Questions and Answers
What impact does the ratio of amylose to amylopectin have in wheat flour?
Rice starch granules are characterized by what shape?
What is the typical size range of potato starch granules?
How are the granules of arrowroot starch described?
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What happens to starch when amylose is absent during an iodine reaction?
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What is a characteristic feature of tapioca starch granules?
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What does the helium of maranta starch resemble?
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How is the starch composition characterized between the inner and outer endosperm of maize?
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What is the significance of amylose and amylopectin composition in starch?
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Starch can generally be referred to with what type of structures?
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Study Notes
General Characteristics of Starches
- Starches are the initial visible product of photosynthesis and act as energy reserves in higher plants.
- They are generally insoluble in cold water and alcohol, and yield a blue-black color when treated with iodine.
- Starch granules vary in shape (spherical, polygonal, elliptical) and size (1-100 µm in diameter).
- The distinct characteristics of starch granules depend on plant species and maturity levels.
- Starch granules exhibit concentric layers with an eccentrically located spot called the helium, which is crucial for identification.
- Upon heating with water, starch granules swell or gelatinize, with the gelatinization temperature ranging between 60-80ºC depending on starch source.
Composition of Starch
- Most starches comprise two polysaccharides: amylose (15-25%) and amylopectin.
- Amylose has a helical structure; in certain plants like wrinkled pear, it can constitute 50-75%.
- Amylopectin is present in higher amounts and reacts differently with iodine, displaying a reddish-brown color without amylose.
Physical Properties for Identification
- Starch grains can appear spherical, elliptical, ovoid, or polygonal.
- The grain size can range from 1-100 µm.
- Helium location can be central or eccentric, with some grains exhibiting two growth points, termed half-compound grains (e.g., potato starch).
- Helium can take various shapes: spherical (potato), cross-shaped (arrowroot), or cleft (maize).
- Starches yield differing iodine reactions, with amylose binding significantly more iodine than amylopectin.
Microscopy and Granule Structure
- Starch granules can be simple or compound, with compound referring to multiple granule components.
- They are produced in leucoplasts, with growth initiating at the helium and layered deposits forming concentric structures.
- Characteristic features such as the position of helium and the presence or absence of striations are important for identification.
Commercial Starches and Their Sources
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Maize Starch (Zea mays)
- Common starch, used in various products and industries.
- Shapes include polyhedral to sub-spherical, with a size of 10-20 µm.
- Structural differences exist between starch from inner and outer endosperm.
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Wheat Starch
- Major component of wheat grain, constituting 63-66% starch.
- Classifications: A-type (lenticular, >15 µm), B-type (spherical, 5-15 µm), and C-type (<5 µm).
- Typically 25-28% amylose and 72-75% amylopectin, affecting texture and viscosity in food products.
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Rice Starch
- Comprised of compound granules with sharp angles, ranging from 3-8 µm.
- Helium is centralized and prominent; no striation present.
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Arrowroot Starch
- Mostly simple with oval or ellipsoidal granules.
- Helium appears as a 2-rayed cleft and shows concentric striations.
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Tapioca Starch
- Characterized as simple, sub-spherical, or polyhedral granules sized 5-35 µm.
- Helium features are punctate or cleft, with concentric striations.
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Potato Starch
- Comprised of simple, irregular, ovoid, or spherical granules sized 30-100 µm.
- Features eccentric helium.
Revision Questions
- General characteristics of starch: organized structures, varying shapes, size range, central/eccentric helium, iodine reaction.
- Separation of amylopectin from amylose via selective precipitation.
- Examples of aggregated starch grains: rice, cardamom.
- Shape of helium in Maranta starch: 2-rayed cleft.
- Two points of growth in a starch grain are termed half-compound.
- Absence of amylose yields a reddish-brown color with iodine.
- Wheat starch consists of 25-28% amylose and 72-75% amylopectin.
- Helium in arrowroot starch is 2-rayed cleft.
- Inner maize endosperm starch is polyhedral; outer is mortar-shaped and polygonal.
- Starches can be referred to as unorganized with organized structures.
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Description
Explore the fascinating world of starches, the primary product of photosynthesis and a vital energy reserve for plants. This quiz covers their characteristics, organization, and the role they play in plant biology. Test your knowledge on the properties and classification of starches.
