An investigation of heat stable milk chocolate by addition of glycerol

Glycerol added to milk chocolate (made via the crumb method), in small amounts, can create a heat stable chocolate. Milk chocolate (MC) is a dense suspension consisting of cocoa solids, sugar and milk powder dispersed in a continuous fat phase of cocoa butter and milk fat. Heat stable milk chocolate (HSMC) is chocolate that does not flow at temperatures between 35-40 °C, when traditional chocolates do. This thesis aims to determine the mechanism of heat stability in solid milk chocolate as afforded by the inclusion of small amounts of glycerol. Chocolate sales in warmer climates is growing exponentially. HSMC is required by these consumers to combat negative effects of premature melting such as “bloom” and misshapen bars that are too soft to handle. When glycerol was added to chocolate it did not completely lose its hardness (i.e. soften) at high temperatures. From indentation experiments, a linear relationship existed between bulk yield stress and the square of the volume of glycerol present in the chocolates. Adding glycerol to chocolate increased its viscosity, which raises the risk of an increase in difficulty pumping and mixing in a factory setting. As determined by DSC and XRD experiments, addition of glycerol did not affect melting or crystallisation properties of the fat phase. The fat phase in HSMC melted over the same temperature range as MC. Tempering chocolate remained the biggest influence on crystal structure of the fat phase, and subsequently its melting temperature range. Adding glycerol before or after tempering did not change this. NMR tests showed that no chemical reactions occurred when glycerol was mixed with the fat blend. This was further demonstrated when HSMC continued to exhibit “bloom” after cooling from melted states, as expected with MC. This is an undesirable effect for the consumer. The reason for induced heat stability was not related to changing the fat phase to remain solid at higher temperatures. The fat in HSMC was melting but the chocolate was not flowing. This indicated there was a structure holding the fat phase in place. By removing fat from HSMC by submerging in hexane, it was confirmed that addition of glycerol to chocolate formed a cage-like structure of solids linked together throughout the entire system. HSMC retains the same heat stability when no lecithin is present. This indicates an interaction between the glycerol and lecithin is not needed to cause heat stability (it remains essential for favourable flow properties). Therefore, glycerol interacting purely with the surface of crumb must be the root cause of the cage structure and heat stability in chocolate. Glycerol did not prefer to wet surfaces of ingredients in chocolate when it was surrounded by the fat blend. However, it weakly wetted sugar surfaces. Through indentation testing on chocolates made with different proportions of sugar, crumb and cocoa powder, it was found that sugar played an important role in the interaction with glycerol. When very small amounts (<0.5%) of glycerol were added, severe agglomeration was seen in chocolates with only sugar solids. Glycerol’s interaction with sugar is the reason for the cage-like structure that imparts heat stability to the chocolate. Glycerol HSMC needs to have sugar in the recipe, so it won’t be applicable for calorie reduced, diabetic chocolates where sugar has been replaced with bulk polyols. Overall, sugar has a large role to play in this cage being formed and deserves further research to confirm if this is due to sugars becoming ‘sticky’ and agglomerating or capillary force induced liquid bridges. The structure of this thesis is as follows: chapter 1 introduces chocolate and its manufacture, chapter 2 reviews relevant academic literature and patents and discusses the current state of knowledge of mechanisms for HSMCs. Four chapters then follow containing experimental results and discussion. The first of these presents physical properties of HSMC and compares them with MC. Three chapters then investigate possible mechanisms for observed heat stability in relation to glycerol’s effect on the ingredients in chocolate: fat blend (chapter 4), lecithin (chapter 5) and solids (chapter 6). The last chapter will discuss the projects conclusions and findings that are relevant to heat stable chocolate manufacture on an industrial scale.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Holian, Jennifer

Advisor dc:contributor.advisor

• Wilson, David

• Chocolate
• Heat resistant chocolate
• Heat stable chocolate

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e1681c27-96f1-4391-8086-ba907da2fa0c/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.108551

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/368239