Sunflower Seeds

When using a planetary mill, samples for comminution should be brittle. Depending on sample moisture content as well as the fat or oil content, most seeds will start sticking rapidly. A longer dry grinding of the sticky sample is not recommended.

For our first trial, we used 100 g of seeds which filled the 500 ml grinding bowl almost to its maximum. The used Planetary Mono Mill PULVERISETTE 6 was set to 500 rpm and the sample was ground with 30 mm Ø steel grinding balls for one minute.

During the operation, a change in grinding sound already indicated that the sample started sticking (mainly to the grinding bowl).

We interrupted the grinding process after one minute. As expected, the balls were located in the middle of the grinding bowl. The sunflower seeds were ground into a mushy consistency, dry enough to stick to the grinding bowl. The grinding balls were coated in oil, the surface was much smoother compared to starting conditions.

We don’t recommend longer dry grinding, with direct contact between grinding balls, abrasion will be higher than usual. Also the sample compressed to the bowl contains larger fragments of sunflower seeds which will not get comminuted any further. Grinding in suspension can be performed to improve the grinding result (see result 2).

For an intensive cleaning, we recommend a short time grinding of a smaller amount of sand in combination with a bit of water. After such a procedure, grinding bowl and balls surfaces become coarser again (beneficial for organic fibers).

For organic or fibrous materials, grinding bowls and balls made of sintered corundum are beneficial. With their coarse surface, fibrous materials are ripped into pieces much faster than smooth-surfaced grinding materials.

Due to its smooth surface, sintered corundum will show higher wear off compared to other grinding materials.

After one minute of dry grinding, the sample was ground similar to the sample in result 1. For grinding in suspension, several solvents (even water) can be considered. We chose Isopropyl alcohol to avoid a rotting or fouling of sample after comminution. Also a sample can be dried much faster when alcohols are chosen for grinding (e.g. by rotary speed evaporation).

We fed about 75 grams of isopropyl alcohol (? ~96 ml IPA) after one minute of dry grinding. We proceeded comminution for a total of 10 minutes. After 10 minutes, further 15 g of IPA were added to maintain a motor oil like viscosity (for an optimum grinding result).

To avoid over pressure, we ground the sample in steps of 3 minutes, followed by a programmed pausing time of 10 minutes. After several cycles, the outside temperature of the bowl should be checked (should remain below 75°C with isopropyl alcohol); grinding time or programmed pausing time can be readjusted afterwards.

After a total of 10 minutes, a few fibrous particles can be spotted with a length of up to 2-3 mm. By diameter, the majority of the sample should be able to pass a 1 mm test sieve already. We took out a small portion for demonstration purposes (see Eppendorf reaction tube) and proceeded grinding for a total of 20 minutes.

After 20 minutes of grinding, we interrupted the grinding process and packed the sample into a plastic bottle. Still some longer fibers (~ 1-2 mm length) were spotted at this point of comminution.

A Variable Speed Rotor Mill PULVERISETTE 14 can be used for comminution also. Depending on the type of seeds, it might be beneficial to embrittle the sample before feeding. With sunflower seeds, the majority of the sample is grindable with 1,5 mm openings after embrittlement.

In order to embrittle, the sample can be placed into a freezer for several hours, placed into dry ice (CO2), or can be placed into liquid nitrogen. Liquid nitrogen is about -178 °C cold; this might work best for a following comminution.

We placed the sample into a plastic bowl and poured liquid nitrogen on top. After one minute of embrittlement, sample was shoveled into the feeding funnel, using a small spoon.

The complete amount of sample (30 g) was fed within 40 seconds. After grinding with 16.000 rpm, rotor and sieve ring are almost clean. Surely higher amounts of sample can be ground this way. By adjusting to maximum rotor speed (20.000 rpm), samples should be comminuted even finer. With higher speed settings it might be possible that fat or oil content will start smearing and perforations of the used sieve ring are clogged faster.

With a standard collecting vessel, about 50 grams of sample should be grindable per batch. After 50 grams are ground, the collecting vessel might need to be emptied. With accessories for grinding large quantities, also 100 grams of sample could be ground in one batch without any problems.

Based on the fact that no sticking of sample was observed in the previous trial, we checked grindability of sample, using a 1 mm perforated sieve ring.

Sample was embrittled for about one minute in liquid nitrogen before feeding of sample was started. It took about one minute to feed the sample a spoonful at a time to the feeding funnel.

After one minute, the entire sample was comminuted successfully. No significant residues were left on rotor or sieve ring. Even a 1,0 mm sieve ring can be used to comminute the sample as shown, with accessories for grinding large quantities in one batch.

Finally we also wanted to testify a grindability without embrittlement. We used the 1,0 mm sieve ring and fed the sample without embrittlement. It took about 40 seconds until the complete sample was fed to the feeding funnel.

After 40 seconds, the sieve ring starts closing at the top and lower level where the rotor has a closed surface / the lid has an edge which is holds the sieve ring in place. This is typical for fatty or oily kind of sample. So far, no sample is blocking the trapezoidal perforations where the main comminution takes place (where the rotor ribs spin). We estimate that a few grams more (e.g. 52 grams) might be fed to achieve an output of 50 grams.

Eventually even higher amounts of sample can be ground without a cleaning step.

After comminution, we recommend flushing the rotor, sieve ring, pan and lid in a sink with hot water to remove sticky residues.