– Onion
– Potato
– Apple
– Peanut
– Benedict’s - presence of reducing sugars
– Biuret’s – presence of proteins
– Sudan IV – presence of fats
– Potassium Iodide presence of starch/polysaccharides

• starch divided up into two fractions
– amylose – helical and unbranched
– amylopectin - branched
• amylose forms a blue-black complex when it combines with the iodide ions found in a potassium iodide solution made for detecting starches
• this potassium iodide solution is also called
Lugol’s reagent
– actually a mixture of potassium iodide
(soluble)and elemental iodine (insoluble)
– results in a soluble solution containing triiodide
• the Lugol’s reagent interacts with the coiled
structure of the amylose fraction of the starch – blue black color
– specifically the triiodine molecule interacts with the polysaccharide
• no color produced by amylopectin or monosaccharides or dissacharides

• used to detect the presence of reducing sugars
• reducing sugar – sugar that in its open form bears aldehyde group
– all monosaccharides (reagent converts fructose to glucose)
– maltose, lactose
• in nature – many foods possess both closed ring and open linear forms of sugars
• under heat - copper of the reagent reacts with the aldehyde of the sugar to turn a specific color aldehyde
Open, reducing form of glucose

• copper is reduced by the sugar to form copper oxide (red, insoluble)
– color denotes amount of reaction (i.e. approximate amount of reducing sugar)
– green - 0.5% sugar
• many dissacharides produce this color
– yellow – 1.0%
– red - 2% or more
• many monosaccharides produce this color
• sucrose is not detected by this reagent – not a reducing sugar
• many polysaccharides such as starch begin with a reducing sugar
– may not be sufficient to turn the reagent’s color aldehyde
Open, reducing form of glucose
Sugar
Benedict’s
Copper
Oxide

• chemical reagent that detects the presence of peptide bonds
• presence of peptide bonds can produce a pink to violet color
– pink – short chain polypeptides
– violet – longer proteins
– spectrophotometers can be used to measure the absorbance of these colored solutions = protein concentration
• biuret = chemical compound with the formula
H
2
NC(O)NHC(O)NH
2
• yet Biuret’s reagent doesn’t contain biuret
– it gives a positive reaction same as the peptide-like bonds in the biuret molecule
• BR = potassium hydroxide and copper
– potassium hydroxide increases the pH of the solution
(alkaline)
– copper forms violet-colored complexes in the alkaline environment of the test
• the copper sulfate of the reagent water BR albumin BR + albumin

• several different kinds of Sudan stains
– all are used to stain lipids
– different colors
– Sudan III = red
– Sudan IV = black
• Sudan IV stains lipid droplets black in slide preparations
• also can stain in liquid preparations
Sudan III staining = oil and water

• Benedict’s
– Onion – reddish color (+ve)
– Peanut – blue (-ve)
– Apple - +ve
– Potato – yellowish color (+ve)
• Biuret
– Onion – +ve (pink)
– Peanut - +ve (purple)
– Apple – -ve (blue)
– Potato – -ve (blue)
• Sudan IV
• Potassium Iodide
– potato, apple – black staining, +ve result
– peanut, onion – little to no staining
• peanut – kind of gray???

• Onion: +ve Benedict’s, +ve Biuret
• 1% protein (sulfurous odor of the onion)
• 4% sugar – glucose, fructose and sucrose
– fructose the most
– sugar content increases with maturation
• 2% soluble fiber in flesh – insoluble fiber in the skin
-presence of reducing sugars
-presence of proteins
• 0.1% fat
• 98% water
• Peanut: +ve Biuret, +ve Sudan IV
– 25% protein – source of the essential amino acids histidine and arginine
– 48% fat (7% saturated, 24% mono, 16% poly) – 41% considered oil
• numerous fatty acids – palmitic, stearic, oleic, linolenic, linoleic
– 0% sugar; 1% cellulose, 9% fiber
– 21% carbohydrates – NO SIMPLE SUGARS
-presence of fats
-presence of proteins

• Apple: +ve Benedict’s
– 85% water
– 0.3% protein – 11 types of amino acids
– 0.4% fats
-presence of reducing sugars
-presence of starch
-not enough proteins or fats
– 12% carbohydrates – all three monosaccharides + starch
– 2.3% fiber
– 0.6% organic acids – e.g. citric acid
• Potato: +ve Potassium Iodide
– 78% water
– 2.2% protein
– 0.1% fat
– 18% starch
– 0.4% fiber
-presence of starch
-not enough proteins or fats