Food starch technology docx

Sources of Starch The most common sources of food starch are corn, potato, wheat, tapioca and rice.. As might be predicted from this data, the cooked characteristics of tapioca lie somew

Food starch technology

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National researchers utilize advanced instrumentation, such as computerized image analyzers and scanning electron microscopes, to determine starch granule size and distribution and to analyze surface morphology.

Food starch technology

a global commitment

National Starch and Chemical Company is the worldwide leader in specialty starch technology and production With technical service facilities in over 20 countries on five continents, National

is constantly striving to develop new food starches with new characteristics for its customers around the globe

We back up our commitment by building state of the art starch production facilities, investing in advanced analytical equipment, expanding our food pilot plants and maintaining an expert staff

of chemists and technologists It is a fully integrated process that also encompasses the genetic breeding of our own seed corn to develop starch characteristics the marketplace demands

What follows is a brief introduction of the technical knowledge obtained in the development of food starches and their applications We have examined the sources and manufacturing

processes, the chemical and physical structure and the phenomenon of gelatinization We have also explained the reasons for modification, specifically crosslinking and stabilization Finally,

we discuss application technology - interaction of ingredients, processing and equipment

Introduction

In nature starch is available in an abundance surpassed only by cellulose as a naturally occurring organic compound It is found in all forms of green leafed plants, located in their roots, stems, seeds or fruits Starch serves the plant as food for energy during dormancy and germination It serves similar purposes for man and animal as well as lower forms of life Man, however, has found uses for starch that extend far beyond its original design as a source of biological energy Practically every industry in existence uses starch or its derivatives in one form or another In foods and pharmaceuticals starch is used to influence or control such characteristics as texture, aesthetics, moisture, consistency and shelf stability It can be used to bind or to disintegrate; to expand or to densify; to clarify or to opacify; to attract moisture or to inhibit moisture; to

produce short texture or stringy texture, smooth texture or pulpy texture, soft coatings or crisp coatings It can be used to stabilize emulsions or to form oil resistant films Starch can be used to aid processing, packaging, lubrication or moisture equilibration

Starch truly serves as a multifunctional ingredient in the food industry

Sources of Starch

The most common sources of food starch are corn, potato, wheat, tapioca and rice Corn is cultivated in warmer climates, with half of the world’s production grown in the U.S.A., its biggest crop China, the second largest producer in the world, grows about 10% Approximately 70% of the world’s potato supply is grown in the cool, moist climate of Europe and Russia Wheat, requiring a more temperate climate, is primarily grown in the former USSR, North America and Europe Approximately 90% of world rice production comes from South and Southeast Asia, while tapioca is cultivated in the narrow tropical band about the equator

Wet Milling of Corn

Corn starch is extracted from the kernel through a process of wet milling The process employs techniques of grinding, screening and centrifugation to separate purified starch from fiber, oil and tightly bound protein (Fig 1)

The wet milling process begins with softening of the kernel by steeping it in a dilute acid

solution Coarse grinding splits the kernel to remove the oil-containing germ Finer milling separates the fiber from the endosperm which is then centrifuged to separate the less dense protein from the more dense starch The starch is then washed and dried or left in a slurry for further processing, such as cross-linking

Structure

The building blocks of carbohydrates are α-D and β-D glucose which contain six (6) carbon atoms and form pyranose rings Through enzymatic condensation, one molecule of water is split out between two molecules of glucose to form a bond This condensation occurs predominantly between carbons 1 and 4 (Fig 2) but occasionally between 1 and 6

Where only the α-1,4 linkage develops, a linear chained homopolymer results which we refer to

as amylose (Fig.3) The length of this chain will vary with plant source but in general the

average length will run between 500 and 2,000 glucose units.*

(*Traditionally, amylose is considered as being only linear in configuration, but recent

investigations indicate the presence of limited branching in some amylose molecules However, for the sake of simplicity and better understanding of the properties of amylose, this presentation will ignore those findings and consider amylose as linear only.)

It is interesting to note that amylose and cellulose are very similar in structure with the single exception of the spatial arrangement of the bridging between the numbers 1 and 4 carbons The beta glucose form found in cellulose results in a rigid molecule with strong intermolecular

bonding which is not digestible by humans The alpha linkage of amylose allows it to be flexible and humanly digestible

The second type of polymer in starch develops when the enzymatic condensation between glucose units occurs at carbons 1 and 6 This occasional linkage, along with the predominant 1,4 bonding, results in a branching effect and the development of a molecule much more massive in size than amylose but with linear chain lengths of only 25-30 glucose units This molecule is called amylopectin (Fig 4)

All starches are made up of one or both of these molecules, but the ratio of one to the other will vary with the starch source Corn has about 25-28% amylose with the remainder being

amylopectin High amylose corn can run as high as 80% Tapioca has about 17% amylose, and waxy maize has virtually none

As might be predicted from this data, the cooked characteristics of tapioca lie somewhere

between those of corn and waxy maize, and the characteristics of corn are greatly accentuated in high amylose corn

The Starch Granule

As the plant produces the starch molecules, it deposits them in successive layers around a central hilum to form a tightly packed granule Wherever possible, adjacent amylose molecules and outer branches of amylopectin associate through hydrogen bonding in a parallel fashion to give radially oriented, crystalline bundles known as “micelles.” These micelles hold the granule together to permit swelling in heated water without the complete disruption and solubilization of the individual starch molecules (Fig.5)

These highly oriented and crystalline micellular areas explain the ability of ungelatinized starch granules to rotate a plane of polarized light to produce characteristic interference crosses This bi-refringent cross is one of the features used in identifying starch source When the radial orientation of the crystalline micelle is disturbed, the bi-refringent cross disappears (Fig 6)

Gelatinization temperatures are considered as ranges covering the temperatures at which loss of bi-refringence is first noticed and less than 10% remains This temperature range is greatly influenced by the binding forces within the granule which vary with species High amylose corn has much greater bonding force than the other maize varieties due to the high degree of linearity within the granule On the other hand, orthophosphate ester groups within the potato granule tend

to weaken bonding and lower energy requirements to gelatinize

When the starch granule is heated in water, the weaker hydrogen bonds in the amorphous areas are ruptured and the granule swells with progressive hydration The more tightly bound micelles remain intact, holding the granule together Birefringence is lost As the granule continues to expand, more water is imbibed, clarity is improved, more space is occupied, movement is

restricted and viscosity increased

With the swelling of amylose-containing granules such as corn, the amylose molecules are solubilized and leach out into solution These molecules will then reassociate into aggregates and precipitate at low concentrations or set to a gel at higher starch concentrations This is referred to

as “set back” or retrogradation The congealed paste will become cloudy and opaque with time and will eventually release water to shrink into a rubbery consistency (Fig 7)

Waxy maize has essentially no linear amylose molecules so its paste will remain flowable and clear It will not gel or weep

Tapioca, having a small amount of amylose, gives a soft gel when pasted Pastes from high amylose starch set to a very stiff gel

To summarize the physical changes during gelatinization: the granule swells and loses

birefringence; clarity and viscosity increase; and smaller linear molecules (if present) dissolve and reassociate to form a gel

Brabender Amylographs

To follow the viscosity behavior of starch while cooking, we use the Brabender

Viscoamylograph The starch suspension is heated in a revolving cup to a designated temperature and held there, as the viscosity is measured through resistance exerted on a stirrer suspended in the pasted starch A continuous recording of this resistance is marked on a moving chart The resulting graphical curve records the point of gelatinization, the rate of viscosity development, the peak viscosity and the rate of viscosity breakdown If desired, further viscosity information can be recorded during the cooling cycle

Corn starch heated in water to 95°C will show a rather rapid increase in viscosity after

gelatinization until it reaches a peak The viscosity will gradually decrease during the holding period, then dramatically increase again as the paste cools and retrogrades

Waxy maize will increase in viscosity at a more rapid rate than regular corn The peak viscosity for waxy maize will be greater and will be attained sooner However, it will also break down in viscosity faster and to a greater extent On cooling, waxy maize shows little increase in viscosity because it does not gel (Fig 8)

Potato starch will absorb more water, showing a high initial peak viscosity, when compared to other native starches The gelatinization temperature is lower, causing the solution to thicken quickly on heating The high peak viscosity falls rapidly during this holding period The solution shows little tendency to retrograde on cooling

Tapioca starch gelatinizes at a temperature between that of waxy maize and corn, and has a slightly lower viscosity than waxy maize The cooled solution retrogrades to produce a soft gel Solutions of cooked tapioca are characterized by a greater clarity when compared to other native starches

Why Modify Starch?

In the unmodified form, starches have limited use in the food industry Waxy maize starch is a good example The unmodified granules hydrate with ease, swell rapidly, rupture, lose viscosity and produce weak bodied, very stringy and very cohesive pastes We modify starch to enhance

or repress its inherent properties as appropriate for a specific application: to provide thickening, improve binding, increase stability, to improve mouthfeel and sheen, to gel, disperse or cloud

Cross-linking

We cross-link to control texture and to provide heat, acid and shear tolerance As a result, we have better control and improved flexibility in dealing with formulation, processing and product shelf-life Cross-linking of starch can be thought of as a means to “spot weld” the granule at random locations, reinforcing hydrogen bonding and inhibiting granule swell (Fig 9)

This cross-linking treatment strengthens the relatively tender waxy starches so that their cooked pastes are more viscous and heavy bodied and are less likely to breakdown with extended

cooking times, increased acid or severe agitation

The change in the swelling characteristics is obvious from the Brabender curves of unmodified and lightly modified waxy maize With very light modification the viscosity breakdown is

reduced dramatically With a moderate level of treatment, the granule swell is restricted to the point where peak viscosity is never reached during the holding period As the cross-linking reaction progresses, the peak viscosities first increase and then decline to very low values as the swelling of the starch granules become progressively more inhibited At the same time, shortness

of string and opacity of paste both increase (Fig 10)

As the degree of cross-linking is increased, the starch becomes more tolerant to acid and less likely to break down This is not to suggest that the most highly cross-linked starch will give the

best viscosity in low pH food systems It must be remembered that cross-linking inhibits granule swell whereas high temperature, extended heating, high hydrogen ion concentration and high energy input all tend to disrupt hydrogen bonding and enhance granule swell With this in mind, one should select the starch which is cross-linked sufficiently to withstand chemical and physical abuses and still give maximum viscosity

If it happens that a moderately cross-linked starch tends to break down when cooked at low pH, the problem can sometimes be solved with a procedural change By cooking the starch at a higher pH, allowing the paste to cool and then adding the acid to reach the desired pH, a

satisfactory viscosity may be reached without changing to a more highly cross-linked starch

Stabilization

Another important starch modification is that of stabilization This modification prevents gelling and weeping and maintains textural appearance

Earlier, it was mentioned that the linear fraction of cooked corn starch will reassociate

through hydrogen bonding causing gelling, opacity and weeping Since the highly branched waxy variety has no amylose, it will not retrograde or gel under normal storage conditions However, under low-temperature or freezing conditions, a waxy paste will become cloudy and chunky and will weep, somewhat like the paste made with regular corn starch This is attributed

to a lowering of kinetic movement as the temperature drops, allowing the outer branches of the waxy starch to associate through hydrogen bonding and bringing about similar but less dramatic conditions that occur with amylose

To prevent the occurrence of this condition, anionic groups are scattered throughout the granule

to block molecular association through ionic repulsion as well as steric hindrance (Fig 11)

The result of this treatment is a stabilized starch which will produce pastes that will withstand several freeze-thaw cycles before syneresis (weeping) occurs

Freeze-thaw stabilized starches are essential to the frozen food industry but have applications in many other areas as well Cold temperature storage conditions of other processed foods such as canned sauces and gravies are commonplace and require stabilized starches to maintain quality