Pulsed vacuum impregnated trehalose to improve the physicochemical quality of frozen-thawed kiwifruit Free

Abstract

Pulsed vacuum impregnated trehalose to improve the physicochemical quality of frozen-thawed kiwifruit was investigated. Kiwifruits were treated by pulsed vacuum impregnation (PVI) with 10%, 20% and 30% trehalose, then frozen and stored at −20 °C. The results showed that PVI with 30% trehalose treatment exhibited minimum drip loss (11.21%), maximum ascorbic acid content (0.93 g kg⁻¹) and firmness (12.3 N), and retained the flavour and taste as well as improved water distribution for frozen-thawed kiwifruit as compared with other treatments: control, PVI with 10% and 20% of trehalose. Therefore, the results illustrated that pulsed vacuum impregnated trehalose could be used as an effective method to improve the quality of frozen-thawed kiwifruit.

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Introduction

Kiwifruit is widely cultivated worldwide and its production is on the rise (Benítez et al., 2013). Kiwifruit loved by consumers owing to its high antioxidant capacity, ascorbic acid content and dietary fibre (Tavarini et al., 2008). However, kiwifruit, as climacteric fruit, is prone to softening and has limited postharvest life at the storage period because of the ethylene production and susceptibility to microbiological infection (Pranamornkith et al., 2012; Burdon et al., 2017). Freezing is considered as a simple and fast preservation method that can prolong the storage life and keep food quality in food processing (Kiani et al., 2011). But freezing may change tissue structure and quality characteristics, such as the deterioration of flavour and texture, resulting in the softening of fruits and vegetables’ tissue (Kong et al., 2016). During the freezing process, ice crystals gradually become larger with the prolongation of time through recrystallisation (Xin et al., 2014). Large ice crystals in frozen food may cause the damage of cell membrane, resulting in irreversible turgor loss that leads to high drip loss, nutrient loss and odour formation after thawing (Chassagne-Berces et al., 2009). Therefore, cryoprotection of frozen food is critical for improving the physicochemical characteristics of frozen-thawed food.

Cryoprotectants, such as sugar, chitin and chitosan, are used for inhibiting ice recrystallisation, thereby presenting ice crystals from growing large causing mechanical damage in products tissue (Zhang et al., 2019). Among cryoprotectants, sugars have stabilising effects on stress tolerance of organisms. Sugars due to their hydrophilic properties are known as cryoprotectants that are widely applied to stabilise the cell structure with hydrogen bonds (Velickova et al., 2013). Sugars such as sucrose and trehalose are commonly used to present dehydration effects in membranes that can be applied for improving the quality of frozen food (Phoon et al., 2008). Especially, it is noted that the trehalose has only 45% sweetness compared with sucrose and has exhibited a superior effect on the conservation of nutritional components such as polyphenols and ascorbic acids of frozen food products (Xin et al., 2014; Song et al., 2017). Additionally, the current price of trehalose was about 1.5 dollars kg⁻¹, which allowed for widespread use in expense sensitivity (Kopjar et al., 2008). Thus, trehalose is given priority as cryoprotectant in this study.

Pulsed vacuum impregnation (PVI) is applied to promote impregnation solution into plant tissues, which helps change the structure and composition of frozen products. PVI is known as a quick and effective method in food industry that is composed of sub-atmospheric pressure stage for shorter period first, then immersed in the cryoprotectant at atmosphere pressure for a period of time (Cheng et al., 2014). PVI is used to boost cryoprotectant infusion into plant tissues owing to hydrodynamic mechanism through pressure changes (Fito et al., 1996). Thus, PVI may alter the composition and physicochemical properties of the products, which is helpful for achieving desired food characteristics to some extent containing the improvement of texture, taste, flavour and so on (Igual et al., 2008; Song et al., 2017). Hence, this is necessary to add PVI with trehalose to minimise the damage to frozen tissue. It is currently reported that the important remains lacking on improving the physicochemical quality of frozen-thawed kiwifruit through PVI with trehalose. In view of such a purpose, this research was conducted to investigate influence of pulsed vacuum impregnated trehalose on the physicochemical quality of frozen-thawed kiwifruit. The physicochemical quality including drip loss, ascorbic acid content, firmness, flavour, taste, water distribution of kiwifruit after freezing and thawing were analysed.

Materials and methods

Raw materials

Kiwifruits (Actinidia deliciosa) were bought from local fruit market (Jingzhou, China). Kiwifruits (commercial maturity) were picked with uniform shape and size, without defects and 6.7 ± 0.2% of soluble solids content as well as 14.3 ± 0.4 N of firmness. Kiwifruits were placed into the refrigerator at 4 °C for 12 h until their use. Kiwifruits were washed and drained, then hand-peeled and manually cut into the slices of 1 cm. Three batches kiwifruit (1.2 kg) were used to conduct triplicate experiments independently. Kiwifruit slices with 400 g of mass were obtained for each batch samples and equally divided into four portions. Kiwifruit slices with 100 g of mass were used for each treatment. These kiwifruit samples were applied for the next experiments.

Pulsed vacuum impregnation

Kiwifruit slices (100 g) were placed into 1 L of beaker filled with 400 mL of trehalose (99% purity, Hyashibara Co., Okayama, Japan) solution with different concentrations (10%, 20%, 30%) based on the study of Song et al. (2017) at one cycle of vacuum and room temperature. The beaker was put into the vacuum drying chamber. PVI treatment according to the method of Igual et al. (2008) at the vacuum pressure of 50 mbar for 5 min, then the vacuum pressure was restored to atmospheric pressure and kiwifruit slices were remained in trehalose solution for 5 min. After PVI treatment, samples were removed from trehalose solution, and then blotted with adsorbent paper to remove the excess solution of samples’ surface. Kiwifruit slices without PVI treatment were used as the control. The control and PVI treatment with 10%, 20% and 30% of trehalose solutions samples were put into polyethylene (PE) packaging bag, respectively.

Freezing and thawing

The samples in PE packaging bag (100 g of kiwifruit slices in each packaging bag) were frozen in a deep freezer (BCD-186KB, Haier Co., Qingdao, China) at −20 °C for 1 week as described by Kong et al. (2016). Subsequently, samples were taken from the deep freezer and thawed in a refrigerator of 4 °C overnight. Frozen-thawed kiwifruit slices for each treatment were used for the analyses of physiochemical properties. All measurements of physicochemical properties were repeated three times.

Measurements of physicochemical properties

Drip loss

Drip loss in thawing process of kiwifruit was measured according to Tu et al. (2015). The below formula was applied to compute drip loss:

where M₀ is the mass of kiwifruit before freezing and M is the mass of kiwifruit after thawing.

Ascorbic acid content

The measurement of ascorbic acid content was conducted according to Harrison & Were (2007). Ten grams of samples and 10 mL 20 g L⁻¹ metaphosphoric acid were mixed and ground together. The volume of mixture was increased to 100 mL using 20 g L⁻¹ oxalic acid. The mixture was filtered with 0.45 mm diameter of filter paper. Ten millilitre of the filtrate was tested through 2,6-dichloroindophenol titration. When pink colour appeared and did not fade within 15 s, the titration was completed.

Firmness

The measurement of firmness was performed according to Wang et al. (2012). Tests using texture instrument (TA-XT plus, Stable Micro Systems Co., Surrey, UK) were conducted with a 2 mm diameter of cylindrical probe. Each kiwifruit slice was measured three times, thus triplicate experiments were performed resulting in nine measurements. The probe at pre-test speed of 1.0 mm s⁻¹, test speed of 0.5 mm s⁻¹ and post-test speed of 1.0 mm s⁻¹ penetrated into samples for 1 mm of depth, and a maximum force (N) was showed that was recorded as firmness.

Flavour

The measurement of flavour of frozen-thawed kiwifruit was conducted through an e-nose analyzer (Ruifen Trading Co., Shanghai, China) according to Chen et al. (2018). The electronic nose system included 14 metal oxide sensors (as seen from Table 1). Five grams of frozen-thawed samples were placed in glass bottle, then equilibrated for 0.5 h at 25 °C. Reference air was utilised to wash gas path for 0.5 h before the e-nose detection. The e-nose parameters were set as 1 L min⁻¹ flow rate, 150 s detection time, and 300 s recovery time.

Principal component analysis

Principal component analysis (PCA) was performed by SPSS software to analyse responding values of e-nose, and distinguished whether there were differences in volatile patterns from different treated samples. PCA as a dimension reduction method retained the most information on original data, and each observation was assigned to a specific group (Chen et al., 2018).

Taste

The measurement of taste of frozen-thawed kiwifruit was conducted through an e-tongue analyzer (Intelligent Sensor Co., Kanagawa, Japan) according to Fan et al. (2020). E-tongue system included eight sensors: sourness, bitterness, astringency, aftertaste-bitterness (aftertaste-B), aftertaste-astringency (aftertaste-A), umami, richness and saltiness. Fifty grams of kiwifruit samples and 100 mL of distilled water were mixed and ground, then filtered with 0.45 mm diameter of filter paper. The filtrate was transferred to special cup of the e-tongue for test experiment.

Water distribution

The measurement of water distribution of frozen-thawed kiwifruit was conducted through a LF-NMR analyzer (Niumag Electric Instruments Co., Suzhou, China) according to Xin et al. (2013). Two grams of fan-shaped kiwifruit samples after thawing were put in a 10 mm diameter of glass tube, then inserted in the magnet chamber of 32 °C. The samples were used to create proton density images through a magnetic resonance imaging (MRI).

Statistical analysis

Experimental results were analysed through analysis of variance (ANOVA). Duncan’s multiple range test in the IBM SPSS statistics 19 software (Chicago, IL, USA) was applied to assess significant difference at 95% confidence level.

Results and discussion

Drip loss

Drip loss can be a problem in the frozen food industry. The water in the food converts into ice in the freezing process, generating physical pressure on food matrix. During thawing process, the water in the food easily leaks out through the matrix, resulting in a large amount of drip loss (Kong et al., 2017). Figure 1 presents that effect of pulsed vacuum impregnated trehalose on drip loss of frozen-thawed kiwifruit. Drip loss of kiwifruit subjected to PVI with different concentrations of trehalose decreased significantly (P < 0.05) compared with the control. Similar results were seen for spinach leaves (Dymek et al., 2015). Results pointed out that trehalose in the tissues helped reduce the damage of ice crystals to decrease drip loss. PVI with 30% trehalose-treated samples exhibited lower drip loss compared with other treated sample, indicating that PVI with 30% trehalose improved water-holding capability of samples in thawing. The reason was that 30% trehalose as cryoprotectant through PVI treatment had high content inside the cell of samples, resulting in high water-holding capability, thereby reducing drip loss during thawing (Ramallo & Mascheroni, 2010).

Ascorbic acid content

Ascorbic acid content is used as a major indicator that may reflect the changes of quality of vegetables and fruits. Figure 1 shows that effect of pulsed vacuum impregnated trehalose on ascorbic acid content of frozen-thawed kiwifruit. Ascorbic acid content of kiwifruit after different concentrations of trehalose impregnation was significantly (P < 0.05) higher than that of control. The results were similar with strawberries as reported by Oszmiański et al. (2009). The reason was that the increase of trehalose as cryoprotectant in the cell through PVI treatment reduced drip loss leading to the decrease in ascorbic acid content of samples, thereby preserving a higher retention of ascorbic acid content. Especially, PVI with 30% trehalose-treated samples had a higher ascorbic acid content in comparison with other treated samples. This may due to higher trehalose concentration in samples to reduce drip loss, thus decreasing ascorbic acid leakage with the water (Fan et al., 2020).

Firmness

Firmness is a key quality attribute in view of consumer's preferences. During the freezing process, the formation of ice crystals in the tissues easily cause cell lysis, resulting in obvious tissue softening after thawing (Alonso et al., 1994). Figure 2 shows that effect of pulsed vacuum impregnated trehalose on firmness of frozen-thawed kiwifruit. Firmness of samples after trehalose impregnation gradually increased compared with the control with increasing the concentration of trehalose. Similar results were also observed for cherries (Kong et al., 2017). Firmness of samples through PVI with 30% trehalose had reduced compared with fresh kiwifruit samples. Nevertheless, firmness of PVI with 30% trehalose-treated samples was significantly (P < 0.05) higher than those of other samples, indicating that PVI with trehalose treatment in a suitable condition helped improve the firmness of samples. This was because the interaction of the trehalose with water minimized ice crystal to the damage of cell structure as well as raised water-holding capability (Song et al., 2017), thereby improving the firmness of kiwifruits.

Flavour

Ice crystal formation induces the damage of cell structure, which causes the loss of volatile compounds in the freezing-thawing process (Zhao et al., 2016). Figure 3a shows that effect of pulsed vacuum impregnated trehalose on flavour of frozen-thawed kiwifruit using electronic nose. It can be seen that sensors S1 and S5 of samples exhibited higher responding values compared with other sensors. As seen from Table 1, sensors S1 and S5, respectively, represent alcohols, ketones, aldehydes and aromatic compounds. The changes in responding values of sensors S1 and S5 illustrated that frozen-thawed kiwifruit samples underwent the changes in flavour including alcohols, ketones, aldehydes and aromatic compounds. The responding values of sensors S1 and S5 of PVI with different concentrations of trehalose-treated samples were lower compared with control, indicating that PVI with trehalose treatment helped conserve the flavour of frozen-thawed samples. PVI with 30% trehalose-treated samples exhibited lower responding values in view of sensors S1 and S5 as compared with other samples. The results were in accordance with the volatile profile of kiwifruit as reported by Talens et al. (2003).

Principal component analysis was utilised to discriminate the differences in responding values from electronic nose between various treated samples. Figure 3b shows that principal components 1 and 2 were, respectively, 86.52% and 7.79% of total sensor response variance. Principal components 1 and 2 accounted for 94.31% of cumulative contribution rate, illustrating that the differences of main information between different samples may be discriminated. Flavour characteristics of PVI with trehalose-treated samples kept away from control, indicating that PVI with trehalose-treated samples had the loss of flavour as compared with control. Hence, the use of electronic nose may distinguish different treated samples, including control, 10%, 20% and 30% trehalose.

Taste

Figure 4 shows that effect of pulsed vacuum impregnated trehalose on taste of frozen-thawed kiwifruit using electronic tongue. It can be seen that the responding values of the sourness of samples presented an obvious change as compared with other taste components. The sourness of PVI with different concentrations of trehalose-treated samples was obviously lower than that of control. This may be because trehalose molecules through PVI treatment infiltrated into the cells of samples causing a decrease in sourness. The sourness of PVI with 30% trehalose-treated samples was lower than that of other treated samples. Additionally, the responding values of bitterness, astringency, aftertaste-A, aftertaste-B, umami, richness and saltiness of samples had no obvious difference. The results demonstrated that PVI with trehalose treatment helped improve the taste of samples. The decrease of sourness was in agreement with the results of cranberry as reported by Nowacka et al. (2018), who also found that sucrose during osmotic dehydration effectively reduced the sourness of cranberry.

Water distribution

In order to observe the change of water distribution in kiwifruit samples, MRI was used to present signal intensity of water in various samples. The different colours presented in the pseudo-colour image represent different signal intensity of water in the sample, and the redder in the image indicates the higher the water proton density in a given region (Shen et al., 2019). Therefore, the colour distribution of pseudo-colour image can directly reflect water distribution of kiwifruit samples. Figure 5 shows that water distribution of PVI with trehalose-treated samples exhibited a stronger signal intensity compared with control. This demonstrated that PVI treatment promoted trehalose infiltration into the samples, thereby raising the interaction of trehalose and water to improve water-holding capability of samples. PVI with 30% trehalose-treated samples exhibited redder colour distribution in pseudo-colour image as compared with other samples, indicating that PVI with 30% trehalose-treated samples had higher water proton density.

Conclusion

The application of PVI with trehalose was effective to improve the physicochemical quality of frozen-thawed kiwifruit. PVI with 30% trehalose exhibited a superior effect than other treatments. Such a treatment effectively reduced drip loss, maintained ascorbic acid content and firmness, preserved the flavour and improved the taste and water distribution of frozen-thawed kiwifruit. Hence, these results demonstrated that PVI with trehalose treatment was applied to improve the quality of frozen-thawed kiwifruit.

Acknowledgments

The authors acknowledge the Life Science Instrumentation Center of Yangtze University for the use of analytical equipment.

Conflict of interest

The authors have no conflict of interest.

Author contributions

Li-Bing Chen: Investigation (equal); Methodology (equal); Writing-original draft (equal). Kai Fan: Investigation (equal); Methodology (equal); Writing-review & editing (equal).

Ethical guidance

Ethics approval was not required for this research.

Peer review

The peer review history for this article is available at https://publons.com/publon/10.1111/ijfs.15399.

Data availability statement

Research data are not shared.

References