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Jan 17, 2022 / in Experiments Biology
Among the major laboratory experiments in the class XI-XII biology practical are the Macro Nutrient Tests. Also commonly known as food tests for class 11 or 12, this is actually a group of five tests ‒ one each for starch, glucose, sucrose, protein, and fat. Today, we will describe each of those tests for your convenience.
The macro nutrient tests or food tests for classes 11-12 are highly important in all education boards including CBSE, ICSE, ISC, IGCSE, IB, and state boards like WBCHSE biology practical syllabus . These tests are included in all of their curriculums, with slight or no variation.
There are more than one way to do some tests in this group. Some schools do all of the tests, and some just prefer one. We will show you the most common food tests done in school. These are:
Note that in the syllabus you may note more tests like xanthoproteic test, Millon’s reagent test, and Sudan III test. These are very uncommon for various reasons ‒ materials are hard to come by, the test is very difficult to conduct, or as in the last case ‒ reagent is prohibitively expensive. The tests above are the most common experiments you will find in school biology labs.
Now let’s get into each test.
Starch is among the most important carbohydrates, so this test is also called the carbohydrate test (though not all carbs are starches) due to oversimplification. Starches are made of polysaccharide and glucoside, and they are generally found as food reserves in plants ‒ in roots or seeds. The most common examples of plant starch reserves are potato, rice and wheat ‒ our main bulk of food. You will also find plenty of prepared foodstuff rich with starch, like bread or biscuits. We will use biscuits today.
Iodine solution is generally yellow or amber colored. When it comes into contact with starch-like compounds, the smaller iodine atoms slip into the larger starch molecules. They have a complicated relationship. Anyway, these starch-iodine complex molecules look blackish blue ‒ that is the color you see in the tube.
Glucose is called a ‘powerhouse chemical’ for the body, because it is THE compound which breaks down to provide energy for the body. It is naturally present in sweet fruits and vegetables like bananas. Most foodstuffs do not naturally have glucose, but those few that do, can be detected quite easily with the Benedict’s Reagent.
The Benedict’s Reagent’s main ingredient is copper sulphate solution, but it is spiked with sodium citrate and sodium carbonate. You can find it easily in our biology lab packages .
The principal element in Benedict’s reagent is copper, specifically, Cupric ions. With two empty spaces in the outer orbit, highly charged. And in glucose-type compounds there are aldehydic or ketonic groups, which are reducing elements. Meaning, they are ready to reduce highly-charged cations to a lower level.
Obviously, these two opposites attract each other strongly. The glucose gives one oxygen to the copper cations, reducing them from cupric to cuprous. It itself changes into gluconic acid. And the copper? It happily attaches to the free electron and forms cuprous oxide, which is brick-red in color and insoluble in water. That is what you see falling to the bottom of the test tube.
The two sodium salts in the reagent act as intermediary agents for these interactions.
Another way to detect reducing sugars is the Fehling’s test, with Fehling’s solution A and B. Solution A is basically a 7% CuSO4 solution. Solution B is potassium sodium tartrate made highly alkaline using KOH.
They are available in the market, but they are also easy to prepare on site which is the precise reason Fehling’s test still exists. Otherwise it would be overshadowed by Benedict’s. Anyway, Fehling’s test is still important in schools and so we will explain it here.
It isn’t much different from Benedict’s test. The difference being in the number of steps where you have to prepare the final solution. We used glucose powder as the sample.
Fehling’s test works more or less the same way as Benedict’s, with red cuprous oxide precipitate. The two Fehling’s solutions combine to make an unstable bistartarocuprate complex, which would turn into black cupric oxide precipitate if left alone for a while. That’s why we don’t let it stay; the Fehling test has to be done fast.
When a reducing sugar is present in the solution, it reacts with the bistartarocuprate complex to release the copper ions. They take an oxygen or hydroxyl ion from the aldehyde or ketone groups of the sugar, and turn into cuprous oxide which is brick red in color and not soluble in water.
Fehling’s is the older experiment, from the 1850’s. It worked well till Benedict’s came, but then went out of favor at research labs because of its unstable nature. First of all, the mixed Fehling’s solution is unstable and will break up to produce cupric oxide soon. Secondly, Fehling’s B is also unstable and doesn’t last long in storage.
But above all what lets down Fehling’s test is that it works only in a strongly alkaline environment. So, if you want to find out if there is glucose in an acidic or even neutral solution, you’re fresh out of luck with Fehling. In a non-highly-alkaline environment, the copper ions won’t be easily reduced with Fehling, and there will be false positives, like from alcohol.
There are other issues with Fehling’s test too. For example, it cannot detect aromatic aldehydes. Instead, you will have to use Benedict’s test in case you are serious about glucose tests.
Sucrose and similar sugars are non-reducing sugars, such as normal cane sugars that we consume every day. They won’t give a positive result in a standard glucose test like Benedict’s or Fehling’s. The trick is to first convert the non-reducing sugar to reducing sugar, and then do Benedict’s test (Fehling’s would be hard to do here ‒ you’ll understand why in a minute).
The basic idea of the sucrose test is to break it down to glucose and fructose which gives positive results in the Benedict’s test. To do this, we hydrolyse the sample with hot HCl. This produces the reducing sugars ‒ but in turn also highly acidifies the solution which is not good at all for good Benedict’s results. So, we neutralise it using simple hydroxide or bicarbonates, and add Benedict’s reagent. Heating this will reduce the cupric ions of the reagent into cuprous oxide, which you get to see as the reddish-brown precipitate.
Proteins are the building blocks for much of our body, especially the muscly parts. That is why you need protein food most if you are bodybuilding and gaining muscle mass. Also, proteins repair any damaged body part and are very important while growing up.
To detect the presence of proteins in foodstuff there are various tests, but the biuret test is the most popular by far. The reason for it lies in the availability of its materials. Also, some of the other tests use very dangerous chemicals, not recommended for school use.
Technically, you would be detecting peptide bonds present in the sample, which is the base of common edible proteins. It is basically an acid formed with carbonyl and amino groups. Our goal is to see if our testing chemicals (producing cupric ions) can form a complex compound with this.
Also called paper spot test, this is by far the simplest method of detecting there is any fat present in a given food item. All you need is a piece of thin (but opaque) piece of paper and a cover glass, with the sample. Since fats are basically oils, when you place the sample on the paper and press, the contact place on the paper should absorb the fat and turn somewhat transparent.
Note: Of course, it is a little bit more work than that in the case of some samples. If your sample is very dry (like a peanut that we took), then just putting it flat on top of the paper won’t display anything. You will have to crush the hard sample and press it against the paper, so it can soak in the oil.
All of the tests mentioned above, including those not described in the article, are possible using Labkafe’s higher secondary level biology lab package . This package has all the chemicals, reagents, glassware, and support apparatus required for the class 11 food tests for CBSE ICSE State boards. If you need this package, or specific items from the package, then you can contact us at 9007218364 any time ‒ our sales rep will help you out. We can ship anywhere in India, and even supply outside India via resellers. Just fill up the contact form below and someone will have a chat with you and hash it out.
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Tags: food test starch test glucose test fructose test protein test fat test Biology Laboratory Experiment Practical class 11