Food Chemistry Identification Of Natural Pigments Based On .

Food ChemistryIdentification of natural pigments based on chemical behaviorBackground: Proper handling and preparation of fruits and vegetables includes preserving theirnatural, attractive colors. Red and orange pigments come from many sources, and while theymay appear similar in color they respond in very different ways to common food preparationconditions. This experiment is designed to familiarize high and junior high school students withthe main types of reddish plant pigments, and to provide insight into proper handling of fruitand vegetable sources containing different pigment classes.This lesson gives students the opportunity to conduct hands-on chemistry experiments;make observations based on chemical reactions, including pH-induced color changes; makeobservations based on visual changes, including whether pigments are soluble in oil or water;and classify pigments based on their observations.Students should be divided into groups of 2-4. Each group will receive 1 test tubecontaining a reddish fruit or vegetable juice (each representing a different type of naturalpigment) and 3 test tubes containing: 1) Baking soda; 2) Cream of tartar; and 3) Salad oil.Students will add 4 ml of their juice to each of the 3 test tubes, observing the changes thatoccur for each. For a longer experiment, each group can be given a tube of each juice and 3sets of test tubes containing the items listed above.Pigment Classes:Anthocyanin – (an-tho-SIGH-a-nin) Type of water soluble pigment found in many plants. Theyare the main pigments found in red onions, red cabbage, and most berries.Anthocyanins are very sensitive to pH. At low pH (acidic), anthocyanins are red to brightpink, while at high pH (basic), anthocyanins are blue to purple. This limits the kinds offoods in which they can be used. At very high pH, anthocyanins become yellow tocolorless as the molecule starts to degrade.Most recipes calling for berries include lemon juice or citric acid. This is important topreserve berry color, especially when they will be combined with high pH foods. Acommon example of anthocyanin discoloration is blueberry muffins. Any juice fromdamaged blueberries can tint the muffin batter green (the batter has a high pH due tobaking soda and/or baking powder that does not react until the muffin is baked). Toprevent this, use dry berries or coat them in cornstarch, and fold berries into the battergently to keep them from becoming damaged.Stains from foods high in anthocyanins are typically very easy to remove from hands,cutting boards, and linens due to their high water stability and sensitivity to pH (mostsoaps and sanitizers have a high pH).Betalain – (BAY-ta-lin) Type of water soluble pigment found in some reddish to yellowish plants.The most common food source is beets, but they are also found in cactus flowers andSwiss chard. They do not change dramatically with pH, though at high pH beet juicemay show a slightly brown tinge.There really no cooking considerations that need to be followed to preserve the color ofbetalains. For this reason, this class of pigment is used extensively in the food industryas a natural source of red and yellow dye.

Stains from foods high in betalain can be difficult to remove because they are oftenhighly concentrated. However, because they are water soluble, they can be removedfrom hands, cutting boards, and linens with soaking or repeated washes.Carotenoid – (ca-ROT-in-oid) Type of oil soluble pigment found in red, orange, and yellowplants. They are the main pigments in carrots, tomatoes, citrus, and squash, as well asspices such as paprika and turmeric. Some carotenoids are precursors to Vitamin A, sothey are very important nutritionally. Because they are oil soluble, carotenoids are notat all sensitive to pH.Even though carotenoids are oil soluble, vegetables containing this pigment (e.g.carrots, squash) usually don’t visibly loose color when cooked in oil. However, the oilthey are cooked in will change. This is especially noticeable for cooking techniquesusing small amounts of oil, such as sautéing. Students should be cautioned to carefullyremove sautéed vegetables or to wipe the colored oil from the plate so it does notdetract from the presentation.Stains from foods high in carotenoids can be extremely difficult to wash off from cuttingboards and linens. Treating stains with a small amount of cornstarch can help pull theseoil soluble pigments out, but do not put additional water on the board or fabric unlessthe stain is already dry. Detergents with a high pH (higher than is common in foods) willalso help break carotenoid pigments down – the molecule isn’t removed from the fabric,it just becomes colorless so it is no longer visible.This lesson incorporates the following State of Utah Course Standards:Foods and Nutrition IStrand 5, Standard 4: apply food selection and preparation guidelines related to fruits andvegetables.ChemistryStandard 3, Objective 2: Explain that the properties of a compound may be different from thoseof the elements or compounds from which it is formed.Standard 6, Objective 3: Differentiate between acids and bases in terms of hydrogen ionconcentration.

MaterialsTest tubes, 15 or 20mL size – 4 per group (12 per group for longer experiment)Test tube racks – 1 per groupEyedroppers – 1 per group (3 per group for longer experiment)pH paper – 2 strips per group (6 per group for longer experiment)Salad oilBaking soda (NOT baking powder)Cream of tartarRed JuicesCranberry or cherry juice, 20 mL or ¾ ounce per test. Make sure the type you choose does notlist “beet juice”, “vegetable color”, or “vegetable concentrate” in the ingredients list.Because betalains (from beets) are so stable, they are commonly added to juicecocktails and blends. Organic juices and concentrates without added color are availableat many grocery stores, or from Walmart online. If you use a juice concentrate, mixwith water as directed on the package before you measure the juice into test tubes.Beet juice, 20 mL or ¾ ounce per test. Canned beets are the best source for this. Avoid pickledbeets if possible, as this will affect the outcome of the pH test portion of theexperiment. Look for beets that do not list vinegar, acetic acid, or citric acid in theingredient statement. If pickled beet juice is the only kind available, baking soda can beused to adjust the pH to 5 or 6. Fresh beet juice can also be used, but be sure to strainall pulp from the juice first (this can interfere with the oil test). Regardless of thesource, if the beet juice is very dark it should be diluted with an equal volume of waterbefore being measured into the test tubes.Citrus or tomato juice, 20 mL or ¾ ounce per test. Ruby red grapefruit juice from Ocean Spray works well for this experiment. Other type of citrus juice don’t have a naturally reddishcolor, but can be used if you are not concerned about having students “guess” thepigment based on the color of the juice. Make sure the juice you choose does not list“beet juice”, “vegetable color”, or “vegetable concentrate in the ingredients list. Juicesthat include some carrot juice are fine to use. Tomato juice or hot sauce can be used,but these are usually quite pulpy so it makes the results of the experiment harder tointerpret. For any of these juices, they can be filtered through cheesecloth (4 or 5 layers)or a coffee filter to remove some of the pulp before portioning into the test tubes.Preparation:1. Prepare juices and measure into test tubes up to 1 day in advance. Do not leave juiceunrefrigerated for more than 2 hours. Label tubes with the juice type (e.g. cranberry,pink grapefruit, beet), and transfer 20 mL of juice into the corresponding test tube(s).2. Add test items (see below) to test tubes labeled with their names. Make up onecomplete set of 3 tubes per group. It is best to have extra on hand in case of spills or ifthe experiment needs to be repeated. If you are going to have each group test morethan one juice source, they will need a complete set of test items for each one.a. Baking soda – 0.5 grams per test tubeb. Cream of tartar – 0.5 grams per test tubec. Salad oil – 3 mL (1/2 teaspoon) per test tube

3. Make enough copies of the data sheets to give one to each student. Make enoughcopies of the instruction sheets to give one to each group.Instructions (with teacher notes):1. You have been given a fruit or vegetable juice solution. It is your job to identify themain type of pigment that gives the juice its reddish color. Have the students compare their chemical solutions. Are some more red, orange,or purple than others? Do some have more sediment in them?2. Using a pH strip, take the pH of your juice solution. Make sure to cover all 4 raised lineson the pH strip. Compare the colors on the test strip to the pH chart you have beengiven. The closest match to all four lines shows the closest pH to your sample. If it isbetween two pH values, add 0.5 to the lower pH (for example, if it is between 3 and 4,the pH is read as 3.5). Record your result and observations. How would you describe thecolor of the juice solution?3. Add 5 milliliters of your juice to the tubes with baking soda, cream of tartar, and saladoil. Put the lids back on and gently rock the tubes back and forth to mix the baking sodaand cream of tartar. Shake the tube with oil vigorously for 20 seconds. Return thetubes to the rack and let them sit for 2 or 3 minutes. Show the students how to read the graduations on the dropper. They do notneed to add exactly 5mL, but they should be careful to add the same amount ofjuice to each tube. Alternately, instruct the students to add 1 teaspoon of juice toeach tube (1 tsp 5 mL).4. Examine the tube with baking soda. How does the color compare to the color of theoriginal juice solution? Using a new pH strip, measure the pH as described in step 1.Record the pH and your observations. The groups with anthocyanins (cherry or cranberry juice) will see a distinct colorchanges in this step. It should appear more blue/purple compared to the originaljuice. The groups with betalains (beet juice) may see a darkening of the color in thisstep, depending on how concentrated the juice is.5. Examine the tube with cream of tartar. How does the color compare to the color of theoriginal juice solution? Using a new pH strip, measure the pH as described in step 1.Record the pH and your observations. The groups with anthocyanins (cherry or cranberry juice) may see a brighteningof the color in this step. Depending on the original pH of the juice, it may appearmore hot pink than red.6. Examine the tube with the salad oil. If it is still cloudy, use the dropper to add a littlemore juice to the tube. Do not shake the tube again – rock it back and forth GENTLY toget rid of the cloudiness. Record your observations on how the color of the juice and/oroil have changed. Groups experiencing cloudiness in this step have formed an emulsion duringshaking. By adding additional juice and rocking gently, the emulsion should breakup enough that the color of the oil can be easily observed. Filtering juice through