Mortar Grinder Sample Processing of Cocoa Beans

The seeds of the cocoa tree are cocoa beans, which are mainly cultivated in South America, West Africa, and Indonesia, and exported globally. Cocoa beverages and chocolate are the key products of cocoa beans. The key characteristic feature of these products for the consumers is the taste.

Prior to chocolate production, the cocoa beans need to be roasted, peeled and broken down into pieces (nibs). These nibs (Figure 1) are then comminuted into a semi-fluid cocoa mass, which is then pressed patty ground for the cocoa powder production. The ensuing cocoa butter and the cocoa mass are then processed into chocolate.

Figure 1. Sample of broken cocoa bean pieces.

Comminution Process

In the laboratory, the nibs are comminuted several times repeatedly. For this purpose, the Mortar Grinder PULVERISETTE 2 and the Planetary-Ball Mill PULVERISETTE 6 classic line are used. In this process, the broken pieces, embedded fat, and the cocoa butter are comminuted into a dark brown mass depending on the temperature and energy applied.

The comminution with a mortar grinder is proven for sensory testing. Industrial roller plants are predominantly used for chocolate production. The Mortar Grinder PULVERISETTE 2 transfers the energy with friction and pressure onto the material to be milled. The applied energy per unit time of the mortar grinder is much lesser when compared to a planetary ball mill, which is designed to provide the maximum impact energy. Hence, the mortar grinder is not suitable for the large-scale technical process.

The sample stays put in the mortar grinder with surrounding temperature. The partial squeezing of the fat from the grain results in a clod shaped mass. The characteristics of the mechanical strains as well as the consistency of the sample (Figure 2) need to be reproduced at higher temperatures for sensory tests. For this purpose, a zirconium oxide mortar bowl and pestle is recommended.

Figure 2. Consistency testing with a finger of a hazelnut spread.

Inside the heating cabinet, the mortar bowl and the pestle are warmed to 50°C. The retained energy is adequate to produce a homogenous mass capable of flowing within the 10-minute process period. Then distinguished palates decide the quality of the supplied cocoa beans for further use.

Selection of Suitable Instrument

The Planetary-Ball Mill PULVERISETTE 6 classic line is ideal to prepare samples for the analysis of pesticides, mycotoxins, and heavy metals. This Mill produces an absolute homogenous mass in an extremely short amount of time. In this example, a 250ml zirconium oxide grinding set equipped with 20mm balls was utilized to produce chocolate from 50g weighed-in-quantity material within 2 minutes.

A glance into the opened grinding set shows nothing to be desired with respect to the homogeneity and fineness. However, the application of high energy does affect the sensory testing.

In the task of processing up to 2kg of a sample, which could be further processed into a chocolate mass in the experimental lab, the nibs need to be ground into a preferably fine pourable powder.

Considering the desired final product, a Variable Speed Rotor Mill PULVERISETTE 14 was selected for this task (Figure 3). The Mill with a 2mm sieve comminuted 100g nibs in less than a minute. A glance into the opened grinding chamber demonstrates the capability of the chamber to accommodate additional material. The ground sample is shown in Figure 4.

Figure 3. Comminuted cocoa bean nibs inside Variable Speed Rotor Mill now a fine powder.

Figure 4. Ground sample.

The required 2kg sample can be obtained either by a batch-wise process or by equipping the mill with the conversion kit for comminuting large quantities. However, the fineness obtained is sufficient enough. By using a 2mm sieve, even finer material can be obtained. Nevertheless, a comminution with the 1mm sieve is failed. Tests to further increase the fineness through the addition of dry ice were not performed. This approach shows promise to produce an even finer, pourable powder.

This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.

For more information on this source, please visit FRITSCH GMBH - Milling and Sizing.