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Practicals

Properties of Chocolate
AKA: Chocolate structure

Summary

Students observe the structure of chocolate by melting samples.

Operation

This practical is very popular with students as they get to taste the chocolate. It should be noted however that there should be no eating in a science laboratory as advised by CLEAPSS.

Students taste samples of the chocolate that have been melted and re-hardenend and samples fresh from the packet and compare the taste and physical properties.

Technician preparation before the lesson may be needed if pre-melted and set chocolate is required. Any milk chocolate is fine for this practical. Dairy Milk and similar types can be purchased in big bars and only a couple of squares per student are usually required.

For pre-melted chocolate, take a sealed packet and place on a radiator or warm place until soft and re-harden by placing in a food refrigerator. Once set, remove and store at room temperature. Ensure that the re-hardened chocolate is the same make as the other samples.

Equipment required (per set):

  • fresh chocolate squares (2)
  • melted and re-hardened chocolate samples (2)
  • 2 boiling tubes
  • 250ml beaker
  • stopclock
  • thermometer or datalogger with temperature probe
  • water at around 50°C
Students usually begin by tasting the fresh chocolate and looking at its texture and colour. They then place a piece into a boiling tube which is partially submerged in the beaker of hot water. A thermometer is used to stir the chocolate as it melts and observations recorded.

The re-hardened sample is investigated in the same way and melting point and physical appearances noted.

The two samples are compared after results and observations recorded.

The fresh sample should melt at around 32-36°C. (form 4 in the table below).

The re-hardened sample should melt at a lower temperature (form 2-3 in the table).

During chocolate manufacture, the cocoa butter can solidify into one of six polymorphs listed in the table below. A form 5 polymorph can be found in the best tasting quality chocolate. Other forms may feel sticky and unpleasant when eaten.

polymorph melting point (°C) comments
form I 17.3 Produced by rapid cooling of melt. Successive polymorphs are then obtained by heating at 0.5 °C/min.
form 2 23.3 Produced by cooling melt at 2 °C/min or rapid cooling of melt followed by storing from several minutes up to one hour at 0 °C. This form is stable at 0 °C for up to 5 hours.
form 3 25.5 Produced by solidification of melt at 5-10 °C or transformation of form 2 by storing at 5-10 °C.
form 4 27.3 Produced by solidification of melt at 16-21 °C or transformation of form 3 by storing at 16-21 °C.
form 5 33.8 Produced by tempering (cooling then reheating slightly while mixing). The most desirable form with good gloss, texture, and "snap".
form 6 36.3 The transformation of form 5 after spending 4 months at room temperature. Leads to the white, dusty appearance.

Safety

 CautionDo not eat in the science lab.

The contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.