4.4 Starch and modified starches in food applications

Starch, a crucial carbohydrate in food science, consists of amylose and amylopectin. These components give starch its unique properties, influencing texture, stability, and functionality in various food applications. Understanding starch structure is key to manipulating its behavior in cooking and processing.

Starch undergoes important transformations like gelatinization and retrogradation, affecting food quality and shelf life. Modified starches offer enhanced functionality, allowing for improved stability, texture, and nutritional benefits in processed foods. These modifications expand starch's versatility in the food industry.

Starch Composition and Structure

Amylose and Amylopectin: The Building Blocks of Starch

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• Starch consists of two main components: amylose and amylopectin
• Amylose is a linear polymer of glucose units connected by α-1,4 glycosidic bonds
• Amylose typically makes up 20-30% of starch granules and contributes to gel formation and film-forming properties (cornstarch)
• Amylopectin is a branched polymer of glucose units with α-1,4 and α-1,6 glycosidic bonds
• Amylopectin accounts for 70-80% of starch granules and provides structure and stability to starch (potato starch)
• The ratio of amylose to amylopectin varies depending on the botanical source of the starch and influences its functional properties in food applications

Starch Granule Structure and Properties

• Starch exists as granules, which are dense, semi-crystalline structures
• Starch granules contain alternating amorphous and crystalline regions formed by the arrangement of amylose and amylopectin
• The size, shape, and composition of starch granules vary among plant sources (wheat, rice, corn)
• Starch granules are insoluble in cold water due to their compact structure and hydrogen bonding
• When heated in the presence of water, starch granules undergo irreversible changes, leading to gelatinization

Starch Gelatinization and Retrogradation

The Gelatinization Process

• Gelatinization occurs when starch granules are heated in the presence of water, typically between 60-70°C (140-158°F)
• During gelatinization, water penetrates the starch granules, causing them to swell and lose their crystalline structure
• As the temperature increases, amylose leaches out of the granules, increasing the viscosity of the starch suspension (thickening sauces and gravies)
• Gelatinization is crucial for the development of texture, viscosity, and digestibility in starch-based foods (puddings, pie fillings)
• Factors influencing gelatinization include starch type, water content, temperature, and the presence of other ingredients (sugar, fat)

Retrogradation and Staling

• Retrogradation is the recrystallization of starch molecules after gelatinization, leading to the formation of a gel network
• During retrogradation, amylose and amylopectin chains realign and form hydrogen bonds, resulting in a more ordered structure
• Retrogradation is responsible for the staling of bread and the firming of starch-based gels over time (rice cakes)
• The rate and extent of retrogradation depend on factors such as starch type, storage temperature, and moisture content
• Retrogradation can be minimized by using modified starches, controlling storage conditions, or adding ingredients that interfere with starch recrystallization (emulsifiers)

Starch Hydrolysis and Digestibility

• Starch hydrolysis is the breakdown of starch molecules into smaller units by enzymes or acid
• Amylases, such as α-amylase and β-amylase, are enzymes that catalyze the hydrolysis of starch into maltose, glucose, and dextrins
• The rate and extent of starch hydrolysis depend on factors such as starch type, granule size, and the presence of inhibitors (phytates, tannins)
• Starch digestibility is influenced by its structure, gelatinization state, and the presence of resistant starch
• Slowly digestible and resistant starches have lower glycemic responses and can provide health benefits (improved gut health, blood sugar control)

Modified Starches

Cross-Linked Starches

• Cross-linked starches are modified by creating covalent bonds between starch molecules using bifunctional reagents (phosphorus oxychloride, sodium trimetaphosphate)
• Cross-linking improves the stability and resistance of starch granules to high temperatures, shear, and acidic conditions
• Cross-linked starches have higher gelatinization temperatures and reduced swelling compared to native starches
• Applications of cross-linked starches include canned foods, salad dressings, and bakery fillings, where stability under harsh processing conditions is required

Pregelatinized Starches

• Pregelatinized starches are modified by pre-cooking and drying native starches, resulting in instant cold-water solubility
• The pre-cooking process disrupts the crystalline structure of starch granules, allowing them to hydrate and swell rapidly in cold water
• Pregelatinized starches provide instant thickening and texture without the need for heating (instant puddings, cold sauces)
• The viscosity and texture of pregelatinized starch dispersions can be controlled by adjusting the degree of pregelatinization and particle size

Resistant Starches

• Resistant starches are starch fractions that resist digestion in the small intestine and are fermented in the large intestine
• There are four main types of resistant starch: physically inaccessible (RS1), granular (RS2), retrograded (RS3), and chemically modified (RS4)
• Resistant starches can be used as functional ingredients to increase fiber content, lower glycemic response, and improve gut health (prebiotics)
• Food sources of resistant starch include legumes, unripe bananas, and high-amylose starches (Hi-Maize)
• Processing techniques, such as cooking and cooling, can increase the formation of resistant starch (potato salad, sushi rice)

Viscosity Modification

• Viscosity modification involves altering the thickening properties of starch to suit specific food applications
• Acid-modified starches are treated with dilute acid to reduce their viscosity and improve clarity (fruit pie fillings)
• Oxidized starches are treated with oxidizing agents (hydrogen peroxide, sodium hypochlorite) to reduce viscosity and improve whiteness (paper coating, textile sizing)
• Thin-boiling starches are acid-modified starches with very low viscosity and high clarity, used in confectionery and beverage applications
• Viscosity can also be modified by controlling the degree of starch hydrolysis using enzymes or acid, resulting in dextrins and maltodextrins with varying dextrose equivalents (DE)