Food & Beverage Analysis
Safe and Effi cient Cryogenic Grinding of Food Samples Author details: Dr Tanja Butt, Retsch GmbH,
t.butt@retsch.com
Most sample materials can be ground to the required analytical fi neness at room temperature. However, there are limits, for example when even a small temperature increase affects the sample in a negative way; or when the material is very elastic and will only be deformed. Moreover, food samples which are fatty or sticky may block the mill. Therefore, cryogenic grinding is the best way to pulverise food samples like cheese, raisins, wine gum or marzipan which simply clump together when ground at room temperature.
Another important aspect is the preservation of volatile sample components. The heat which is generated during the grinding process would cause volatiles like alcohol or residues of softeners from plastic wrappings to escape. This effect is prevented by using
grinding aids like liquid nitrogen (LN2; -196°C) or dry ice (solid CO2; -78°C) which also embrittle the sample and make it break more easily. In the following, the special requirements for cryogenic grinding in different mills will be discussed as well as which other aspects need to be taken into consideration (Table 1).
Table 1. Overview of mills suitable for cryogenic grinding Mill
Feed size and max. feed quantity *
Mixer mill
<8 mm 2 x 20 ml
Remark
• Sample is placed in leak-free grinding jar of steel or PTFE and
embrittled before grinding, LN2 preferred over dry ice
• Intermediate cooling may be required
CryoMill
<8 mm 1 x 20 ml
• Continuous grinding at -196 °C with LN2
• User comes at no point in contact with LN2
• Zirconium oxide grinding jar available for cryogenic grinding
Knife mill
<40 mm 2000 ml
• Embrittlement with dry ice
• Dry ice cools sample during grinding
• Use of full metal knife, grinding container of stainless steel and specifi c lid mandatory
Ultra centrifugal mill
<10 mm 4000 ml
• Embrittlement with dry ice or LN2
• Dry ice preferred if sample material is < 1 mm or has low thermal capacity
• Use of cyclone mandatory Cutting mill
<80 mm 4000 ml
• Cryogenic grinding with dry ice or LN2
• Use of 6-disc rotor and cyclone mandatory
• Bottom sieves 2 – 20 mm suitable *both depending on sample material
Small Sample Volumes and Feed Sizes in the Mixer Mill MM 400 and the CryoMill
Sample volumes like a few gummy bears, some strips of chewing gum, a small piece of meat or small amounts of fatty spices are best homogenised in the MM 400. This ball mill is perfectly suited for homogenising sample volumes up to 2 x 20 ml in less than 1-2 minutes. It is important to fi ll the jar fi rst with the grinding ball(s) and with the sample and close it tightly before embrittling. Care must be taken that no LN2 is enclosed in the
Figure 2. Gummy bears before and after cryogenic grinding in the Mixer Mill MM 400 Cryogenic grinding in the CryoMill offers the advantage of continuous cooling of the
grinding jar with LN2, thereby reducing the temperature of jar and sample to -196°C within minutes. This consistent temperature is guaranteed even for long grinding times without the need for intermediate cooling breaks. An automatic pre-cooling function ensures that the grinding process does not start before a temperature of -196°C is reached and maintained. Moreover, operation is particularly safe as the user comes at no point into
Figure 1. The CryoMill offers the advantage of continuous cooling of the grinding jar with LN2
grinding jars because the evaporation of LN2 would result in a considerable pressure increase inside the grinding jar. The closed grinding jars, and thus the sample, are
embrittled in a LN2 bath for 2-3 minutes. Suitable grinding jars for cryogenic grinding are made of steel or PTFE; it is not recommended to use jars made of different materials (e. g.
steel jar with lining of zirconium oxide). This is important, as these may react differently to extreme temperatures of -196°C which may lead to damages of the jar. Single-use vials of 1.5, 2 and 5 ml are also available for cryogenic grinding when it comes to sample amounts below 0.7 ml. Due to the high energy input and the resulting frictional heat, the grinding process should not take longer than 2 minutes to prevent the sample from warming up and to preserve its breaking properties. If longer grinding times are required, these should be interrupted by intermediate cooling of the closed grinding jars in the MM 400.
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