable texture; creating a smooth or pulpy texture;developing a soft or crisp coating; or stabilizing an emulsion.
Second, starch is abundant and readily available. Third, starch can provide an economicadvantage in many applications where higher priced items such as gums otherwise mustbe used.
Problems with Native StarchesTo further illustrate the functionality of a modified starch, it will be helpful to look atwhat happens when a modified food starch is not used.
Narrow Peak Viscosity RangeModified waxy maize is a typical starch used in the food industry to impart viscosity. Theamylogram in Figure I depicts the rapid hydration of unmodified waxy granulesaccompanied by a sharp increase in viscosity (A). With continued cooking, the granulesrupture, leading to a very rapid decrease in viscosity (B). As can be seen, there is anarrow range at which waxy corn develops and loses its viscosity (C). These rapidchanges in viscosity are accelerated by heat, acid and shear. This narrow viscosity range
along with the starchs susceptibility to processing conditions would make manufactureof an acceptable product virtually impossible.
Texture and Stability LimitationsIn addition to the narrow peak viscosity range, cooking waxy corn in water (6% solids)results in a weak bodied, stringy, cohesive paste. In our world where quality is of utmostimportance to the consumer, such a cohesive pudding would be far from desirable.
As with waxy corn, other native starches such as those from dent corn, potato, andtapioca have inherent disadvantages that would make them unsuitable for food systemsand processing. These include a narrow peak viscosity range, undesirable textures, andpoor stability and processing tolerance. In many cases the native starch would make anuneconomical, poor quality product. However, by appropriate structural modification,these undesirable properties can be turned into desirable ones. Such modified productscan provide the needed controlled peak viscosity, improved tolerance to rigorousprocessing conditions, desirable texture, and prolonged stability.
Principal Types of Modified Food Starches
BackgroundStarch consists of amylose and amylopectin, which are polymers of glucose, linear andbranched respectively. Each glucose unit potentially has three reactive hydroxyl groupsthat are the basis of all derivatizations. Starch gelatinization is the collapse or disruptionof molecular orders within the starch granule, manifested in irreversible changes inproperties such as granular swelling, native crystallite melting, loss of birefringence, andstarch solubilization. Starch retrogradation is a process that occurs when the moleculescomposing gelatinized starch begin to reassociate in an ordered structure (1).
Starch modification usually affects these phenomena, and can include hybridization aswell as actual chemical and physical modification of native starch. Chemicalmodifications include conversion and derivatization. Typical physical modificationsinclude pregelatinization, particle size adjustment, and moisture adjustment. Each ofthese areas will be briefly discussed here.
HybridizationHybridization is a process whereby corn is selectively bred to yield products withdifferent percentages of amylose and amylopectin. High amylose starches have uniqueproperties for gelling and film forming and have been used in applications such asproviding structure to candies and reducing oil pick-up in fried foods. Waxy maizestarches, composed of nearly 100% amylopectin, have built-in viscosity stability due tothe branched nature of the polymer molecule. Waxy maize is useful in a wide array ofapplications in the food industry because of its stability.
Chemical ModificationConversion. Conversion is a process that is used to reduce the viscosity of raw starches.Its main objectives are to allow the use of starches at higher percentages, increase thewater solubility, control gel strength, or modify the stability of starch.
Typically, native starch cannot be used at much more than 6% solids because it impartssuch a high viscosity. However, in the confection industry in a product such as a soft gumcandy, a low-viscosity starch at high solids is needed to obtain the desired gel structureand set. Thus, a converted starch is required.
Methods of conversion include acid hydrolysis, oxidation, dextrinization, and enzymeconversion. Each method of conversion provides starch products with distinctivefunctionality.
Other chemical modifications fall in the area of derivatization, including crosslinking,stabilization, and the addition of unique functional groups.
Crosslinking. This is a treatment whereby small amounts of compounds that can reactwith more than one hydroxyl group are added to the starch polymers. Crosslinking yieldsstarch granules with increased resistance to overcooking and other variations inprocessing conditions. Its main purposes are to provide a short texture to cookeddispersions of starch and to impart resistance to processing conditions such astemperature, acid and shear.
Stabilization. This is a process whereby blocking groups are reacted with starchpolymers to inhibit retrogradation, the alignment of polymers that causes a change in thestructure of the food product. Inhibiting retrogradation imparts textural, and freeze-thawstability, thus prolonging the shelf life of the food product. This modification is mostimportant in frozen foods, since retrogradation of starch polymers is accelerated at coldtemperatures, leading to an opaque, gelled, and/or chunky texture with eventual syneresisor weeping of liquid from the gel.
Addition of Lipophilic Groups. In addition to crosslinking and stabilization,derivatization of the starch molecule can also include the addition of other functionalentities such as lipophilic groups to starch molecules. These starch products are used inencapsulation and emulsion stabilization.
Physical ModificationStarch can also be physically modified using a number of techniques to provide desirableproperties. Such techniques include pregelatinization of starch for quick viscositydevelopment in instant systems, cold water swelling starches for instant systems wheremore process tolerant products are needed such as in microwaves, and adjustment ofparticle size to control dispersability and hydration.
Where Modified Food Starches Are UsedThe modifications listed in the preceding section make starch an important functionalingredient for numerous food systems. Starch can be used in numerous possiblefunctional application areas, including adhesion, antistaling, binding, clouding, dusting,emulsion stabilization, encapsulation, flowing aid, foam strengthening, gelling, glazing,moisture retention, molding, shaping, stabilizing and thickening.
For example, starch can be used on fried fish where it binds the breading to the fish piece,in processed meats where starch binds the juices, in orange soda where it providesemulsion stability, in candy where starch provides structure, and in numerous otherapplications where starch is used as a thickener.
Choosing the Correct Starch
Questions to AskIn selecting a starch for a food system some basic questions should be answered that willhelp in selecting the proper starch. First, what are the properties of the targeted foodsystem? Is it cook-up or instant? Will it be frozen or canned? What is the desired shelflife? In what physical form will the starch be used? Second, what ingredients are presentin the system: sugar? acid? fats? proteins? How will these affect the starch and itsfunctionality? Finally, what are the processing conditions such as time, temperature andshear, and what type of starch is needed to tolerate these conditions?
Physical Forms of StarchTypically starch will be used in one of four physical forms: dry, swollen, moleculardispersion or dry film. When used in the dry state or ungelatinized granular form, starchtypically serves as a dusting agent, as in coating bakery products to provide easy releasefrom pans after removal from the oven. In the swollen or gelatinized state, starch is usedto provide viscosity, texture, and mouthfeel in products such as gravies, sauces, fillingsand creams. A molecular dispersion of starch is used for encapsulation and emulsionstabilization. As a dry film, the starch is basically a binder - for example, in adheringsesame seeds to crackers.
Evaluation of Starch CooksDepending on what form of starch is used, particularly when in the swollen state, it isimportant to understand how to evaluate starch cooks microscopically. This sectionbriefly reviews how to evaluate a starch cook.
Uncooked corn starch when viewed under a microscope shows small intact granules, 10-20 microns in diameter (Fig. 2A). If observed under polarized light, birefringent Maltesecrosses would be exhibited due to the crystalline structure of the granule. If added towater, the starch would settle.
When properly cooked (for applications requiring maximum viscosity), the majority(approximately 80%) of starch granules are highly swollen with small percentages (10%each) of uncooked and ruptured granules (Fig. 2B). The granules would not exhibitbirefringence under polarized light since the crystalline nature has been disrupted.Such granules provide excellent viscosity and textural characteristics, and the starch cookcan show good clarity, excellent sheen, a heavy-bodied short texture and good stability.In applications where highly swollen granules are desired, the food processor must aimfor just the right cook. Either undercooked or overcooked starch may lead to poorfunctionality.
When starch is undercooked, the granules are slightly swollen and highly intact (Fig. 2C).A slurry of such starch would be low in viscosity, have poor clarity, a starchy taste andpoor stability. To improve the cook might involve using a less crosslinked starch orincreasing the cooking time and temperature.
If the starch is overcooked, the majority of the granules are ruptured with a largepercentage of fragments (Fig. 2D). Even though this cook will have good clarity, it iscohesive and usually lower in viscosity than desired. A more crosslinked starch or lower
cooking times and temperatures could be used. Characteristics of starch cooks forthickening purposes are summarized in Table 1.
Table I. Characteristics of Starch Cooks for Thickening Purposes Characteristic Appropriateness of Cook
Undercook Optimal cook OvercookAppearance Cloudy Clear ClearTexture Thin, starchy taste Heavy bodied Cohesive,
short textured long texturedStability Poor Good FairViscosity Low Good Viscosity drop
Factors Affecting Degree of CookFactors that affect the degree of cook of a starch include pH, process conditions, andother food ingredients in the formulation. Deviations from a neutral pH tend to affect thedegree of cook of a starch and its breakdown. Since many food systems are acidic, it isimportant to choose the properly crosslinked starch that will give the desired viscositywhen exposed to low pH conditions.
Acidic Conditions. A lightly crosslinked waxy starch cooked at a neutral pH underappropriate conditions will result in a short, heavy-bodied texture. If this same starch iscooked at an acidic pH below pH 3.5, it will overcook with a substantial loss of granularintegrity, yielding a long texture with a thin viscosity. A simple modification to yield anacceptable product would be to cook the starch first and then add the acid. The finalacidic product would look like the neutral product with all the properties indicative of agood cook. However, addition of acid at a separate time may not be possible duringmanufacturing; thus, a moderately crosslinked starch that can withstand the acidicconditions should be used. A highly crosslinked starch that does not cook out in a neutralsystem may be properly cooked in an acid system.
In a real food containing acid, such as a cherry pie, selection of the appropriate starch isvery important. If an inappropriately crosslinked starch is used, a low-viscosity fillingwith a runny appearance may result. The proper starch will give a high quality pie with aheavy-bodied filling.
Other Food Ingredients. Other ingredients can also affect the degree of cook of a starch.For example, sugars have a marked effect on starch gelatinization, and this effect is anexample of the complexity of combining ingredients with starch. Figure 3 shows theeffect of different percentages of sucrose on the gelatinization temperature of moderatelycrosslinked starch. The higher the percentage of sugar, the higher the gelatinizationtemperature. A slurry containing 60% sugar with starch raises the gelatinizationtemperature to above 100C.
Ingredients such as fats and proteins also increase the gelatinization temperature ofstarch. These tend to coat the granule, limiting water penetration and thus hydration andswelling. In all cases, either the starch must be cooked before the addition of theingredients or the appropriate selection of a starch that will gelatinize under theseconditions must be made.
Time and Temperature. The third set of factors to consider when selecting a starchwould be the processing conditions, including time, temperature, and shear. If a starch isnot cooked at the right temperature for the correct amount of time, the degree of swellingwill be inappropriate, and the starch will not impart the desired properties.
Two starch slurries of a crosslinked starch cooked at 160F (71C), one for 5 min and theother for 60 min, will both yield cooks with low viscosity, poor clarity and a starchytaste. Since the temperature is too low, the starch will not swell sufficiently even withextended cooking. The same starch cooked to 180F (82C) and held for 5 min developsa desirable, smooth, short texture; however, at the same temperature for an extendedperiod of time the starch begins to break down, giving lower viscosity and a cohesivetexture. Thus, a properly crosslinked starch must be selected that can work optimallywithin the given temperature and time constraints.
Shear or Mechanical Work. Shear or mechanical work exerted on a starch cook canincrease or decrease paste consistency. It is important to select a starch that can toleratethe processing conditions exerted on it. A lightly crosslinked starch dispersion, whenexposed to low shear, most likely will maintain its granular integrity and little change inthe product will be noted. However, when exposed to high shear, starch granules willfragment and yield product with the properties of overcooked starch-low viscosity andlong texture.
A slurry of a highly crosslinked starch that is exposed to low shear is too inhibited toswell adequately under such processing conditions, and the resulting cook is poor.However, under high shear conditions, this starch may be the appropriate one to provide aheavy-bodied, short-texture product.
Since many types of equipment are used to process food products that involve a range ofprocessing conditions, the starch must also be correctly selected for the equipment. Forexample, a steam-jacketed kettle exerts low shear on starch; however, it has long cookingand cooling times. A plate heat exchanger exposes the food product to high shear butshorter cooking and cooling times at medium temperatures. A colloid mill such as usedby the salad dressing industry represents very high shear. Table II lists typical pieces ofprocessing equipment and conditions associated with them.
Table II. Comparison of Different Kinds of Processing Equipment in Terms ofShear, Cooking and Cooling Time and Temperature Equipment Processing Conditions Steam-jacketed kettle Low shear, long cooking and cooling timesSwept surface cooker and cooler Medium shear, medium cooking and cooling timesTangential steam High shear, high temperature, short cooking timeColloid mill Very high shearSteam infusion cooker Low shear, high temperature, short cooking timePlate exchanger cooker and cooler High shear, short cooking and cooling times,
medium temperatureFlash cooling High shear (caused by vacuum cooling)Pumps Varying degrees of shear
Whether it be time, temperature, pH, sugar, other ingredients or processing equipment, itis essential to select a starch that can tolerate the conditions and provide the desiredproperties.
Applications of Modified Starch ProductsIn this section, several application areas will be highlighted showing where starch can beused and what type of starch will provide the best functionality. The areas to be reviewedinclude dry mixes, emulsion stabilization and encapsulation, confections, low-caloriefoods, frozen foods and microwaved foods.
Dry MixesConvenience plays a very important role in our lives. It is not only the consumers buzzword, manufacturers also want convenience. For consumers, convenience meanspreparation of a food item with minimal time and effort. To the food processor, it meansenergy savings during processing, faster operation and no special equipment becauseingredients mainly require blending and packaging. Instant starches can play a role inboth cases, especially in the area of dry mixes.
Dry-mix foods include both cook-up and instant products. Both are typically preparedand used immediately. In a cook-up system such as a cheese sauce or white sauce, thestarch needed must be able to tolerate the cooking step and provide the desired texture.Thus, a crosslinked starch will be needed. The starch may also be stabilized if the finalprepared product will be refrigerated.
An instant dry mix such as for an instant pudding inherently provides greaterconvenience than the cook-up variety. A pregelatinized starch is typically used herebecause it develops viscosity immediately. A conventional starch of this type consists ofa cookup starch that has been cooked, dried, and ground to the desired particle size.Generally, crosslinked pregel types are the most effective because of their resistance toheat, acid, shear and processing conditions. However, several properties of pregels mustbe considered when selecting the right starch product.
Particle Size. The particle size of pregelatinized starch determines to a large extent itstextural attributes and dispersion properties. For example, a finely ground pregel tends togive a smooth texture, but is difficult to disperse because of rapid hydration and lumping.A finely ground pregel starch is generally blended with other dry ingredients such assugar to achieve good dispersion in the final dry mix food product. In some applications(such as salad dressing production) where oil is used, the starch can be incorporated inthe oil phase to slow down hydration when water is added later.
A coarsely ground pregel is slower to hydrate and disperses quite easily, but imparts agrainy, pulpy appearance to the system. It should be used wherever pulpiness is desired,such as in apple sauce, tomato sauce or fruit purees.
Cold Water Swelling Products. Until recent technological breakthroughs, pregelatinizedstarches showed more graininess, less sheen and less flexibility to processing conditionsthan cook-up products. Thus, a sacrifice in quality had to be made for convenience.However, with new cold water swelling (CWS) starches, the properties of cook-upstarches can be obtained in an instant system. When comparing a dispersion made with atraditional pregelatinized starch to one made with the new CWS starches, a very grainytexture will be noted, while the CWS dispersion will have a very smooth texture, like acook-up starch.
In addition to texture, the CWS starch has more processing tolerance than a traditionalpregel. The amylogram in Figure 4 shows that the pregel breaks down upon extendedheating, while the CWS starch maintains its viscosity.
The cook-up-like properties of CWS starches and their superior performance totraditional pregels can be explained by examining the granular structure of the two starchproducts. The electron micrograph in Figure 5A shows a traditional pregel. Note thefragmented structures and complete loss of granular integrity, similar to an overcookedstarch. Figure 5B shows a CWS starch. Note the highly swollen intact granules, whichare similar to a cook-up starch. The intact granules are responsible for imparting superiortexture and processing tolerance.
Fig. 5. Scanning electron micrographs of traditional pregelatinized waxy corn starch (A)and new cold water swelling waxy corn starch (B).
Emulsion Stabilization/EncapsulationReaction of certain lipophilic substances with starch polymers imparts a slightlyhydrophobic character to the starch and makes a specialty product that can be used inemulsion stabilization and encapsulation. The main purpose of a starch here is to provideviscosity and stability. Liquid emulsions stabilized with a lipophilic starch includebeverage emulsions, liquid clouds, creamers, salad dressings and special water-in-oilemulsions.
Beverage EmulsionsSince the general properties required to make an emulsion are similar in mostapplications, beverage emulsions will be discussed to be representative of the emulsionarea. In beverages, a concentrated flavor emulsion is often used to prepare soft drinkssuch as orange or lemon. A typical process consists of preparing an emulsion of a flavorblend in a solution of a lipophilic starch that also contains citric acid, sodium benzoateand color. This emulsion must be homogenized to obtain the desired particle size. Asmall amount is then used to flavor a finished beverage that contains mostly sugar andwater. Emulsion stability is very important, since the flavor can separate over time.
Such lipophilic starches have been successful in replacing a large portion of importedgum arabic, offering advantages of consistent supply, excellent emulsion stability andfinished beverage quality, with improved microbiological quality and economics.
Encapsulation. Lipophilic starches are also used in encapsulation where emulsificationis one of the steps, followed by drying (generally spray-drying). Typical areas includeflavors, beverage clouds, creamers and vitamins. In each of these areas, emulsion-forming properties are very important.
Figure 6 illustrates how a lipophilic starch works in the encapsulation of lemon oil. Thelipophilic starch will enrobe and protect higher levels of oil than traditional gum arabic.As the amount of oil approaches 50%, the lipophilic starch used in this experimentencapsulates substantially more oil.
Resistance to oxidation of the encapsulated substance is also important. Figure 7compares the oxidative resistance of gum arabic with that of a lipophilic starch. The areasrepresented by the bars are the areas under the peak of a gas chromatography spectrumand show the oxidized component. Immediately after spray drying, the same amount ofoxidized material is present with the starch or the gum. However, after aging, the oil
encapsulated with gum arabic develops more oxidized material than the oil encapsulatedwith a lipophilic starch.
ConfectionsIn the confectionery industry, starch is used for a number of functions, such as a gellingagent, thickener, textural stabilizer, foam strengthener, crystallization inhibitor, adhesive,film former, glazing agent, dusting agent, flow aid and molding medium.
Two areas of starch-based confectionery are soft jelly gums and hard gums. In bothapplications the main function of the starch is to provide structure. In soft jelly gums, theretrogradation properties of amylose are utilized to give a texture ranging from a gelled,short, clean bite to a long and somewhat chewy bite. Examples of soft jelly gums includegum drops, orange slices, cinnamon bears, and jelly centers. In hard gums, a starch thathas controlled retrogradation properties is required. Applications include lozenges, chewycough drops, and wine gums. Only jelly gums will be further discussed.
Fluidity Starch. In soft jelly gums, starch is converted to a low viscosity so that highsolids can be used to obtain structure. Candy made with this type of fluidity starchrequires 48-72 hr to dry. If the starch is over-converted or too thin, a low viscosity slurryresults, yielding splashing of candy, dislodging of molding starch, and increasing set-timein the drying room; if too low of a conversion, a higher viscosity candy slurry is obtainedwhich causes tailing problems and toughness.
Blends with High Amylose Starch. In soft jelly gums, a second generation of starchproducts using high amylose starches has been developed. Because of the high amylosecomponent, these starches can gel more quickly. Figure 8 shows that a fluidity starchblended with a high amylose product provides a faster set as compared to a traditionalfluidity starch alone. These products allow removal from starch molds in approximately24 hr. However, blends must be cooked at considerably higher temperatures thanconventional corn fluidity starch, 335F (168C) vs. 285F (140C), to completelydisperse the molecules for best results.
Special High Amylose Starch. With recent developments, a third generation of productshas become available including a high amylose starch that can be cooked at lowertemperatures and still set rapidly. Figure 9 compares the gel strength of a conventionalfluidity starch, a high amylose blend and such a special high amylose starch. The newhigh amylose product has higher gel strength than the other products and offers theadvantage of being cooked at either low- or high-cooking temperatures and faster dryingtimes.
Low Calorie FoodsThree approaches can be taken to make low calorie foods; for two of these, specialtystarch products have been developed.
Sweetener Replacement. In the first approach, sugar levels can be reduced by usingsuper sweeteners. Such sweeteners can be used at drastically reduced levels compared tosucrose and still impart the desired sweetness at only a fraction of the calories. However,a net caloric reduction for the consumer is possible only if the weight can be made up byusing a correspondingly increased amount of water in preparing the final product actuallyconsumed.
Fat Replacement. Other approaches deal with fat replacement. In one case, fat can bereplaced with compounds that are not digested by the body. Another approach is to use aspecial maltodextrin, a carbohydrate that when dissolved in water gives gels with fat-liketextural characteristics. One gram of a special maltodextrin gel at 25% solids providesone calorie as opposed to nine calories for one gram of fat. It is thus possible to replaceup to 50% of fat in products such as salad dressings, spreads, and sauces without losingthe mouthfeel and meltaway characteristics of the original product.
The formulation in Table III shows a salad dressing where one-third of the oil has beenreplaced with a carbohydrate. The final product has 21% fat. In this application the starchproduct is able to impart mouthfeel, body, and stability over time.
Table III. Typical Reduced Fat Spoonable Salad Dressing Formulations
30% FatControl, Replacement,
Ingredients % %
Soybean oil 30.05 21.35Water 27.14 30.74Distilled white vinegar 19.50 22.00Sugar 11.50 11.50Crosslinkcd and stabilized corn starch a 4.70 4.70Egg yolk 4.50 4.50Maltodextrin b -- 2.60Salt 1.70 1.70Mustard powder 0.70 0.70Onion powder 0.07 0.07Garlic powder 0.07 0.07Paprika 0.07 0.07Calories / 100 g 352.00 284.00 a Purity 420-A, National Starch and Chemical Co.b Instant N-Oil II, National Starch and Chemical Co.
It is also possible to make nonfat frozen desserts such as the formulation shown in TableIV. In this formulation, the carbohydrate provides all of the desired textural propertiesneeded for a high quality product, and yet there are no calories contributed by fat. A 37%caloric reduction (from 187 cal / 100 g for 10% fat ice cream to 117 cal / 100 g for thisproduct) is the result.
Table IV. Typical Non-fat Frozen Dessert Formulation % Fluid skim milk 75.25Cane sugar 10.00Corn syrup (36 D.E.) 6.25Non-fat dry milk 5.90
Maltodextrin a 0.85 a Instant N-Oil II, National Starch and Chemical Co.
In another area of fat replacement, specialty formulated products using the technology ofmodified starches have been developed that are capable of providing those characteristicsof shortening and fats normally incorporated into bakery products, mainly volumecontrol, uniform cell structure, and mouthfeel. The formulation in Table V shows a cakewhere all of the shortening has been replaced. The resulting product has excellent riseand a uniform cell structure. This product has approximately 60 calories less per servingthan a comparable full-fat control.
Table V. Typical Shortening-free Yellow Cake Mix a % Sugar 45.0Cake flour 40.5Special modified starch b 7.6Leavening agent 3.4Dextrose 2.5Salt 0.6Whole egg solids 0.4 a Dry blend, add 1.6 parts water + 1 part whole egg to 3 parts cake mix, beat, whip, bakeat 350F (177C) 28-35 min.b N-Flate, National Starch and Chemical Co.
Frozen FoodsTwo starch modifications of importance in frozen foods are crosslinking, to provide ashortened texture, and stabilization, to provide stability in storage. The effect ofstabilization will be discussed.
Freezing accelerates the retrogradation of the starch molecule. Typical frozen foods todayare exposed to a series of freezing conditions long before the product actually getsthawed for consumption. A typical frozen food may first be frozen in a blast freezer, thenstored in the processing plants warehouse, transported to an intermediate frozenwarehouse by refrigerated truck, and trucked again to a retail outlet. Here, the product isstored in a retail freezer (opened frequently by consumers), then purchased by theconsumer and transported home, stored in a home freezer subject to temperaturefluctuations, then thawed and possibly heated before ingestion.
Effect of Freeze-thaw Cycles. Although the product rarely thaws out completely in thissequence, localized thawing is possible with each temperature change. These minifreeze/thaw cycles cause amylose molecules and straight segments of amylopectin toassociate by hydrogen bonding leading to a more rigid structure with a chunkyappearance, a loss in clarity, and eventually the squeezing out of liquid called syneresis orweeping.
The effect of freeze/thaw cycles on different starches can be easily seen. Figure 10Ashows a cook of unmodified corn starch containing 27% amylose that has been exposedto three freeze-thaw cycles. Because of the retrogradation of the amylose, a gelled texturewith syneresis has formed. Waxy maize that contains 100% amylopectin has inherentlymore freeze/thaw stability than dent corn due to the branched amylopectin molecule.However, it also begins to develop drastic textural changes after freezing and thawing(Fig. 10B).
Stabilization. Figure 10C shows a stabilized waxy maize product after three freeze/thawcycles. This product resists retrogradation and provides excellent freezer stability. Byselecting the appropriate starch, the desired freezer stability can be achieved.
The requirements of a food system will determine the degree of stabilization needed.However, most foods that undergo long-term storage or that may be subjected totemperature fluctuations require a stabilized starch. Table VI shows a formulation for afrozen cookup fruit filling. The starch needed must have heat and acid resistance as wellas low temperature stability. Thus, in this case it is best to use a crosslinked and stabilizedstarch.
Table VI. Typical Frozen Cherry Pie Filling Formulation Using 5:1 Sugar Pack Ingredients % Drained juice plus water after defrosting 29.00Drained cherries 22.00Granulated sugar 7.38Crosslinked and stabilized starch a 3.12Water 8.13Salt 0.06Granulated sugar 8.00 bDrained cherries (approximately) 22.00 b
a COLFLO 67, National Starch and Chemical Co.b Added separately after the initial mixture is brought to 190F (68C)
Microwave FoodsMicrowave heating/cooking continues to gain in popularity and is establishing itself as anecessity in todays convenience-oriented culture. Over 70% of Americas householdsare said to have microwave ovens, and by the year 2001 this percentage will increase to95%. It has been estimated that by then 50% of households will have two microwaves,and $10 billion will be spent annually on microwaved foods, with 85% of dinners havingat least one microwaved component. In any case, the optimal starch for microwaveproducts must be selected with even more discrimination than for conventional products.
Problem Areas. A number of problems have been noted with microwaving that havebeen detrimental to the quality of microwave reconstituted foods, including nonuniformheating, sogginess, lack of crispness and browning, and nonuniform volatilization offlavors. In each of these areas a modified food starch can help. For example, highamylose starches and flours are less sensitive to water than those from dent corn, and canlimit sogginess while maintaining a crisper coating.
In convection ovens, heating occurs from the outside to inside of the product, withgelatinization of the starch occurring in the same way. Starch gelatinization occurs withina narrow range of temperature giving relatively uniformly swollen granules. However,microwave cooking results in not only a broad range of temperatures but also varyingdegrees of granular swelling. This can produce undesirable phase differences andseparations within a food product.
Factors to Consider. Factors that can influence microwave heating include size, shape,density, thickness, and physical state of the food. Irregularly shaped objects heat non-uniformly. As the density of a food product increases, the microwave absorptionincreases with less penetration. Microwaves seem to be transparent to ice, with wavespassing through the ice, heating and overcooking some areas while others are still frozen.Thus, numerous factors must be considered when selecting a starch for a microwavefood. Two specific applications will be discussed: the prepared frozen food and theinstant food prepared by the consumer such as from a dry mix.
Prepared Frozen Foods. In a frozen microwave food, starch properties needed include asmooth short texture, freeze/thaw stability, and microwave tolerance. This can beaccomplished with a crosslinked and stabilized cook-up or CWS starch.
Of particular importance is selecting a microwave-tolerant starch. A lightly crosslinkedstarch incorporated into a sauce may provide good viscosity if microwaved for only ashort period of time, but if microwaved for a longer period of time, the granules becomeovercooked, breaking down and producing a thin sauce. However, by using a slightlymore crosslinked starch, the product will maintain viscosity in the microwave even withextra heating and will not break down.
Mixes for Consumer Preparation. In dry mixes, instant dispersion, rapid viscositydevelopment, and a smooth short texture along with microwave tolerance are needed.These properties are best obtained with a pregelatinized starch, preferably the CWS type.
If a dry mix is reconstituted using a cook-up starch, it may settle out during microwavingunless mixed every few seconds during heating, which can be very undesirable andinconvenient. A stratification of ingredients can occur, and a gelatinous mass of starchmay form on the bottom of the container. To prevent such a problem, it is importantto use a rapidly hydrating starch that will provide viscosity to keep other solids suspendedand to maintain uniform heating.
A traditional pregel will give instant viscosity, but will break down quickly withincreased microwave time. Modified CWS starches are optimally suited for microwavingbecause of the exceptional tolerance to heating imparted by their intact granular structure.They can provide early viscosity and then develop optimal viscosity without breakdownwhen the product is sufficiently warmed (see Fig. 11).
1. Atwell, W. A., Hood, L. F., Lineback, D. R., Varriano-Marston, E., and Zobel, H. F.The terminology and methodology associated with basic starch phenomena. Cereal FoodsWorld 33: 306, 1988.
1990. The American Association of Cereal Chemists, Inc.
Click here to load reader