Ch. 9: Starches (part 1)
Terms in this set (53)
What are the 2 types of complex CHO's?
What is the structure of amylose?
D-glucose linked by a-1,4-glycosidic linkages (linear)
-slightly soluble but not sweet
-most foods have 17-30% amylose
What is the structure of amylopectin?
D-glucose linked by a-1,4-glycosidic linkages AND a-1,6-glycosidic linkages (linear for 10-25 units then branches)
-majority 80% of starch in foods
What are starch granules?
concentric layers of amylose and amylopectin molecules formed in the leucoplasts
What are leucoplasts?
plastids in the cytoplasm of plant cells; storage site of starch in plants as granules
What is the innermost layer of a granule? What are they held together by?
What parts of concentric rings are highly ordered?
What part of concentric rings are amorphous and less dense?
Describe birefringence. What does it form?
-refraction of light in 2 slightly different directions
-ordered portions of RAW starch granules cause this under polarized light
List the functional properties of starches.
4. gelation and syneresis
What happens when starch is added to cold water?
Define gelatinization. What is its results?
-swelling of starch granules and migration of some amylose into cooking water when starch is heated in water
-thickened food; makes starch more easily digestible
changes that occur with continued heating of a gelatinized starch (forms a paste)
What happens to viscosity during pasting?
initially increases, then decreases when maximum is reached
Starch gelatinization manifested in irreversible changes in what properties?
-loss of crystalline structure
-loss of birefringence
-loss of amylose from granule
Continuation of gelatinization through additional heating involves...
-further loss of amylose from the granule
-increase (then decline) in viscosity
-granules are even more swollen and softer
-highest thickening capacity of the starch
What factors affect gelatinization and pasting?
-other ingredients (sugar, acid, lipids, proteins)
Does starch become less or more viscous when held at 95*C?
List more effective thickeners. Examples?
-root starches (more translucent)
-potato (most effective but undesirable texture); tapioca (undesirable texture)
List less effective thickeners.
-cereal starches (less translucent; wheat least effective cereal starch)
-waxy cereal starches more effective than non-waxy starches
What is the affect of sugar on gelatinization and pasting? Results?
-is hygroscopic and competes with starch for water needed for gelatinization
-delayed gelatinization; increased final temp; increased translucence of starch mixture; less viscosity; less gel strength
What is the affect of acid on gelatinization and pasting? Results?
-acid + heat causes hydrolysis of the starch molecules, which move more freely in starch mixture
What is the affect of lipids and proteins on gelatinization and pasting? Results?
both reduce temp of max gelatinization and viscosity
If a starch mixture becomes hot, what is it?
sol (solid trapped in a liquid)
When a starch mixture is cooled, what is it? What is this process called?
-gel (liquid trapped in a solid)
-gelation (hydrogen bonds form between escaped amylose molecules, eventually forming a stable cross-linked network
What are some factors affecting gelation?
1. starch type
2. starch concentration
5. other ingredients
What is required for significant gelation? What starch gels effectively?
-cereal starch (cornstarch)
Flours vs pure cereal starches are less effective in gelation. Why?
due to protein content and impurities
What are the effects of temp on gelation?
-too much heat = granules collapse
-optimal gel strength requires enough heating so that amylose molecules escape
What are the effects of agitation/stirring on gelation?
thins the paste by breaking granules and disrupting hydrogen bonding (resulting in a weakened gel)
What are the effects of sugar on gelation? Acid?
-decreases gelatinization and amylose release (softer)
-decreases gelatinization by hydrolysis of granules (softer)
What are the effects of aging on gelation? What happens during aging?
-under normal conditions, some free water is trapped between water molecules within the gels structure
-amylose molecules draw together, water is squeezed out
-amylose molecules draw together
-water is squeezed out
Define retrogradation. Give an example.
-realignment of amylose molecules into more orderly crystals (results in gritty texture)
-ex: pudding, stale bread
-occurs during dry-heating of starch
-temp rises rapidly and causes chemical degradation of starch
-amylose/amylopectin split into shorter molecules called dextrins.
What are the 3 main components of cereal grains?
-endosperm (source of complex carbs; b vitamins, proteins)
-bran (fiber, b vitamins, phytonutrients, minerals)
-germ (essential fatty acids, vitamin B & E, minerals)
Define a native starch. What are they used for & some examples?
-used without being modified from their original form
-thickening, commercial food production, flours (home use); ex: corn starch, rice, wheat, potato, waxy maize
What is a waxy starch?
almost 100% amylopectin (has no amylose because it won't form gel during thickening); ex: waxy cornstarch, waxy sorghum
How do different native starches vary?
-gelatinization and gelation characteristics
What are root starches?
-potato: large starch granules, does not gel
-tapioca: cassava plant, processed into pellets, very small starch granules
Define a modified starch. What is its use?
-physically or chemically changed from its native state
-more stable and consistent during processing, wider range of textures, prolonging maximum viscosity
Define a pre-gelatinized starch.
has been gelatinized and then dehydrated; can be re-thickened by adding water (physical modification, no heating needed, less thickening ability)
Define thin-boiling starches. What is another name for it?
-formed by treating a starch with an acid to hydrolyze starch molecules
What are possible results from thin-boiling starches?
-decreased thickening power (decreased viscosity)
-increased solubility of starch
-increased debranched amylopectin molecules
-increased gel strength when cooled
Define resistant starches. What are the 4 types?
-not digested (acts like fiber) and does not contribute to caloric intake (possible health benefits: colon health, weight management)
-RS1, RS2, RS3, RS4 (modified starches created chemically - cross linked starches)
Define oxidized starches.
thin-boiling starches produced by alkaline treatment (treatment degrades into smaller molecules; powder-like consistency)
Define cross-linked starches.
-undergo minimal retrogradation
-benefits: larger size molecules=more heat and acid resistant=less likely to rupture, delayed gelatinization, and more viscous; decreased retrogradation
Define starch phosphates.
-starches esterified with phosphate groups (often form sodium tripolyphosphate)
-benefits: increased stability of thickened starches, increased viscosity, decreased retrogradation, decreased syneresis
Define starch acetate.
-formed when hydroxyl groups on starch are substituted with acetyl groups
-benefits: decreased retrogradation, increased heat stability
What are some functions of starch in food products?
-water or fat binders
What are the basic components of a thickened sauce?
What are the 3 methods for preparing thinking agents?
1. Roux - made by cooking equal parts flour (usually wheat) and fat
2. Buerre manie - equal parts of uncooked flour and soft butter are mixed into paste
3. Slurry - starch mixed into cold water, then slurry is added to a simmering liquid base
What must be considered when selecting a starch for use?
-will item be refrigerated and eaten without reheating
-equipment will be used to manufacture product