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Freeze Drying of food samples

prior to Pressurized Solvent Extraction


Freeze-drying (lyophilisation) is a very gentle method of preparing food samples for subsequent fat extraction using the SpeedExtractor. As the SpeedExtractor E-914/E-916 can only be used for solid or semi-solid samples, it is normally necessary to mix liquid or pasty samples with a carrier material such as diatomaceous earth. In addition, this step also acts as a way of drying the sample in order to enable efficient extraction using a non-polar solvent.

Alternatively, the samples can be freeze-dried prior to extraction. In that case, drying is effected primarily by sublimation of ice at low temperatures. For the experiments described in this article, chicken breast, hen's eggs and basa fish were chosen as examples. The freeze-dried samples were extracted using the SpeedExtractor E-914 and the fat content determined was compared with levels according to literature.

All the chosen samples were homogenized for a few seconds using the Mixer B-400. The fresh chicken breast meat and basa fish samples were cut into pieces prior to mixing. One set of triplicate samples was prepared for the fat extraction, a second set of triplicate samples was prepared for the determination of the moisture content. The weights of the homogenized samples, shown in Figure 1 (a, c, e), were recorded for later fat determination. All samples were placed on a plastic dish and frozen for 24 hours in a deep freezer at -24°C.

After 24 hours of deep freezing, the samples were transferred with the plate into the Lyovapor L-200 Pro for freeze drying. In Table 1, the Lyovapor L-200 Pro general settings are presented.

Table 1: General settings for drying in LyovaporTM L-200 Pro.

The steps of the primary and secondary drying process were programmed on the Lyovapor L-200 Pro as listed in Table 2. The parameters can also be programmed on the computer using the Lyovapor software.

Table 2: Parameters using LyovaporTM L-200 Pro.

The freeze dried samples, shown in Figure 1 (b, d, f), were homogenized using a mortar, transferred quantitatively into a weighing boat and weighed. The sample was mixed with 10 g of diatomaceous earth and transferred into an 80 mL extraction cell of the SpeedExtractor E-914 [1].

Figure 1: Fresh homogenized fish (a), chicken (c) and egg (e) samples before and after Freeze Drying (b, d, f).

The extraction of the freeze-dried samples was performed using the parameters listed in Table 3. Pre-weighed 240 mL vials were used to collect the extracts.

Table 3: Parameters using SpeedExtractor E-914 [1].

The extracts were dried using the Multivapor P-6 with the parameters listed in the Table 4, then placed in a drying oven for 30 min at 102 °C. The extracts were cooled down to ambient temperature in a desiccator for 60 min and the weight was determined.

Table 4: Parameters using Multivapor P-6.

The freeze-dried triplicates for the moisture content determination were dried in a drying oven for 120 min at 105 °C, cooled down in a desiccator for 30 min and weighed [3]. The difference to the initial weight was defined as moisture content.

Results and Discussion 
The determined moisture contents of the freeze dried triplicate samples are shown in Table 5. The results correspond well to the expected range of 1 – 5 % [2].

Table 5: Moisture contents of freeze dried samples.

The results of the fat extraction of the freeze dried samples using SpeedExtractor E-914 are shown in Table 6. They corresponded well with the expected fat content mentioned in literature [4, 5].

Table 6: Extracted fat using SpeedExtractor E-914.

With the Lyovapor L-200 Pro, a high drying efficiency was achieved. The analyzed samples of chicken, egg and fish had a residual moisture content of 3.51 %, 3.29 % and 3.08 % (n=3) after freeze drying which corresponds well to the typical range of 1 – 5 % for freeze dried biological samples mentioned in literature [2]. The initial moisture content of the samples was 70.44 % for chicken, 73.53 % for egg and 86.57 % for fish (n=3). Hence, the freeze drying method on the Lyovapor L-200 Pro can lead to an average moisture removal of 96 – 97 %. In summary, the Lyovapor L-200 Pro enables to dry food samples such as chicken, fish and egg quickly and effectively prior to Pressurized Solvent Extraction. 

After freeze drying, the samples were extracted using the SpeedExtractor E-914 and the fat content of the original samples was determined: 1.38 % for chicken, 9.44 % for egg and 3.56 % for fish (n=3). The obtained fat contents are comparable to reference values in literature [4, 5].

References: BUCHI Short Note No. 288/2017
[1] BUCHI Application Note, No.069/2012, Extraction of PCDD/Fs and PCBs in fish using SpeedExtractor E-914.
[2] Determination of Moisture in Foods, GB 5009.3-2016. Chinese National Food Safety Standard.
[3] FDA Guideline for the Determination of Residual Moisture in Dried Biological Products, 89D-0140. US Food and Drug Administration.
[4] Swiss Federal Food Safety and Veterinary Office. http://www.naehrwertdaten.ch (accessed Apr 10, 2017).
[5] United States Department of Agriculture, Food Composition Databases. https://ndb.nal.usda.gov/ndb (accessed Apr 12, 2017).

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