topic7 starch Flashcards
what is starch?
A carbohydrate, It is a polysaccharide made up of glucose molecules
2 major categories of starch?
Native Starch
Modified Starch
what is native starch?
pure starch isolated from original plant material
what is modified starch?
native starch that has undergone chemical or physical processing to change its properties
2 structures of starch
amylose 直链淀粉
amylopectin支链淀粉
how to distinguish amylose and amylopectin
Amylose bule
Amylopectin red purple
amylose is responsible for gelling or thickening?
gelling
amylopectin is responsible for gelling or thickening?
thickening
what is waxy variety?
only contains amylopectins
sources for starch
Cereal grains (corn, wheat, rice, oats)
Legumes (dried beans)
Roots and tubers (potato, tapioca, arrowroot)
basic ingredients for bubble tea?
black tea, water, tapioca pearls, milk, syrup or sweetener
why is the tapioca pearls have a dark appearance?
added brown sugar or caramal
wet milling process
- Cleaning
- Grinding or Grating
- Slurry
- Separation/Refining
- Drying
maize starch = corn starch
what’s the difference between whole grain and the white grain
no germ(for oil), no bran(for fibre)， so has a little nutritive value
what’s the use of refined starch?(white grain)
Added to other foods to act as a thickener增稠剂, stabilizer稳定剂, bulking agent膨松剂 and anticaking agent抗结剂()
Terminology
Gel and colloid(sol)
Gel:colloidal dispersion that holds shape保持形状的胶体分散物
Sol:intermediate between small particles in solution and large particles in suspension介于溶液中的小颗粒和悬浮液中的大颗粒之间
Starch is deposited in plants in organized units called granules
in Seeds, roots and tubers
Birefringence of starch
When the starch molecules are observed microscopically under polarized light当在偏振光下显微镜观察淀粉分子时Light is rotated so that the cross-pattern on each granule is observed,光被旋转，以便观察到每个颗粒上的交叉图案This pattern disappears once the starch is heated一旦淀粉被加热，这种图案就会消失
what is native starch
Developed through traditional plant breeding or genetic engineering No physical or chemical modification of the starch
what is modified starch?
Chemical or physical modifications may be used to change function in food preparation
how to modify starch?
- Hydrolysis or acid-converted–>used in the confectionary糖果，甜食
- Cross-linked or cross-bonded:improved viscosity and texture
- Substitution or stabilization prevents retrogradation凝沉
the use of hydrolysis or acid converted starch
produces Low viscosity paste, stiff gel, Useful in confectionary industry
the use of cross linked or cross bonded starch
Increased temperature for hydration therefore greater tolerance to heat, Resistant to shear or stirring,Useful for canned soup, spaghetti sauces, and pie fillings
steps of hydrolysis starch
- mix starch with water
2.add acid - dry
steps for physically modified starch
1.Instant or pregelatinized –> instant dry mixes, gravies, sauces
2. Cold-water swelling –> pregelatinized\
3. Heat treated —> greater viscosity and stability
what is resistant starch
Resistant starch is not digested by the human body
4 main types of resistant starch
RS1 resistant to digestion ex whole grains, seeds
RS2 resists digestion due to compact structure ex green bananas
RS3 retrograded ex cooked and cooled potatoes
RS4 only one chemically modified ex modified starches
dry heat and starch
Starch becomes more soluble
Reduced thickening ability
Dextrinization –>when starch is broken down into dextrins糊精化——淀粉被分解成糊精
Brown, nutty, toasted flavor develops
examples of the use of dry heated starch
Examples: Brown Roux and Espagnole Sauce
moist heat and and starch
Starch granules insoluble in cold water
so
Colloidal dispersion produced with heating
produces Starch paste
Changes with Moist Heat and Cooling
Gelatinization凝胶化:With heat, starch granules absorb water and swell
Pasting浆糊
Gel formation or gelation –>during cooling
Retrogradation
when would retrogradation happen?
Occurs upon standing after gel formation
what is retrogradation like?
Amylose molecules associate more closely, the gel netnork syneresis(脱水缩合)
Factors Affecting Starch Pastes
- Temperature and time of heating
- Agitation and stirring
- Acidity
- Addition of other ingredients
Temperature and Time of Heating —> starch pastes
Thicker if cooked quickly
undercooked pastes will have a raw starch flavour, and less smooth and silky
Agitation or Stirring and starch pastes
Stirring is desirable to disperse starch,If excessive – starch granules may rupture破裂 and develop a Slick and pasty mouthfeel口感柔滑，糊状
Acidity (pH) and starch pastes:Add acid late in cooking process(lemon pie)
May cause fragmentation and hydrolysis,pH below 4 will lead to a thinning of paste
cross-linked starch has a better resistance of acid
Addition of Other Ingredients and starch pastes
Sugar
Raises temperature of gelatinization,May decrease viscosity
Fats and proteins
Delay hydration of starch
Lowers rate of viscosity development
Starch Cookery: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 to thickening a liquid with a roux?
(a) add cold liquid or stock to a hot
roux or
(b) add cold roux to a hot liquid
or stock.
why not recommend to blend a hot roux into hot liquid?
it is very difficult to smoothly blend the roux into the liquid before lumps are formed due to the rapid gelatinization of the
starch. 由于淀粉的快速糊化，在形成团块之前，将面粉糊顺利地混入液体是非常困难的。
Five Basic Mother Sauces
- Bechamel 白酱with milk
- Veloute 白酱with chicken stock
- Espagnole 红酱with beacon
- Tomato
- Hollandaise 蛋黄酱
sauce that thickened by starch
- Bechamel (white sauce) —>fat, flour, milk, seasonings
- Veloute —> fat, flour, veal/chicken/fish stock, seasonings
- Espagnole —> fat, flour, beef stock, seasonings
sauce that May be thickened with starch
tomato
