Teacher Notes
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Teacher Notes![]() Bacteria in MilkSuper Value Laboratory KitMaterials Included In Kit
Powdered milk, 450 g
Resazurin, 1%, 40 mL Test tubes, 13 x 100 mm, 90 Additional Materials Required
Marker
Milk storage containers, covered, 3 Test tube rack Water bath (shared) Prelab PreparationThe milk provided in this kit is powdered, dry milk. It will need to be reconstituted with water at least two days prior to the laboratory. Place the dry milk (approximately 90 g) in 1 L of distilled water. Mix the milk solution thoroughly until it all stays in solution and it “looks” like regular milk again. Divide the milk into three covered containers, label them clearly, and treat them as follows:
Safety PrecautionsOnce food-grade item have entered the laboratory, they are considered chemicals and should not be consumed. Resazurin is a permanent stain and is not easily removed from skin or clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. DisposalPlease consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions from this activity can be disposed of down the drain with volumes of water according to Flinn Suggested Disposal Method #26b. Teacher Tips
Correlation to Next Generation Science Standards (NGSS)†Science & Engineering PracticesAnalyzing and interpreting dataPlanning and carrying out investigations Constructing explanations and designing solutions Developing and using models Disciplinary Core IdeasMS-LS1.C: Organization for Matter and Energy Flow in OrganismsMS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems MS-PS3.D: Energy in Chemical Processes and Everyday Life HS-LS1.C: Organization for Matter and Energy Flow in Organisms HS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems HS-PS1.A: Structure and Properties of Matter HS-PS1.B: Chemical Reactions Crosscutting ConceptsCause and effectStructure and function Energy and matter Performance ExpectationsMS-LS2-3: Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem. Answers to Questions
ReferencesThanks to Jeannette Miller, Schalmont High School, Schenectady, NY, for this activity. Recommended Products
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Student Pages
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Student Pages![]() Bacteria in MilkIntroductionAre you willing to share your milk with bacteria? Leave your milk unrefrigerated for a while and the bacteria will consume it faster than you want them to! Concepts
BackgroundMilk has often been described as the “near-perfect” food for growing humans. It is probably the most nutritionally complete food that is found in nature. Since milk is the only food young mammals usually consume in the weeks following birth, it is important for their early development that milk is so nutritionally complete. Mammals’ milk contains vitamins, minerals, proteins, carbohydrates, and lipids, all essential for early growth and development. A “typical” chemical composition of milk is somewhat difficult to pinpoint. In an individual cow, for example, the actual content varies from breed to breed, herd to herd, and even month to month. Since milk is near “perfect” as a food source it is logical that microbes would also thrive on milk. In fact, they do! Milk provides a potentially ideal growth media for many pathogens. Hence, contamination of milk by any pathogen can produce a very serious health problem. Contamination of milk due to unsanitary handling or processing has caused many milk-borne epidemics of human disease. Undesirable microbes can be introduced into milk not only by diseased cows, but more frequently during processing by dirty hands, unclean or contaminated utensils, flies, polluted water supplies, etc. Milk that is sold commercially is carefully handled and treated from the cow to the supermarket shelf in order to keep the bacteria population under control. Testing of milk for microbes and the careful processing of milk are crucial to good public health. Dairy products derived from milk are manufactured with the controlled use of microbes. Specific microbes produce tasty cheeses and other dairy products. One chemical test for microbial activity in milk will be used in this activity. The test is based upon a simple principle— bacteria in milk use up oxygen as they grow and multiply. The amount of bacteria in the milk affects the amount of oxygen used; oxygen concentration is therefore a relative reflection of the bacteria population size. To measure the amount of oxygen in the milk, an oxidation–reduction indicator will be used. Oxygen is a good oxidizing agent and will oxidize an indicator, changing its color. Resazurin is an ideal indicator for oxygen. Resazurin is blue in its completely oxidized state. Upon reduction it goes through a series of color shade changes from purple-red to pink to colorless, as illustrated in Figure 1. The color continuum allows for a more quantitative comparison than is possible with other indicators that exist in only two color forms. {11163_Background_Figure_1}
Tests with resazurin solution can be interpreted as follows:
{11163_Background_Table_1}
Materials
Resazurin 1% solution, 3 drops
Milk, refrigerated for 2 days, 5 mL Milk, standing at room temperature for 2 days, 5 mL Milk, boiled and then refrigerated for 2 days, 5 mL Marker Test tube rack Test tubes, 13 × 100 mm, 3 Water bath Safety PrecautionsOnce food-grade item have entered the laboratory, they are considered chemicals and should not be consumed. Resazurin is a permanent stain and is not easily removed from skin or clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Procedure
Student Worksheet PDF |