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. 2020 Oct 1;58(8):3192–3198. doi: 10.1007/s13197-020-04822-7

Freezing and freeze-drying of strawberries with an additional effect of micro-vibrations

G V Semenov 1,✉, I S Krasnova 1, S I Khvylia 2, D N Balabolin 3
PMCID: PMC8249526  PMID: 34294981

Abstract

Our research focuses on the formation of ice crystals and evaluating the structure of preserved frozen and freeze-dried strawberries. Strawberries were frozen in two ways. One-half of strawberries were frozen at − 30 °C under conditions of convective heat exchange. The other half of strawberries were frozen under the same conditions with an additional effect on the strawberries of micro-vibrations created in the air of the freezer according to a specific program. A digital frequency synthesizer that generates 250 W/m3 electromagnetic field rectangular pulse packets in the frequency bands of 2500–5000 kHz creates micro-vibrations. The microstructure of strawberries, the number of cells that have retained their structure and firmness were determined in frozen strawberries. The strawberries retained 25–30% of the cell structure of their total number during traditional freezing, and 65–70% of the cell structure when frozen under micro-vibration. The data of the penetration and shear stress showed that the strawberries frozen under micro-vibration conditions were 10–15% stronger. Then researched strawberries were vacuum freeze-dried. The primary drying temperature was 30 + 1 °C below zero and at the secondary drying the temperature was 38–40 °C. The microstructure and firmness of strawberries were researched in dried samples also. Freeze-dried strawberries frozen under micro-vibration had small and evenly distributed capillaries and their firmness was 8–10% higher than freeze-dried strawberries frozen by the traditional method. Thus, freezing strawberries with the additional effect of micro-vibration have a positive effect on the firmness of both frozen strawberries and freeze-dried strawberries.

Keywords: Strawberry, Micro-vibration, Microstructure, Firmness, Freezing, Freeze-drying

Introduction

International market of food products is associated with the need to transport them over long distances and the development of the wholesale, network, and small-scale trade systems. This has given rise to the need for an extensive use of different techniques to extend the shelf life of many foodstuffs and raw materials (Sagar and Suresh Kumar 2010). Nowadays, it is recognized that vacuum freeze-drying allows successfully solving this problem (Cortés Rodríguez et al. 2009; Verma et al. 2015). However, the disadvantage of this technology is the duration of the drying process and high-energy costs, which limits its wide application in the food industry. Therefore, in many countries, researches are being carried out to optimize the freezing and freeze-drying, reduce its duration and obtain a high-quality dried product (Menlik et al. 2009; Semenov et al. 2016; Tarafdar et al. 2017).

Freezing is known to have a main effect on the quality of freeze-dried products and the intensity of dehydration. The ice crystal forms during freezing, while the shape and size of ice crystals, their distribution in the frozen material, and the change of their properties depend on the freezing rate (Shafiur Rahman and Velez-Ruiz 2007). Researches of freezing and vacuum freeze-drying of foods containing compounds that are thermally sensitive have been reported widely in literature (Djaeni et al. 2014; Ratti 2008). At the same time, researches on the formation of small and homogeneous ice crystals and the preservation of the cell structure remains actual. This especially applies to fruits and berries of delicate structure, such as strawberries. Harder and denser strawberries are better processed and transported; they have a longer shelf life and a more attractive appearance, as compared to soft berries (Abbott 1999). In this connection, the search for different methods that provide a more firmness structure during the storage of strawberries is relevant.

Air blast freezing with natural or at forced convection is widely common (Celli et al. 2016; Dempsey and Bansal 2012). The velocity of the process is quite low during freezing at natural convection and this, therefore, results in formation of large ice crystals and decrease the quality of foodstuffs (Nagy et al. 2013). Quick-freezing process ("shock freezing"), using a medium at very low temperature is preferred for high quality products. It can assure the quickly turning of the water contained in the food into minute ice crystals, avoiding damaging the food cell membranes (Kim et al. 2015), as it may happen during freezing at standard temperature (− 40/− 20 °C) (Biglia et al. 2016). It is achieved by using high air velocities (more than 10 m/s) in freezers with air temperature under (− 40)–(− 45 °C). Shock freezers are quite complex and expensive and require high-energy costs (Dempsey and Bansal 2012).

Immersion freezing is another type of freezing for many kinds of foods (e.g., fish, poultry, fruits, berries, and vegetables) (Salvadori et al. 2000). Celli et al. in review reported that the immersion freezing in liquid nitrogen damage the structure of fruits, for example, apples. It is noted that the negative effect on the quality can also be due to the recrystallization process, which may occur during freezing and storage (Celli et al. 2016).

In addition, there are technologies of ice crystal formation during freezing with the additional action of ultrasound on the frozen material (Ercan and Soysal 2013, Xu et al. 2015). It is noted that a significant effect of ultrasonic exposure is achieved by immersing objects of freezing in water, and ultrasound is transmitted through it (Li and Sun 2002; Nowak et al. 2019; Olmo et al. 2008). This technology is suitable for products with a dense structure, such as meat or fish (Jayasooriya et al. 2004; Ravishankar 2019). For strawberries, this technology is unacceptable; water will have negative affect on their quality, texture and microbial safety.

The effect of micro-vibration (acoustic) on berries during freezing in air can be an alternative method for the formation of small ice crystals. There are evidence of a positive effect of vibration effects on freezing objects, where the advantages of the method for achieving small ice crystals, the effect on nucleation and size distribution of crystals in frozen products are noted (Acton and Morris 2001; Ganina et al. 2019; Owada et al. 2001). The use of vibration during freezing of foodstuffs can reduce the duration of freezing and improve the quality of frozen foods, for example, preserve the freshness at high standards after thawing (Owada et al. 2001). However, there is no evidence of the effect of this method on the texture of strawberries, although strawberries are one of the first places for growing berries on an industrial scale, and consumed fresh, frozen or freeze-dried (Simpson 2018). Therefore, the aim of this research was to evaluate the micro-vibration effect on the texture of frozen and freeze-dried strawberries.

Materials and methods

The object of research was strawberries (the “Asia” variety, Russia) collected in June 2018. The size of strawberries was 25–35 mm in diameter; the strawberries had a pronounced colour and a characteristic aroma. The strawberries were washed under running tap water, dried with paper tissue and sent to freezing.

Freezing

The strawberries were frozen in two ways. Half of the strawberries were frozen at − 30 °C under conditions of forced convection in a freezer. The second part was frozen in a lab scale freezer—ABAT-20/1-AEF—under the same conditions, but with additional effects on the strawberries of micro-vibrations created in the air environment of the freezer according to a special program. The digital frequency synthesizer of the original design, able to generate electromagnetic fields with a power of 1 to 500 W/m3 with packets of one- and two-polar rectangular pulses within the frequency range of 10 MHz–5000 kHz was used in this freezer. At the end of the freezing stage, frozen strawberries were taken and the microstructure, penetration and shear stress determined. The remaining frozen strawberries were subjected to vacuum freeze-drying.

Vacuum freeze-drying

For vacuum freeze-drying, trays with frozen strawberries were placed in a lab scale freeze dryer (Semenov and Krasnova 2018), and treated as follows. Frozen strawberries were placed on two metal trays, each freezing variant on a separate tray. The primary temperature was minus 30 + 1 °C. Secondary drying was carried out at a temperature of 38–40 °C. The total duration of the drying cycle was 16 h. The final moisture content of the freeze-dried strawberries was 1.5–1.7%. Microstructure and firmness were also studied in freeze-dried strawberries.

Research methodology is shown in Fig. 1.

Fig. 1.

Fig. 1

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Research methodology

Microstructural analysis

The following method was used for the microstructural analysis of frozen strawberries in order to determine the degree of tissue destruction under different methods of freezing (Khvylia 2016; Khvylia and Giro 2015). Pieces of 20 × 20 × 10 mm were cut from the berries without thawing and warming. The pieces were mounted on the cryostat subject tables. The samples were placed quickly in the chamber of the freezing microtome MK-525 with a set temperature of minus 20 °C to avoid thawing and changes in the microstructure of the tissues. The thickness of the sections produced was about 25 microns. The obtained sections were mounted on glass slides, placed in a room with a temperature of 18–22 °C and dried. Then, these sections were stained with Ehrlich hematoxylin and fresh aqueous-alcoholic eosin. The preparations were enclosed in glycerine–gelatine. The analysis of the microstructure of the freezing and freeze-drying strawberries was performed by light microscopy under a microscope AxioImager. A1 (Carl Zeiss, Germany) with a polarized light filter and coupled to a video capture system. The images were processed using ACDS 8 PRO.

Texture analysis

The firmness of strawberries was evaluated based on the penetration and shear stress. Frozen berries were thawed. Thawing of frozen berries was carried out by means of their natural heating at room temperature 22–23 °C. Strawberries were placed at some distance from each other on two sheets of white paper.

Full thawing of berries was achieved in 2.5–3 h. Penetration and shear stress were determined at room temperature. Strawberries were fixed on a platform and measured on the highest point of each berry. The shear stress and penetration were determined on a penetrometer PMDP, with a strain gauge at 100 H and a cylindrical cone with an apex angle of 60°. Under the action of the mass of the cone and the rod connected to it, the cone was immersed in the product to a certain depth, the value of which was fixed using an indicator.

Penetration (kPa) was determined by the formula P.A. Rebinder:

P=k×mh2

where k—cone constant depending on the angle at its vertex (k = 2.1 H/kg); m—mass of the cone with the rod (50.69 × 10–3 kg); h—depth of immersion of the cone, m.

Shear stress (kPa) was determined using the same formula, where h—is the depth at which the cone is immersed for 60 s. The texture analysis were determined on 15 berries of each sample.

Freeze-dried strawberries were subjected to rehydration in glasses of water for researching their texture. Each berry was in a separate glass. The time required for complete rehydration was 2.5–3.5 h, depending on the strawberry size. Strawberries were removed from the glasses and placed on a mesh substrate to allow excess moisture to drain from the surface layer upon completion of rehydration. Further, the penetration and shear stress were determined according to the method described above, as with frozen strawberries.

Statistical analysis

Results were processed by one-way analysis of variance (ANOVA) in Data Analysis in MS Excel (Excel 97–2003. Microsoft office). Significant difference between means were tested using Fisher Snedecor Test and Tukey’s test with a probability level fixed at p < 0.05. Differences at p < 0.05 were considered significant. The most significant differences were determined by the Scheffe Multiple comparisons tests.

Results and discussion

Analysis of the microstructure of traditionally frozen strawberries tissues showed that large ice crystal structures formed over the entire thickness of the sample. These structures were not stained with histological dyes. During the freezing of strawberries from the cytoplasmic content formed material does not perceive the used dyes (Fig. 2a, b).

Fig. 2.

Fig. 2

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Light microscopy image: a the deep layers of strawberry tissues after traditional freezing; b the surface of strawberry tissues after traditional freezing; c the deep layers of strawberry tissue after freezing with micro-vibration; d surface tissue of strawberries after freezing with micro-vibration

Partial damage of the cell membranes of both the surface layer of the strawberries and in their cells, forming the main weight of the strawberries after traditional freezing is observed (Fig. 2a, b). Traditional freezing causes fragmentation of cell membranes in the visual assessment of samples of approximately 60–70% of the total number of external cellular structures. Ice crystals formed during traditional freezing induced significant changes in the structure of strawberries, which led to severe cell damage and water loss (Delgado and Rubiolo 2005; Nowak et al. 2019). According to the literature, moisture loss is caused mainly by cell wall deterioration, which leads to the loss of the ability to act as a semipermeable membrane or diffusion barrier (Delgado and Rubiolo 2005). Firmer cells of the vascular system of the strawberries are better preserved.

The analysis of changes in the microstructure of strawberry tissues frozen with micro-vibration showed that smaller ice crystal structures form throughout the whole volume of frozen strawberries. This is observed both on the surface and in the deeper layers of strawberries. This effect is similar to the effect of quick-freezing (Biglia et al. 2016), but is achieved at higher air temperatures in the freezer.

In addition, cell membranes is damaged as well as during traditional freezing. At the same time, the degree of destructive changes during freezing with micro-vibration is significantly less compared to traditional freezing. This is approximately 25–35% of the total number of surface layer cellular structures. This is attributed by the higher freezing rate obtained by applying micro-vibration and, therefore, the domination of intracellular small ice crystals. Strawberries after freezing with micro-vibration had a thicker surface layer cells than traditional frozen strawberries and they were better protected against moisture loss. This applies both to the surface and to the deeper layers of strawberry tissue (Fig. 2c, d).

The microstructures of freeze-dried strawberries after traditional freezing and after freezing with micro-vibration are shown in Fig. 3.

Fig. 3.

Fig. 3

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Freeze-dried strawberry: a after traditional freezing; b after freezing with micro-vibration

The Fig. 3 data show that the significantly more preservation of the cell membranes in freeze-dried strawberries frozen with micro-vibration, even in its depth. Cell membranes were shaper, intact, more contrasted compared to strawberries after traditional freezing and cells contained smaller cytoplasmic cavities dehydrated during freeze-drying.

The texture analysis of strawberries after freezing and rehydrated after freeze-drying to confirm microstructural changes were determined. The results are presented in Tables 1, 2.

Table 1.

The texture analysis of strawberries after freezing

Groups Samples Sum Mean Variance
Penetration of thawed berries after traditional freezing 15 9.15 0.61 0.000214286
Penetration of thawed berries after freezing with micro-vibration 15 10.65 0.71 0.000785714
Shear stress of thawed berries after traditional freezing 15 6.15 0.41 0.027057143
Shear stress of thawed berries after traditional freezing (Shear stress of thawed berries after freezing with micro-vibration) 15 7.4 0.49 0.009766667
ANOVA
Source of variation SS df MS F p value F crit
Between groups 0.778125 3 0.259375 27.4298 4.80002E−11 2.769430
Within groups 0.529533 56 0.009455952
Total 1.307658 59
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Table 2.

The texture analysis of strawberries rehydrated after freeze-drying

Groups Samples Sum Mean Variance
Penetration of rehydrated freeze-dried strawberries after traditional freezing 15 7.35 0.49 0.001114286
Penetration of rehydrated freeze-dried strawberries after freezing with micro-vibration 15 8.18 0.545333 0.001455238
Shear stress of rehydrated freeze-dried strawberries after traditional freezing 15 6.23 0.415333 0.000626667
Shear stress of rehydrated freeze-dried strawberries after freezing with micro-vibration 15 7.1 0.47333 0.00992381
ANOVA
Source of variation SS df MS F p value F crit
Between groups 0.12886 3 0.042953 13.0955 1.36797E−06 2.769430
Within groups 0.18368 56 0.00328
Total 0.31254 59
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Raw strawberries were much more rigid and more difficult to deform than defrosted and rehydrated freeze-dried berries. The penetration was determined at 15.5 kPa and the shear stress was determined at 10.4 kPa.

In comparison with raw strawberries, the firmness of berries thawed after freezing was significantly lower. Higher values of penetration are noted in thawed strawberries after freezing with micro-vibration and they were determined at 0.71 kPa. The shear stress thawed strawberries after freezing with micro-vibration was determined at 0.48 kPa and it was higher than shear stress of thawed strawberries after traditional freezing. The values of penetration and shear stress indicates better preservation of the texture of thawed strawberries after freezing with micro-vibration. However, the difference in values is small and it determined at 10–15%. Statistical analysis showed that the null hypothesis is not confirmed. Statistical differences are determined between ways of freezing, as evidenced by the Fischer–Snedecor test (F = 27.43 ≫ Fcrit = 2.77). Additional data analysis by Scheffe's multiple comparisons tests showed that freezing with micro-vibration is the preferred way.

A similar dependence was obtained in the research of the texture of freeze-dried strawberries.

It is determined that the penetration of rehydrated freeze-dried strawberries after traditional freezing was 0.49 kPa and it was less on 0.06–0.07 kPa than rehydrated freeze-dried strawberries after freezing with micro-vibration. The shear stress of rehydrated freeze-dried strawberries after traditional freezing was determined at 0.42 kPa and it was less the shear stress of rehydrated freeze-dried strawberries after freezing with micro-vibration. The firmness of rehydrated freeze-dried strawberries, previously frozen with micro-vibration, is 8–10% higher than freeze-dried strawberries, previously frozen by the traditional method.

It should be noted, what freeze-dried strawberries were characterized by lower firmness (on 0.8–1.0%) in comparison with frozen fruits. Therefore, freeze-drying does not practically effected on the texture of the product, all changes in the texture occur during freezing, and it was confirmed by statistical analysis.

The texture analysis results confirmed the data obtained in the analysis of microstructure. Ice crystals formed during freezing damaged strawberries cell membranes and induced significant changes in texture of strawberries.

The results showed that freezing with micro-vibration in air positively affects the texture of strawberries. However, these changes are not so significant and their effect is similar to the effects of low-intensity ultrasound on fruits in water. Therefore, polish scientists researched the effect of ultrasound on raw and frozen/thawed blueberries and developed the conclusion that exposure to low-intensity ultrasound caused minor changes in the texture of the raw berries (Nowak et al. 2019).

Similar results were obtained by Olmo et al. (2008). They researched the influence of ultrasonic waves on the freezing of distilled water, and no significant influence on the freezing parameters of the droplets when ultrasound is applied at non-cavitational intensities (0.15 W/cm2).

Chandrapala et al. showed that the ultrasonic treatment process has little effect on the structure of proteins in reconstituted whey protein concentrate (Chandrapala et al. 2010).

A feature of the proposed freezing technology is the effect of acoustic waves on the tissues of the object of freezing, transmitted through the air environment, and causing micro-vibrations in the tissues. This effect is described in Abbott 1990, who reported that when an object is excited at acoustic frequencies, it vibrates. Small ice crystals are formed in frozen strawberries, providing high preservation of tissue structures (Zhang et al. 2020; Owada et al. 2001). The surface layer of the frozen object perceives vibrations, that penetrate into depth the object and decreases further away from the surface layer. The use of micro-vibration makes it possible, to significantly reduce the influence of the shape and relative position of the frozen materials on the result.

Conclusion

Our researches have shown that freezing strawberries under convective heat transfer with additional exposure to berries by micro-vibration contribute to the preservation of their cell membranes. As a result, their mechanical properties increase.

In freeze-dried strawberries, previously frozen with micro-vibration, the tissues structures of the berries are also better preserved. After vacuum freeze-drying, berries frozen with micro-vibration became less breakable compared to the freeze-dried strawberries after traditional freezing.

The results show that both frozen and freeze-dried strawberries will better retain their structure and appearance during transportation and packaging.

Researches, conducted using strawberries, allow us to make a reasonable forecast that freezing with micro-vibration of small-sized berries with a dense shell (currant, gooseberry, blueberries, etc.) will provide more significant benefits in their freeze-drying in comparison with the traditional freezing.

Footnotes

Publisher's Note

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