Analiza żywności i paszy Odpowiednie młynki laboratoryjne do przygotowywania próbek i produkcji na małą skalę

Powstanie próbek swobodnie płynących, ziarnistych lub kryształowych (nie tłustych), takich jak kukurydza lub cukier, jest zwykle proste. Odpowiedni jest szeroki zakres młynów, w tym różne Młyny z rotorem. Próbki kryształowe, takie jak cukier, mogą być pomalowane do mniej niż 0,05 mm za pomocą linku 151 - wewnętrzny link "Otwiera wewnętrzny link w bieżącym oknie">Młyn ultra odśrodkowy ZM 300/link>. Użycie cyklonu pomaga wydostać próbkę z Piece komorowe do pomalowania, chłodzi próbkę i pozwala na przetwarzanie do 4,5 l próbki na partię.

Materiały takie jak kukurydza lub zioła są zwykle zmielane do wielkości cząstek około 500 μm lub nawet 250 μm za pomocą ZM 300 lub Młyn bijakowy SR 300. Przy zmielaniu nieznanej próbki, zaleca się również rozpocząć siew ze średnią wielkością otworu. Gdy próbka nie blokuje siewu, wielkość może być dalsza zmniejszona. Ta procedura ma zastosowanie do wszystkich Mn z rotorem. SR 300 jest również korzystny dla większych ilości próbek do 30 l, jeśli cyklon jest przymocowany.

Cukiernictwo występuje w bardzo różnych teksturach: może być twarde, klejące, tłuste lub wilgotne. Typowy proces homogenizacji ciężkiej cukierni o wysokiej zawartości cukru i syropu skrobiowego jest wykonany w Knife Mill GRINDOMIX GM 200: 100 g twardej cukierni jest najpierw przybliżenie zmielone przez kilka sekund przy 2000 obr./min. w trybie odwrotnym z odwrotną stroną noża w celu ochrony ostrej strony i zmniejszenia zużycia. Po tym następuje odstępowe zmielanie w trybie przodu przez kolejne 15 sekund przy 4000 obr./min.

Twarde materiały próbne, takie jak tłusty, pasmowy bacon , stanowią wyzwanie dla procesu homogenizacji przed analizą. Jeśli większe części skórki lub skóry pozostają nierozcięte, próbka nie jest homogeniczna, a analiza może dać fałszywe wyniki. Młyny nożowe okazały się najlepiej odpowiednie do dokładnej homogenizacji próbek mięsa. Silny silnik zapewniający wykorzystanie pełnej zdolności cięcia ostrzy jest korzystny. Aby uzyskać całkowicie homogenizowaną próbkę (w temperaturze pokojowej), noże z ostrzy są idealnie odpowiednie do homogenizacji twardych próbek mięsa w bardzo krótkim czasie, ze względu na dodatkowy efekt rozerwania, który ułatwia redukcję wielkości włókien mięsa.

250 g of pork shoulder is processed in a GRINDOMIX GM 200 with interval mode at 3000 rpm for 30 s, using a serrated blade knife. The first step is followed by two cycles of 30 s, each at 7000 rpm. Thorough homogenization of the sample is achieved after another 30 s at 10,000 rpm. The sample would bounce too much if the maximum speed was selected right from the start. Nonetheless, full speed is required at some point to achieve optimum results. It is also important to use a standard lid during the first step, as other lid types might put too much pressure on the sample. For the fine-grinding-step, a volume-reduction lid  is beneficial to fully homogenize the sample. The material sticking to the grinding container wall above the blades needs to be removed from time to time and returned to the grinding process.

Cheese samples can also be tricky as they are fatty and sticky. 130 g sample were homogenized to < 0.5 mm in 2 x 10 s grinding steps at 10,000 rpm using the GM 200. It was important to employ the volume reduction lid 0.25 l to force the sample towards the blades. Between the grinding steps, the sample can be mixed manually with a spoon to loosen sticky parts. Samples like raisins are even more sticky than cheese, but 200 g can be homogenized in a similar way as described for the cheese sample within about 20 s.

Free flowing samples like seeds or coffee, which are oily, can be processed in a knife mill, but usually a rotor mill like the ZM 300 is used to pulverize such samples. Distance sieves (or a distance rotor if the SR 300 is employed) help to minimize shearing effects and also fat release from the sample, which would cause agglomeration and blocking of the sieve. An aperture size of 0.5 mm or larger should be used for the same reasons. Reducing the speed can have a beneficial effect and lead to less warming and consequently to less fat release.

Vegetables often contain moisture (like stem cabbage) or even consist predominantly of water (like tomatoes). Processed products like noodle soup also contain a lot of water and are prepared similar to tomatoes. In the latter cases, complete homogenization is facilitated by the high water content, as sample pieces are too wet to stick on the walls of the grinding container in a GRINDOMIX GM 200 or GM 300 where they no longer come into contact with the rotating knifes. For example, 180 g tomatoes are homogenized in the GM 200 for 10 s, first at 4000 rpm, then 8000 rpm. The gravity lid with overflow channels reduces the volume of the grinding chamber, preventing the sample from spilling out. For larger sample portions up to 4.5 L the GM 300 is the perfect choice.

Samples like stem cabbage have a lower water content. The sample pieces tend to stick on the wall of the grinding container, thus evading contact with the knife blades. A few sample pieces may remain in the mostly homogeneous sample, even if maximum speed is used. Employing the gravity lid with overflow channels helps to improve the grinding effect, but full homogenization is often achieved by adding a bit of water to the sample. 280 g stem cabbage was cut manually in four pieces. Grinding was performed in two steps. It is recommended to use a low speed of 2000 rpm for the first 10 seconds. For fine grinding at 5000 rpm, 50 ml water was added to achieve a good homogeneity after 20 seconds. The gravity lid with overflow channels was used to ensure thorough homogenization. The interval mode during the fine grinding step improves mixing of the sample and thus increases grinding efficiency.

For fibrous samples such as dried herbs or other plant materials - but also freeze-dried fish, for example, - cutting effects are best suited to homogenize the samples. Usually, large sample amounts are required to ensure representative sample preparation, as fibrous materials tend to be light, voluminous materials and can be very heterogeneous. Sometimes manual pre-cutting is necessary to obtain a suitable sample size for feeding the cutting mill and avoid formation of nests and clusters which tend to remain in the hopper and are therefore not efficiently homogenized.

The Cutting Mill SM 100 is suitable for basic preliminary grinding, where the plant samples are effectively cut with a parallel section rotor. Usually, a fineness down to 4 to 6 mm can be easily achieved in a cutting mill. For obtaining particles < 1 mm, the use of a cyclone is recommended, for example in the SM 300. The Ultra Centrifugal Mill ZM 300 produces even finer particles but has the drawback of accepting smaller sample pieces. Thus, if large fibrous samples need to be homogenized to less than 0.5 mm, the combination of pre-cutting in a cutting mill followed by fine grinding in a ZM 300 is the best choice. A standard ring sieve rather than a distance sieve should be used in the ZM 300 as fibrous samples require shearing forces. Another solution might be a rotor mill like the SR 300 which accepts larger initial feed sizes and is capable to pulverize fibrous samples down to < 0.5 mm using the standard rotor which generates sufficient shearing forces.

A cyclone helps to improve the discharge of light sample materials from the grinding chamber. It also cools the sample to minimize loss of volatile ingredients such as terpenes. If volatile ingredients should be preserved, it is recommended not to use too fine bottom sieves – as this causes warming and thus loss of volatiles. Depending on the sample properties, the pulverized materials tends to remain fibrous, as long fibers may pass the bottom sieves of the cutting and rotor mills lengthwise. If this needs to be avoided, ball mills like the MM 400 or the PM 100 are a better choice.

The feed size is the original particle size of the sample. For choosing a suitable mill, it makes a great difference whether large samples, like a whole fish, or small particles, such as grain kernels, are to be homogenized.  The homogenization of a complete fish is a challenge; scales, skin and bones are fairly resistant to size reduction so that the sample still contains larger pieces after grinding in most mills (e. g. fresh fish in a knife mill). A high fat content makes the process more difficult, as fatty particles stick together to form large lumps which block the mill and keep the sample inhomogeneous. Freeze-drying the fish and grinding it in the Cutting Mill SM 300 help to solve the problem. 125 g (4 fishes, pre-cut once) of carp or turbot were pulverized in the SM 300 at a speed of 3,000 min-1, using a V-rotor which also cuts the scarp and fish bones. The cyclone was employed to cool the sample. After 2 min of grinding with a 0.75 mm bottom sieve, a particle size of 0.75 mm is obtained without significant heat-built-up.

Another example for large sample pieces is cocoa cake. 1 kg with sample pieces of up to 80 mm can be easily homogenized to a fineness < 10 mm in the Cutting Mill SM 300 using a 10 mm bottom sieve and the parallel section rotor at 1,500 rpm for 1 min.

Rock salt not only consists of sodium chloride but may also contain other minerals and silicates. To analyze the composition of salt, the sample needs to be sufficiently homogenized, considering that larger lumps of rock salt are usually very inhomogeneous. The element concentrations in salt are typically very low so that quantities in the kilogram range are required for sample preparation. In principle, a cutting mill can cope with large quantities and sample pieces, but wear would have a much greater impact than in a rotor beater mill, as the cutting bars of the cutting mill are not designed to process large amounts of abrasive materials. With a rotor beater mill, charges of several kilograms can be pulverized easily. A distance rotor is recommended to reduce frictional heat. Thanks to a 5-L collecting vessel, 5 kg of sample with a feed size up to 25 mm is pulverized in one run at a speed of 10,000 rpm in the SR 300. The complete sample is reduced to a final fineness of <200 µm.

For some analyses, large sample amounts are required to detect traces of analytes or to find clusters, e.g. of mycotoxins or GMO. Mycotoxins are produced by fungi, which form clusters in a sample. Open systems with an inlet and outlet, like rotor mills, accept large feed quantities of bulk material and are therefore ideally suited for sample preparation prior to mycotoxin or GMO analysis.

The first step is the preliminary size reduction of a representative amount of, for example, 2 kg per ton of nuts with the Cutting Mill SM 100 to a particle size of 3 mm. The sample is then divided into representative sub-samples with the help of the Rotary Tube Divider PT 100 which provides a very high division accuracy.

The subsequent fine size reduction is ideally carried out in the Ultra Centrifugal Mill ZM 300. For the processing of hazelnuts, the use of distance sieves is recommendable which have been specially developed for grinding temperature-sensitive, brittle materials. As mycotoxins are lipophilic, the grinding process should be as gentle as possible to avoid the release of fat from the sample.  A cyclone helps to quickly discharge the sample from the grinding chamber and provides cooling by generating an air stream. A fineness of 300 µm is sufficient for the subsequent extraction of the mycotoxins from the sample. In the same way, soy beans can be processed.

If a sample is already fairly homogeneous or if the subsequent analysis deals with PCR, only small sample volumes are required. For this type of application, ball mills are often the ideal choice.

An allrounder like the Mixer Mill MM 400 features two grinding stations and accepts sample amounts up to 20 ml. It pulverizes, for example, 6.5 g dried peas to a fineness of 0.4 mm in 30 s using a 50 ml stainless steel grinding jar  and 1 x 25 mm grinding ball.

In a similar way, 8 g dried hibiscus is pulverized within 2 min to 100 µm. In 50 ml steel jars, usually a 25 ml ball is used. A rule of thumb is: the grinding ball needs to be three times larger than the biggest sample piece for efficient sample crushing. Therefore, a particle size of about 8 mm can only be effectively crushed in the 50 ml jar which leaves enough room for a 25 mm ball. The MM 400 also accommodates adapters, e.g. for single-use vials, to process samples with particle sizes of 3 mm max.

The MM 400 can be equipped with a variety of adapters, accepting, for example, 2 ml single-use vials, 2 ml steel vials or 5 ml steel jars. Thus, homogenization can be done in batches of 8 or 20 samples, which is a benefit for PCR analysis of, e.g., a single grain or pea. Here, 2 x 7 mm to 10 mm grinding balls made of steel or tungsten carbide are added to each tube. Single-use vials have the benefit of preventing cross contamination.

The Mixer Mill MM 500 vario allows for higher sample throughput thanks to 6 grinding stations for grinding jars or adapters. In total, 50 x 2 ml single-use tubes or stainless steel jars or 24 x 5 ml stainless steel jars can be used per batch.