Lecture 7 Flashcards
What is starch?
A Carb
-polysaccharide made up of glucose molecules
How do plants store energy?
As starch
What are the 2 major categories of starch?
Native Starch: Pure starch isolated from original plant material
Modified Starch: Native starch that has undergone chemical or physical processing to change its properties
What is the difference between amylose and amylopectin?
Amylose: Linear molecule of glucoses
Amylopectin: Branched molecule of glucoses stuck together
How are glucose molecules joined together in starch?
No. 1 carbon atom of one glucose molecule and the no.4 carbon atom of the next one
How does amylopectin join the branching glucose molecules?
Joined between No. 1 atom of one glucose unit and the No. 6 carbon atom
How is starch broken down?
Amylase beaks down the glucoside bond of maltose to form glucose
What are sources of starch?
Cereal grains (corn, wheat, rice, oats)
Legumes
Roots and tubers
What are the steps in the wet milling process?
- Cleaning
- Grinding or Grating
- Slurry
- Separation/Refining
- Drying
What is refined starch?
Refined to remove other components of the food such as fat, protein vitamins and minerals
What is refined starch added too?
Added to other foods to act as a thickener, stabilizer, bulking agent and anti caking agent
-little nutritive value
What is a gel?
Colloidal dispersion that holds shape
What is a colloid?
Intermediate between small particles in solution and large particles in suspension
What is a starch granule?
Starch is deposited in plants in organized units
-seeds, roots and tubers
What is Birefringence?
When he starch molecules are observed microscopically under polarized light
- light is rotated so that the cross pattern on each granite is observed
- This pattern disappears once. the starch is heateed
What is native starch?
No physical or chemical modification of the starch
-Developed through traditional plant breeding or genetic engineering
What is modified starch?
Chemical or physical modifications may be used to change function in food preparation
What are the steps to modify starch?
Hydrolysis or acid converted–> used in the confecitionary industry
Cross-linked or cross-bonded–> improved viscosity and texture
Substitution or stabilization–> Prevents retrogradation
In step one of modifying starch what is the result?
Low viscosity paste
Produces stiff gel
Useful in confectionary industry
In step 2 of modifying starch, what is the result?
- Increased temp for hydration therefore greater tolerance to heat
- Resistant to shear or stirring
- Useful for canned soup, spaghetti sauces and pie fillings
In step 3 of modifying starch what is the result?
- Prevents retrogradation
- Reduces syneresis
- Useful for frozen or refrigerated starch-thickened foods
What is the function of modified starch?
Chemical or physical modifications may be used to change function in food preparation
What are the ways in which starch is physically modified?
Instant or pregelatinized–> instant dry mixes, gravies, sauces
Cold water swelling–> pregelatiniized
Heat treated–> greeter viscosity and stability
What is resistant starch?
Not digested by the human body
What are the 4 main types of resistant starch?
RS1- Resistant to digestion (whole grains, seeds)
RS2- Resists digestion due to compact structure (green bananas)
RS3-Retrograded (cooked and cooled potatoes)
RS4- Only one chemically modified (modified starches)
What can modified starch be used in food for?
Increase fibre content
What effect does dry heat have on starch?
Starch becomes more soluble
Reduced thickening ability
Dextrinization: when starch Is broken down into dextrins
Brown, nutty, toasted flavour develops
What effect does moist heat have on starch?
Starch granules insoluble in cold water
Colloidal dispersion produced with heating
Starch paste
What happens during gelatinization?
With heat, starch granules absorb water and swell
Increased viscosity and increased translucency
What happens during get formation/gelation?
Occurs with cooling
What occurs during retrogradation?
Occurs with standing after gel formation
Amylose molecules associate more closely
Gel network (syneresis)
What are the factors affecting starch pastes?
Temperature and time of heating
Agitation and stirring
Acidity
Addition of other ingredients
How is starch pastes affected by temperature and time of heating?
Thicker if cooked quickly
If undercooked
- raw starch flavour
- less smooth and silky
How is starch pastes affected by agitation or stirring?
Stirring is desirable to disperse starch
If excessive- starch granules may rupture
-Slick and pasty mouthful
How is starch pastes affected by acidity?
May cause fragmentation and hydrolysis
Decrease of thickening powder
Add acid late in cooking process (lemon pie)
How is starch pastes affected by the addition of other ingredients?
Sugar: raises temperature of gelatinization and may decrease sicosity
Fat & Protein: delay hydration of starch and lowers rate of viscosity development
What are the methods of cooking with starch?
Combining starch with hot liquids
Roux: Prepared by blending melted fat with flour
Slurry: Mixing flour or starch with some of the cold liquid which is then added to a hot liquid while stirring
How do you thicken a liquid with a roux?
Add cold liquid or stock to a hot roux
OR
Add cold roux to a hot liquid or stock
What do you need to avoid doing when making a roux?
Blending a hot roux into a hot liquid is not recommended because it is very hard to smoothly blend the roux into the liquid before lumps are formed due to the rapid gelatinization of the starch
What are the 5 basic bother sauces?
Bechamel (fat, flour, milk, seasoning)
Veloute ( fat, flour, veal/chicken/fish stock, seasoning)
Espagnole (fat, flour, beef stock, seasoning)
Tomato
Hollandaise
Which of the sauces are thickened with starch or emulsion?
Starch: Bechamel Veloute Espagnol Tomato (sometimes)
Emulsion:
Hollandaise
What do cream soups usually have?
Mirepoix for flavouring
Thickened with
- starch (roux or slurry)
- Pureed starchy vegetables (potatoes)
What is the thing you dont want to happen with cream soups?
Curdling
- acid (tomatoes)
- Freshness of milk/cream
- Higher fat milk/cream is more stable than low or nonfat
How do you thicken desserts with starch?
Corn starch (clear & glossy)
Flour
Tapioca
Cook starch mixture before adding eggs
-temper eggs
Can starch be microwaved?
Yes, best for small batched
Stop microwave and stir in about every 1 minute
