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NST106-Lec5 - Lecture 5 Carbohydrates 2 Disaccharides...

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Unformatted text preview: Lecture 5: Carbohydrates 2 Disaccharides: Sucrose Structure, Properties, Significance, Production Polysaccharides: Starch Significance, Uses High Fructose Corn Syrup Science & technology of production Sucrose: α-D-glucose(1-2)β-D-fructose Raffinose family oligosaccharides (RFO) Sucrose homologues = raffinose family oligosaccharides: found in legumes (soybean, kidney bean, red bean, pinto,etc) Raffinose (trisaccharide) α-D-gal(1-6)α-D-glucose(1-2)β-D-fructose Stachyose (tetrasaccharide) α-D-gal(1−6)α-D-gal(1-6)α-D-glucose(1-2)β-Dfructose Undigestible: we don’t have α−galactosidase, RFOs end in lower GI tract fermented by bacteria gas (flatulence) Sucrose: Properties Structure: head-to-head (anomeric C1 of glucose to anomeric C2 of fructose) linkage – nonreducing Hydrolyzed by sucrase in the human small intestines Daily per capita consumption in the US: 160 g Sources of sucrose in the US: Sugar cane (Hawaii, Louisiana, mostly imported – Carribean, Indonesia, Malaysia, Philippines, etc) Sugar beets (Idaho, Montana) Sources of sucrose Sugar cane (70%) Sugar beet (30%) Sugar cane plantations Sugar production is tied up with history and culture of many countries Sugar production extraction evaporation filtration evaporation Sugar cane --- juice (12% sucrose)----- clarified juice--------syrup (30% solids) --------- 95% solids molasses (raw or 1st) centrifugation crystals (raw sugar muscovado) Raw sugar – exported to mainland US (i.e. California C&H) for further processing molasses (2nd) redissolved in water centrifugation white sugar Brown sugar and molasses Brown sugar: 2 forms Raw sugar (muscovado) White sugar mixed with 2nd molasses (More minerals than white sugar) Molasses: 2 forms – raw (1st) and 2nd molasses 2nd molasses – sold in US 24% water 65% sucrose (non-crystallizable) rich in minerals - 8% ash, Fe, K, etc (check USDA NDL data base) K = 1063 –2927 mg/100g potato = 600 mg/100g banana = 400 mg/100g Sucrose and other sweeteners Sweetener Sucrose Glucose Fructose Galactose Invert sugar Lactose Maltose Xylose Xylitol Aspartame Saccharin Cyclamates Relative sweetness (5% solution) 1.0 0.58 1.3 0.4-0.6 0.85-1.0 0.2-0.3 0.3-0.5 0.59 1.01 200 20,000-70,000 3000-8,000 Various forms of sucrose as food ingredients Table sugar – white or brown Confectionery sugar – crystals that are ground to very fine powder + 3% corn starch to prevent caking Liquid sugar – dissolved in water – easier to handle than crystal sugar (very soluble, hydrophilic) –shipped in large tanks Syrups (concentrated liquid) – used in pancakes, etc., no need for refrigeration, aw is low (0.65 – 0.75) brown sugar sugar cube liquid sugar Polysaccharides: STARCH Big picture (other polysaccharides): Plant seeds: Starch, cellulose, hemicellulose, pectin Plant exudates: gum arabic, gum tragacanth Seed galacturonans: guar gum, locust bean Seaweed: carageenan, algin Animal: glycogen Starch Unique among carbohydrates – chemical, physical and nutritional properties set it apart from other CHO Plant biology: predominant food reserve in plants Nutritional significance: provides 80% of dietary calories in humans worldwide rice is the staple food of half of the world’s population, eaten by about 3B people starch and starch hydrolysis products make up most of the digestible CHO in the human diet Sources and uses of commercial starches ► Cereal grains: corn, wheat, rice ► Tubers/roots: potato, tapioca (cassava), taro ► Food (functional) uses: natural and modified starches have enormous number of uses: thickening, binding, dusting, film forming, gelling, glazing Chemical structure of starch: amylose and amylopectin Amylose (linear chain) MW ~ 106 Common starch ~25% amylose High amylose starch ~50%, ~70-75% amylose (by plant breeding) Axial-equatorial link of α1-4 gives amylose righthanded helix shape, some branches ~0.4% Iodine test – helix structure allows I2 to be trapped inside –giving blue color- used to test presence of amylose in starch solution Amylopectin (branched chain) MW ~ 107 – 5 X108 Some of the largest molecules in nature Common starch: 75% amylopectin Waxy corn: 100% amylopectin 4-5% α(1-6)linkage with 20-25 glucose units One reducing end per molecule No helical structure (branching disrupts helical structure) Iodine test: add I2 –purplish color- less I2 trapped inside structure Biosynthesis of starch As starch is synthesized –deposited in the amyloplasts - grow concentrically to form granules – unique shapes and sizes for each of the cereal grains Under polarizing microscope –granules exhibit birefringence- refraction of light in 2 different directions rainbow colors Indicates some crystalline structure or highly ordered structure – crystalline structure is ~35-40% of total structure High Fructose Corn Syrup (HFCS) HFCS – now a dominant sweetener in the US and has replaced sucrose in soft drinks, biggest use of sugar in this country. The trend has affected the economies of sugar exporting countries who were dependent on the US as a major market. (23B lbs of sugar/yr) Science and Technology of HFCS Production - Steps 1. Acid-enzyme conversion method: (DE = dextrose (glucose) equivalent) HCl 0.12%, 140oC Corn starch -----------------neutralization,filtration α-amylase DE 45-50 corn syrup DE 62 corn syrup --------------------------------2. Enzymatic conversion α-amylase, β-amylase, glucoamylase Corn syrup DE 62 ------------------- DE 95-98 “glucose” 3. Isomerization (key technology) immobilized glucose isomerase DE 95-98 ----------- glucose (58%) + fructose (42%) equilibrium mixture 4. Equilibrium mixture is passed through chromatography (binds fructose), fructose is recovered and added back to “equilibrium mixture” to get HFCS Final fructose content varies: Typical HFCS 55% fructose (soft drink sweetener) 90% fructose (general sweetener) Commercially Available HFCS Relative sweetness Equilibrium mixture (Glucose 58%, fructose 42%) 55% Fructose (Glucose 45%, fructose 55%) 90% Fructose (Glucose 10%, fructose 90%) 1.00 1.05 1.40 Enzymes used in HFCS production Specificities: α-amylase – an endoamylase – cuts randomly within the oligosaccharide molecule starting from the non-reducing end smaller oligosaccharides β-amylase –exoamylase – cuts at the non-reducing end, one maltose (di-glucose) unit at a time maltose Glucoamylase –exoamylase – cuts one glucose unit at a time at non-reducing end glucose Teamwork! Products of α-amylase become substrates for βamylase and glucoamylase. Immobilized glucose isomerase – converts glucose glucose + fructose - immobilization is a key technology that allowed continuous production of HFCS Immobilized Enzyme (IE) Technology IE = enzyme immobilized on to a solid support through which the substrate is passed continuously and converted to products (p.504-509 Fennema) Advantages: Allows continuous output of products Control of reaction rate by regulating flow rate To stop reaction, enzyme is easily removed from rxn mixture Product is not contaminated with enzyme Usually, IE is more stable than soluble enzyme Ways of immobilizing: Covalent attachment to solid support Entrapment in a gel matrix (cellulose, agar) Adsorption to insoluble matrix Intermolecular cross-linking of enzymes to form insoluble matrix Use of immobilized glucose isomerase in the production of HFCS is the largest and most successful use of IE - a technology that has affected the economies of sugar producing countries and continues to change significantly the sweetener market in the US ...
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