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Skip to 0 minutes and 15 secondsThis activity investigates the antimicrobial properties of spices. Spices, which are dried, aromatic or pungent plant substances, have been used for centuries to mask the bad smell and flavour of rotting food. Before refrigeration was commonplace, sugar was typically added to foods to disguise the vast quantities of the added spices. For example, several spices including garlic, black pepper, clove, ginger, cumin and caraway are used extensively in the Indian diet and in Indian medicine. Research since the late 19th century has documented the antimicrobial properties of several spices, including cloves, cinnamon and oregano, which suppress the growth of E. coli bacteria found in raw meats.

Skip to 1 minute and 4 secondsIn this activity the antimicrobial efficiency of different spices will be investigated by observing visible microbial life on rice pudding, observed over five days. The effect of spices sprinkled on the surface of rice pudding, and on being cooked with the rice pudding, will be explored. Antimicrobial activity will be measured by the appearance of visible microorganisms on the surface of the rice pudding, when compared to a control sample. The longer it takes for signs of mould and rotting to appear, the more potent the antimicrobial property of the spice. You are strongly encouraged not to eat any of the rice pudding under investigation and to dispose of it, in a household food waste bin, after the activity.

Skip to 1 minute and 47 secondsSpices like cinnamon and cloves should work well - the most important antimicrobial compounds in these spices are cinnamaldehyde and eugenol, which can degrade the cell walls of the microorganisms. So, you could try one or both of these, or alternatively, any other spice in your spice rack (such as garlic, allspice, thyme, cumin), or even a combination of different spices. Will the components of one spice aid the other, so improving the antimicrobial effect? For a fair test, it is important to ensure that you use the same

Skip to 2 minutes and 19 secondsratio of rice pudding:spice for each experiment (i.e. that you change one factor at a time while keeping all other conditions the same). Each day, take photos of your control sample, your samples of rice pudding cooked with your chosen spice, and your samples of rice pudding sprinkled with your chosen spice. Compare the appearance of the rice pudding samples and record your findings. Does heating or sprinkling work better? Also, are the results reproducible? It is worth replicating at least one of your experiments to test this. Other experiments could explore the size and shape of the spice particles by grinding them just before use. Also, do you get a different result with the same spice that is passed its expiry date?

Skip to 3 minutes and 0 secondsDon't forget to post your results, so that we can pool them all together, in our quest to further understand the spices of life.

Determining the antimicrobial properties of spices

It has been known for centuries that some spices can have an antimicrobial effect, mainly in their use as preservatives of food.

You are going to test the effectiveness of different spices by observing the growth of microbial colonies on rice pudding.

The following ingredients will be required:

  • A selection of household spices
  • Packaged rice pudding

The following items will be required:

  • Clingfilm or similar plastic wrap
  • A kitchen weighing balance
  • Small spoons to stir spices

Ensure that you read through all of the instructions before beginning the experiment to ensure that you understand exactly what will happen at each step.

Step 1: Measurement of spice

First select the spice that you wish to use. Add 1 g of your chosen spice to 190 g of rice pudding. This corresponds to the average size of a rice pudding microwave pot.

spice weight

In addition to the two rice pudding samples, for each spice, you will also need to create a control sample for the experiment. The control is treated the same way as all of the experimental samples, but, it does not have any spice added to it.

spices

This allows for us to see the effects of adding spices compared to adding no spices at all to our sample. Should we find that there is no significant difference between the microbial growth on an experimental spice sample compared to the control sample we can say that this spice has a comparatively weak or no antimicrobial activity.

Step 2: Heating

Follow the heating guidelines supplied with your rice pudding pots. Our samples were heated in the microwave for 50 seconds at a minimum of 800 watts. Remember to heat your control sample too, it must experience exactly the same conditions that the experimental samples do – with the exception of the addition of spice.

microwave rice pudding

After the rice pudding samples have been heated leave them to cool to room temperature. Once cooled take a cling film wrap and cover the top of the rice puddings. Remember to label each rice pudding sample so that you know which is which.

wrapped rice pudding

Continue to observe your rice pudding samples for between five days to a week. Each day, take a picture of your rice pudding samples and once the activity is finished post your collection so that we can pool data together and have a wide range of spices to analyse (our open Padlet or the Twitter hashtag #FLchemistry).

finished samples

Try experimenting with adding multiple spices to a single pot, is the effect of each spice augmented by the other? Does the same mass of rice pudding spread flat over a plate result in a faster growth rate of microbial colonies?

Safety

  • Take care when warming the rice pudding: Rice pudding can be hot; Ensure that you handle any rice pudding pots with oven gloves or protective mitts.

  • Dispose of rice pudding samples correctly: We strongly advise not to eat any of the rice pudding samples at any stage of the experiment; Ensure that all rice pudding samples are disposed of into a general waste bin.

Experiment adapted from: A.M. Sousa and W.R. Waldman, J. Chem. Educ., 2014, 91, 103.

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This video is from the free online course:

Exploring Everyday Chemistry

University of York

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