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Unlock This CourseFood testing is a crucial skill in marine science that helps us understand what nutrients are present in different organisms and food sources. Just like detectives use clues to solve mysteries, scientists use chemical tests to identify specific nutrients in biological samples. These tests are particularly important when studying marine food chains and understanding how energy flows through ocean ecosystems.
Key Definitions:
Marine organisms contain different amounts of proteins and lipids depending on their role in the ecosystem. Fish muscle is high in protein, whilst fish liver contains lots of lipids. Understanding these differences helps marine biologists study food webs, nutrition and energy transfer in ocean environments.
The biuret test is named after a chemical compound called biuret, which gives a similar reaction to proteins. This test works because proteins contain peptide bonds that react with copper ions in alkaline conditions, producing a distinctive colour change.
When biuret reagent (which contains copper sulphate and sodium hydroxide) is added to a protein solution, the copper ions form complex bonds with the nitrogen atoms in peptide bonds. This creates a purple-violet colour that's easy to spot.
Test tubes, biuret reagent, food samples, distilled water, measuring cylinders, safety goggles and a test tube rack.
1. Add 2ml food sample to test tube
2. Add 2ml biuret reagent
3. Mix gently
4. Wait 2-3 minutes
5. Observe colour change
Purple-violet = proteins present
Blue = no proteins detected
The deeper the purple, the more protein is present
When marine biologists test different parts of a cod fish, they find that muscle tissue gives a strong purple colour (high protein), whilst the swim bladder shows only a faint colour change (low protein). This information helps understand how different tissues function and their nutritional value in the marine food web.
The emulsion test for lipids works differently from the biuret test. Instead of a chemical reaction, it relies on the physical properties of lipids. Lipids don't dissolve in water, so when they're mixed with water, they form tiny droplets that make the solution look cloudy or milky white.
An emulsion is a mixture of two liquids that don't normally mix together, like oil and water. When you shake oil and water together, the oil breaks into tiny droplets suspended in the water, creating a cloudy appearance. This is exactly what happens in the lipid test.
Test tubes, ethanol, distilled water, food samples, measuring cylinders and safety equipment including goggles and gloves.
1. Add 2ml food sample to test tube
2. Add 2ml ethanol
3. Shake vigorously
4. Add 2ml distilled water
5. Observe for cloudiness
Cloudy white emulsion = lipids present
Clear solution = no lipids detected
More cloudiness = more lipids present
Laboratory safety is extremely important when conducting food tests. Both biuret reagent and ethanol can be hazardous if not handled properly.
Contains sodium hydroxide (caustic) and copper sulphate. Wear safety goggles and gloves. If spilled on skin, wash immediately with plenty of water. Never eat or drink in the laboratory.
Highly flammable liquid. Keep away from flames and heat sources. Use in well-ventilated areas. Wear gloves as it can dry out skin. Store safely when not in use.
Good scientific practice requires using control samples to make sure your tests are working properly. A negative control contains no proteins or lipids, whilst a positive control contains known amounts of these nutrients.
For protein tests, distilled water serves as a negative control (should stay blue), whilst egg white solution makes a good positive control (should turn purple). For lipid tests, distilled water is again a negative control (should stay clear), whilst vegetable oil mixed with ethanol serves as a positive control (should form a white emulsion).
Marine researchers studying zooplankton nutrition found that copepods (tiny marine animals) contain high levels of both proteins and lipids. When tested, copepod samples showed strong purple colours in biuret tests and formed thick white emulsions in lipid tests. This high-energy content makes them crucial food sources for fish larvae and explains their importance in marine food webs.
Food testing isn't just a laboratory exercise - it has many practical applications in marine science and the food industry.
Fish farmers use food tests to check the nutritional content of fish feed and ensure their fish are getting proper nutrition for healthy growth. They also test fish flesh to determine the best time for harvesting.
Scientists test marine organisms to monitor ecosystem health. Changes in protein and lipid content can indicate environmental stress, pollution effects, or changes in food availability.
Even experienced scientists sometimes get unexpected results. Understanding common problems helps improve your technique and confidence in interpreting results.
If your biuret test doesn't change colour, check that your reagent hasn't expired and that you've mixed the solutions properly. For lipid tests, make sure you've shaken the ethanol mixture vigorously before adding water - insufficient mixing is the most common cause of false negative results.