ApplePress

Which is the most energy efficient method of preserving apple juice – pasteurisation or freezing?

With the approach of the apple harvesting season, Transition Bath’s energy group were wondering which was the most efficient way of preserving apple juice?

For domestic juicers there seems to be two alternative approaches: pasteurisation and freezing. Both require energy:

  1.  pasteurisation incurs energy heating the juice up to 80C (fast pasteurisation method)
  2. freezing incurs energy reducing the temperature of the juice to 0C, freezing the liquid and then cooling the frozen solid to -18C

Our brief analysis suggests that freezing might be the most efficient

Method Energy Requirement (kWh/litre) Cost (pence/litre)
Pasteurisation 0.0948 1.5
Freezing 0.0403 0.6

requiring about half the energy and cost. Either way processing your own apples into juice at between 0.6p and 1.5p per litre must cheaper and have lower impact on the environment than buying your juice from a supermarket with all the food miles that that entails? We also suspect converting the juice into cider might be even more efficient as it doesn’t require heating or cooling for preservation, however the bubbles of CO2 from the end product might increase its carbon footprint!

If you have apples to spare this year, Transition Bath has an apple press and pasteuriser available for loan and we run training courses in their use. If you are interested in making use of this equipment, please contact us at food@transitionbath.org .


Detailed energy calculations

Pasteurisation

Specific Heat Capacity of water 4.2 J/g/K
Start temperature 15 C
End temperature 80 C
Temp diff 65 C
Energy 273000 j/l
Energy 0.075833 kWh/l
Efficiency 80%
Gross Energy 0.094792 kWh/l

Freezing

Heat capacity of water 4.2 J/g/K
Drop to zero energy 63000 J/l
Heat of fusion of water 334 J/g
Heat of fusion of water 334000 J/l
Heat capacity of frozen water 2.1 J/g/K
Drop from zero to -18C 37800 J/l
Total energy requirement 434800 J
Total energy requirement 0.120778 kWh
Efficiency (COP of freezer) 300%
Net 0.040259 kWh/l

Posted in Energy, Food.