Investigating the Effect of Chilling on the Water Potential of Maris Piper Potatoes

Apparatus

• 5mm cork borer – in order to accurately produce potato cores of the same width.
• Mathematical ruler – to measure the length of each potato core. The mathematical ruler is more accurate than the cheaper plastic ones.
• Potato peeler – to remove the skin to produce uniform cores.
• Scalpel and white tile – to cut the potato cores to a uniform length.
• 42 test tubes–in which to soak the potato cores in salt solutions. They will need to be labelled in order to record correct results.
• Test tube racks – to hold the test tubes
• 14 Maris Piper Potatoes – seven stored at room temperature for a week and seven stored at 2oC for a week prior to testing. All the potatoes will be bought at the same time from the same shop so that hopefully they will all have been harvested for the same length of time. The potatoes will be selected to be approximately the same size.
• Incubator set at 24oC (just above normal room temperature)- to maintain a constant temperature in an incubator.
• Thermometer – to check the temperatures of the incubator.
• A range of salt (sodium chloride) solutions with water potentials ranging from 0 kPa to -2000 kPa – salt solutions will be used instead of sugar solutions because sugars are used as metabolites by potato cells and providing extra sugar could increase the metabolic activity of the cell and again produce unreliable results. Details of how to make these up given in the final plan.
• Electronic top pan balance – to measure the mass of the potato cores easily.
• 25cm3 measuring cylinder – to measure out accurately the volume of salt solution needed.
• Paper towels – to roll the wet potato cores on to remove excess liquid from the surface.
• Stopwatch – in order to time accurately how long to keep the potato cores submerged.
• Sticky labels – to write the water potential onto each test tube.
• Petri dishes – for storing the potato cores to prevent evaporation.

Risk assessment

This is a low risk investigation because it will not involve any dangerous chemicals or equipment. However it will involve some equipment that could be dangerous if it is not used properly, particularly the scalpel, cork borer and glassware. For safety, the following precautions will be used.

1. All glassware should be kept in a place where it cannot be knocked over and broken, preferably at the side of the laboratory once the solutions have been measured.
2.  The scalpel should be used with care and again put in a safe place once it is no longer required for the experiment.
3.  Any spillages should be cleared up immediately.
4. There is also a risk of electrocution as the salt solutions are very good conductors of electricity. To reduce this risk all plugs must be turned off when they are not being used and all the solutions must be kept a safe distance away from the live plugs.
5.  There should be a clearly marked first aid kit to hand in the laboratory and a qualified first aider present.

Controlling the variables        

• Temperature must be controlled because the rate of diffusion of water molecules will change with temperature. Any increase in temperature will lead to an increase in the kinetic energy of the water molecules and the particles in solution. If the potato cores are not all kept at the same temperature this could produce unreliable results. An incubator set to 24oC will produce a constant temperature similar to daytime room temperature. The other potatoes will be stored in the lab fridge, which has a temperature of 2oC.
• Length and width of the potato cores must all be the same or else the surface area of the potato will not be controlled and this could lead to variability in the results. A greater surface area will increase the rate of diffusion of water particles across the plasma membrane of the potato. The potato cores will be cut with a 5mm cork borer to maintain a constant width and cut to 5mm in length with a scalpel on a white tile using the most accurate ruler available.
• Surface liquid will affect the mass of the potato. To make the results as reliable as possible it is important to use a constant drying method to remove excess surface liquid when the cores are removed from the salt solutions. This will be investigated in a pilot experiment. The cores will all be reasonably dry when first cut. As the change in mass is going to be measured it will be important to make sure that the potatoes have been dipped into liquid and dried before they are first weighed as well as after they have been soaked. This is the most difficult variable to control.
• Time of soaking must be constant for all the potato pieces. The rate of osmosis will differ according to the water potential gradient between the potato core and the soaking solution. The greater the difference in water potential, the faster the rate of osmosis. As water diffuses across the plasma membrane from the higher water potential to the lower water potential the difference in water potential will decrease and the rate of osmosis will decrease until equilibrium is established on both sides of the membrane. There is not enough time to carry out pilot investigations to determine the length of time needed for equilibrium so the potato cores will be soaked overnight for 24 hours because this should be plenty of time for equilibrium to occur.
• Species and age of potato (since being harvested) – different varieties of potato may have different water potentials so it is important to use all one variety. Many fruits and vegetables undergo changes after they have been harvested so by using potatoes that are bought at the same time from the same shop should reduce this variation but it is impossible to be sure that the potatoes were all picked at the same time or grown under the same conditions. Maris Piper potatoes have been chosen because they are described on the packaging as suitable for chipping. This means they must contain a fairly low reducing sugar content and any increase should therefore be easy to detect.
• The volume of the salt solutions will be kept constant using a 25cm3 measuring cylinder.
• The concentrations of the salt solution will be controlled by making careful dilutions and using only one batch of salt solution to reduce variation.
• The mass of the potato will be measured using an electronic top pan balance, which weighs accurately to 0.01g.
• After cutting the cores each one will be temporarily stored in petri dishes covered with a lid to reduce evaporation

If all the variables are controlled as carefully as possible then the results obtained should be reliable. To check the reliability of the drying method a pilot will be carried out to investigate whether repeat weighing from the same piece of potato core produce very similar repeats.

Pilot investigations

1. A potato core will be cut. It will be weighed dry. It will then be dipped into water briefly and dried in a systematic way (to be determined during the pilot). The potato will then be reweighed. This will be repeated 10 times to investigate how variable the results are. If the ten repeats are all very similar then it will indicate that the results are reliable and therefore that the method of drying is suitable. If the repeat results show a lot of variability then it will indicate the drying method is not reliable enough and a different method will need to be found.

The two drying methods to try out will be a systematic “flick” method and a rolling in paper towel method. For the “flick” method the potato will be dried by holding it in between the thumb and first finger and then flicking the wrist twice to remove excess water. The potato will be dried in a paper towel by rolling the core up and down a 30cm length of paper towel.

      This will be repeated with cores cut from potatoes that have been stored at 24oC and 2oC.

      A total of 4 cores will be used, two from each temperature, labelled (1) and (2).

2. Two Maris Piper potatoes will be stored for four days before the pilot, one at room temperature and one at 2oC in a refrigerator. Three potato cores will be cut from each and placed into 200cm3 distilled water for 2 hours and left at room temperature. The cores will be removed, dried by the chosen method and reweighed. The % change in mass for each will be determined to test whether there is any detectable difference in the mass of water uptake in the potato cores due to different storage conditions after 2 hours. If so, then it is possible that an even greater change will be measured when the potato has been stored for longer.

A final plan will be made after the pilot investigation.

Pilot results

1. Testing the drying method

% error in measuring mass = mass (g) after drying - original mass (g)   X  100

                                                                      original mass (g)

Table 1 The % error when measuring the mass of a wet potato core dried by two different methods

2. Is there a detectable difference between the water uptake of potato cores stored at 4 and 24oC for 4 days before being soaked in distilled water for 3 hours?

Table 2 The % change in mass of potato cores soaked in water after storing at two different temperatures

Analysis of pilot results

Pilot 1 results show the following trends: firstly, all 20 of the potato cores dried by the “flick” method showed an increase in mass, indicating that excess water remained on the surface of the potato. At the same time 18 of the potato cores dried by the roll method lost mass, one remained the same and one core gained mass. Secondly, the % error for the potato stored at 24oC was greater than the % error of that stored at 2oC when the flick method was used. This was probably because the flick method was changed slightly and a greater force was used with the second core, but it could be for other reasons to do with the difference in temperature. Thirdly, the mean % error at both temperatures is slightly lower when using the rolling method, but the range of % errors is greater by this rolling method.

Both methods give errors. However, I have decided to use the flick method to dry my cores for two reasons. One, the range of results is lower. This indicates that the results will be less variable using the flick method than when using the towel method. Although there is an error, it seems to be fairly constant. Two, I found the towel method awkward and it was difficult to roll the potato core in a systematic way which might account for the variation.

Pilot 2 showed that the potato cores from both storage temperatures gained much more mass than the potatoes that had been simply dipped in the water in pilot 1. This was what was expected. The distilled water used had a very high water potential (0kPa) while both potato cores would have a lower water potential due to the solutes in the cytoplasm or vacuole. Water diffused down the water potential from the distilled water into the potato cores so that the cores increased in mass. They also felt quite turgid to touch.

The results of the pilot 2 also support the hypothesis that potatoes stored at 2oC will have a lower water potential than those stored at 24oC. The potato cores stored at 2oC all increased in mass by a greater % than those stored at 24oC. This may indicate that starch has been converted to reducing sugar in the colder storage conditions so more water will move into the potato by osmosis. However, as this pilot only used 3 cores from one potato more work will need to be done before the hypothesis can be accepted or rejected.

Final Plan

The salt solutions will be made up as follows:

Fourteen Maris Piper potatoes of roughly the same size and age will be used. Seven will be stored at 24oC for one week and seven will be chilled to 2oC for one week. Three potato cores will be removed from each potato using a 5mm cork borer and cut to 30mm lengths using a scalpel and a ruler.  Each core will be dipped into one of the salt solutions, dried in a systematic way using the flick method, weighed on a top pan balance and placed into a test tube containing 10cm3 of sodium chloride solution. By making the cores wet and then drying them before they are soaked, it will ensure that the error caused by excess fluid on the outside of the chip is reduced because the same excess will be present before and after soaking. If the change in mass is measured, the effect of the residual liquid should not affect the results. Each core will be placed into a separate labelled test tube containing 10 cm3 of sodium chloride with a particular water potential. This will be repeated for each water potential. The potato cores will be left in the solutions for 24 hours. A water bath will be used to maintain a constant temperature. They will then be removed, dried systematically and reweighed. The mean % change in mass of potato will be calculated for each different salt solution. The mean % change in mass will be plotted on a graph against the water potential of the salt solution. Where the line crosses the x-axis will represent the approximate water potential of the potato tissue. This method has been chosen because it will measure any change in solute concentration i.e. both reducing and non-reducing sugars as well as other possible breakdown products such as amino acids.

Additional work if there is time

To check that any difference in water potential is due to an increase in reducing sugar content, a Benedict’s test could be carried out. 10g of tissue from potato stored at 2oC and 24oC will be homogenised in a blender with a minimum volume of water. The mixture will then be filtered and 5cm3 of filtrate will be placed into a test tube. 2 cm3 of Benedict’s solution will be added to each and the contents of the test tube boiled for 8 minutes in a heat block. The colour of the two solutions will then be compared. If a reducing sugar is present the colour will change from blue to green then yellow then orange then brick red. The final colour of the solution will depend on the concentration of reducing sugar in the potato tissue sample; the more the concentration of reducing sugar the greater the colour change (Larkcom and Miller, 2001).

Bibliography

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