1. Introduction
The volume of world vegetable production from 2000 to 2021 increased by 40%. Consumption of vegetables is very important for consumers. A 2021 study found that more than a quarter of the German and Australian populations mainly consume vegetables, fruits, and whole-grain products [
1]. Vegetables are an important source of nutraceuticals in the human diet, which are naturally occurring substances in food. Nutraceuticals are important in the daily diet because they benefit the human body, improve physiological functioning, and improve overall health. Nutraceuticals are ingredients such as antioxidants, vitamins, minerals, and natural pigments (e.g., carotenoids) [
2].
Broccoli (
Brassica oleracea L. var.
italica) is a popular vegetable worldwide. Its largest producers are China and India [
3]. In the United States, broccoli consumption per capita in 2022 was 6.51 g per day, and a 2% increase over the previous year was observed [
4]. Broccoli has high nutritional value because it contains bioactive compounds (e.g., sulforaphane, glucosinolate, flavonoids (kaempferol, quercetin), carotenoids (lutein)), and nutrients (i.e., dietary fiber, vitamins (A, C, and vitamin B complex), minerals (e.g., selenium, potassium), and amino acids) [
3,
5]. Phytochemicals and nutrients in broccoli support well-being, reduce inflammation, and prevent cancer. Broccoli is characterized by antioxidant, anti-inflammatory, antimicrobial, and anticancer effects [
6]. In addition, broccoli is low in calories, so it can be a healthy, light snack for people who want to maintain a normal weight, lose weight, or just eat healthy [
7].
Courgette (
Cucurbita pepo L.) is a seasonal vegetable that is popular around the world [
8]. In Australia in 2021/2022, the annual consumption of courgette was 17.520 tons, equivalent to 680 g/person per day [
9]. Courgette is eaten raw, but due to its rapid loss of freshness, it is used for processing. Courgette contains about 93.5–95% water, so it has a very low energy value. Courgette plays an important role in a balanced diet due to its high nutrient content, including carbohydrates, protein, folic acid, fiber, vitamins, and minerals such as potassium, phosphorus, magnesium, and calcium [
8,
10].
Red beetroot (
Beta vulgaris L.) is a valuable raw material used in the food industry. It is used to produce frozen, dried, preserved, and fermented food, as well as concentrates and juices [
11]. The popularity of this vegetable is related to the content of many valuable nutrients, including vitamins (A, K, E, C, and B vitamins), minerals (potassium, sodium, magnesium, iron, zinc, boron, silicon, copper, selenium, and manganese) [
12]. Red beetroot belongs to the group of health-promoting foods due to the content of the above-mentioned nutritional compounds and bioactive ingredients, i.e., phenols, betalains, and inorganic nitrates [
13]. Betalanins, substances contained in beetroot, are used as natural dyes in food production. They are an interesting alternative to artificial dyes. Firstly, because of the possibility of changing the color of food products in a natural way, and secondly, because of the possible health benefits for humans [
14]. Based on literature data, red beetroot has strong antioxidant, anti-inflammatory, anti-stress, anti-hypertensive, anti-viral, anti-obesity, anti-cancer, and anti-bacterial properties, as well as lowering lipid levels [
15,
16]. All the health-promoting properties discussed confirm that red beetroot has great potential as a functional food ingredient. Most importantly, it is used in industry as a raw material for the extraction of a natural food additive, thanks to which it is possible to use beetroot juice to produce innovative, health-promoting food [
17].
The development of techniques to extend the shelf life of fruits and vegetables is increasingly driving the food market [
18]. Most vegetables and fruits are sourced seasonally, and, due to the high cost of storing fresh materials, they brown, lose firmness and rot quite quickly. This is why proper preservation of fruits and vegetables is so important. Some of the most important methods of food preservation are dehydration and drying. These techniques are commonly used not only to preserve food but also because of the reduction in weight for transportation and the smaller volume of the product, which is important for packaging [
2,
8]. Processed products, on the other hand, may show changes in sensory, functional, nutritional, and physicochemical properties, especially those that are more sensitive to changes in light, pH, or heat [
19].
Technological processes such as osmotic dehydration and vacuum impregnation can help produce products rich in bioactive and volatile compounds [
19,
20,
21,
22]. The processes involved in the osmotic drying of fruits and vegetables are now widely studied, as evidenced by recent research papers [
22,
23,
24,
25]. Osmosis involves the transport of water from the interior of the sample into a hypertonic solution, with the simultaneous transport of osmotic matter into the interior of the sample, resulting in the leaching of natural solutes [
26].
Vacuum impregnation is a variation of osmotic dehydration but is carried out under reduced pressure [
20]. Vegetable tissues are well suited for the application of the vacuum impregnation process because their interior is filled with pores containing fluids and air [
27]. Depending on the type and composition of the impregnation solution used to VI vegetables and fruits, different results could be obtained. Literature reports provide examples of changing physicochemical properties, protecting against browning, increasing antioxidant properties, bioactive properties, and volatile compounds, and affecting texture through the use of appropriate impregnating solutions [
21,
28].
In addition, the subsequent use of vacuum drying or freeze drying can favorably extend the storage time and shelf life of fruits and vegetables. This is due to the reduction in water content, which thus makes it possible to inhibit the growth of microorganisms and expand the range of ready-to-eat products offered [
20]. Freeze drying is considered one of the best methods for removing water from plant materials. This technique makes it possible to obtain drought of the highest nutritional and sensory quality [
29,
30,
31].
Creating healthy and practical vegetable-based products is an ongoing challenge in the food industry. The purpose of this study was to produce novel dried courgette and broccoli snacks using vacuum impregnation as a pretreatment method. In addition, organic beet juice was used as an impregnating substance aimed at improving the properties of the vegetables. Then two drying methods (vacuum drying (VD) and freeze drying (FD)) were compared and their effects on physical and chemical properties, i.e., dry weight, water activity, gelation index, shrinkage, density, color, and VOCs.
2. Materials and Methods
2.1. Reagents and Standards
All reagents and organic solvents were of analytical grade and mass spectrometry purity. Undecan-2-one and cyclohexane were obtained from Sigma-Aldrich (Steinheim, Germany).
2.2. Materials
The study material consisted of broccoli and courgette purchased from the local market (commercial products from Lower Silesia, Poland). Before the study, the raw materials were stored in an RL58GRGIH refrigerator (Samsung Electronics Poland, Wronki, Poland) at a constant temperature of 4 ± 2 °C. The vegetables were washed and dried. Broccoli was divided into florets of equal size (approximately 2 cm high and 1.5 cm in diameter), and courgette was cut into slices 5 mm ± 0.1 mm thick, and then subjected to the planned tests. The research used organic, cold-pressed red beet juice. Nutritional value in 100 mL of juice was as follows: Energy value: 155 kJ/37 kcal, fat: <0.5 g, including saturated acids: <0.1 g, carbohydrates: 0.8 g, including sugars: 8.0 g, fiber: <0.5 g, protein: <0.5 g, salt: <0.13 g, potassium: 280 mg (Haus Rabenhorst, Unkel, Germany). Explanations of code designations are given in
Table 1, while visualization of all tests is shown in
Figure 1 and
Figure 2.
2.3. Vacuum Impregnation (VI)
Vacuum impregnation was performed in a prototype plant located at the Institute of Agricultural Engineering, Wroclaw University of Life Sciences (Wroclaw, Poland) [
20]. The vacuum (VI) impregnation process was carried out at a reduced pressure of 6 kPa and lasted 21 min. A perforated stainless steel vessel with 100 g samples was placed in the impregnation chamber and subjected to reduced pressure for 2 min. Then 1000 mL of impregnating solution was added, and the material was kept in the chamber for 4 min. After restoring atmospheric pressure, the material was kept in the chamber for 15 min. The material was drained using filter paper. As an impregnating solution, organic, cold-pressed red beet juice was used at a concentration of 10.1° Brix (Haus Rabenhorst, Unkel, Germany).
2.4. Drying Methods
Two drying methods (vacuum drying (VD) and freeze drying (FD)) were used. For each version of the study, 100 g samples of fresh and vacuum-impregnated broccoli and courgette were used.
2.4.1. Vacuum Drying (VD)
Vacuum drying was carried out at 60 °C, under a vacuum of 10 kPa, in a vacuum laboratory dryer (Memmert, VO101, Schwabach, Germany). The drying time was 24 h.
2.4.2. Freeze Drying (FD)
Freeze drying was carried out at a hotplate temperature of 22 °C under a reduced pressure of 5 Pa, for 24 h in a 4.5 l Free-Zone system (Labconco, Fort Scott, KS, USA). Samples were frozen at −20 °C at a rate of 1 °C/min. A contact method was used to deliver heat to the dried material.
2.5. VOCs Extraction and Analysis
VOCs that were released from the samples to the atmosphere headspace solid-phase microextraction Arrow (HS-SPME Arrow) were used. As sorbent, 1.10 mm DVB/C-WR/PDMS SPME Arrow fiber (Shimadzu, Kyoto, Japan) was chosen. Briefly, 0.5 g ± 0.005 of fresh samples or 0.25 g ± 0.005 of dried samples was weighed. Before the extraction, 0.5 µg of undecane-2-one (Sigma-Aldrich, Steinheim, Germany) was used as an internal standard. The extraction was performed in 20 mL headspace vials, which were pre-conditioned at 45 °C for 5 min; then the VOCs were extracted for 30 min at the same temperature, while the sample was shaken at 250 rpm. Thereafter, the analytes were thermally desorbed for 3 min at the injection port temperature. The analyses were run in triplicate.
The separation, identification, and quantification of analytes were performed on the Shimadzu QP 2020 Plus apparatus (Shimadzu, Kyoto, Japan) equipped with a ZB-5Msi (Phenomenex, Torrance, CA, USA) column (30 m × 0.25 mm × 0.25 µm). Injector conditions: Temperature 220 °C; helium as carrier gas with linear velocity 35.0 cm·s−1; split 10. The analyte separation was carried out with the following temperature program: 40 °C, then 130 °C at a rate of 3 °C·min−1; then 280 °C at a rate of 20 °C·min−1, held for 5.5 min. The MS mode was scanned (40–400 m/z) with an ion source temperature of 250 °C and an interface temperature of 250 °C.
The analyte identification was based on the comparison of linear retention indices (LRIs) and mass spectra. As a reference database, the Flavours and Fragrances of Natural and Synthetic Compounds 3.0 (FFNSC 3.0) library was used. The experimental LRIs were calculated on the basis of simultaneous analysis of the n-alkanes c 7-c 40 mixture (Sigma-Aldrich, Steinheim, Germany). The identification limits were as follows: LRIs ±15; mass spectra similarity ≥90%. Area normalization with an internal standard peak area was used as the quantification method.
2.6. Physical Properties
2.6.1. Dry Matter (DM)
Broccoli and courgette samples weighing 0.5 g were measured using an electronic balance (AS160/C/2, Radwag, Radom, Poland; accuracy of measurement: ±0.0001 g). The fresh and dried samples prepared in this way were dried at 70 °C at 3 kPa for 24 h. A vacuum dryer V0101 (Memmert, Schwabach, Germany) was used for the tests [
32].
2.6.2. Water Activity (AW)
An AquaLab 4TE ± 0.003 apparatus (AquaLab, Warsaw, Poland) was used to measure water activity. The tests were performed at a constant temperature (25 °C) according to the manufacturer’s instructions. The result is the average of three measurements.
2.6.3. Bulk Density (ρb)
Bulk density was performed using a measuring cylinder filled with material [
33]. The measurement was made in triplicate and calculated using the formula:
gdzie:
ρb—bulk density [kg · m−3],
ws—weight of samples [kg],
V—volume [m3].
2.6.4. Volumetric Gel Index (VGI)
Volumetric gel index (
VGI) was determined according to the method developed by Kim et al. [
34] with our own modifications. Two malletiers of crushed sample was suspended in 20 mL of distilled water at 20 °C in a measuring cylinder and stirred over a 2 min period. The solution was allowed to swell for 15 min, and then the gel volume was noted. The volumetric gel index of the samples was calculated from the formula:
gdzie:
2.6.5. Shrinkage (S)
Drying shrinkage (
S) was determined as the ratio of the volume of material after drying (
Vk) to the volume of material before drying (
V0) according to the given formula [
35]:
2.6.6. Color
The Color Minolta Chroma Meter CR-200 colorimeter (Minolta Corp., Osaka, Japan) was used to measure the color. A color measurement was performed using a colorimeter with a measuring aperture of 0.008 m in diameter. The CIE-Lab scale was used to evaluate L* for brightness, a* for (+) redness/(−) greenness, and b* for (+) yellowing/(−) blueness, respectively. A D65 light source and a standard colorimetric observer with a field of view of 10° were used. The result is the average of 10 measurements.
The total color change (∆
E) between dried and fresh vegetables was calculated according to the equation [
36]:
where:
L*control, a*control, and b*control—color of fresh materials,
L*sample, a*sample, and b*sample—color of dried samples.
The color saturation (
C) was calculated according to the equation [
37]:
The browning index (
BI) was calculated based on the equations (Equations (6) and (7)) provided by Dziki et al. [
37].
where:
2.7. Statistical Analysis
Statistical analyses, multiple regression equations, determination, and correlation coefficients were performed using Statistica version 13.1 (StatSoft, Tulsa, OK, USA). The results were presented as the mean ± standard deviation. Bidirectional analysis of variance (ANOVA) was performed in this study. HSD Tukey’s least significance test (p < 0.05) was used to determine homogenous groups. For VOCs before the creation of the heatmaps, the numerical data were standardized by software algorithms. The results were expressed as mean values (n = 3 ± 10) ± standard deviation.
4. Conclusions
During the study, it was observed that both the addition of beet juice, the vacuum impregnation process, and the use of different drying methods had a significant effect on the physicochemical properties.
The results confirmed that the use of vacuum impregnation as a pretreatment method before the drying process is an effective method for improving the properties of the tested vegetables. The use of VI resulted in lower drying shrinkage, VGI, lower water activity, and dry matter. In addition, the use of beet juice during VI directly influenced the saturation of volatile organic compounds, especially evident in the case of courgette, which was characterized by a softer texture, making it possible to introduce more impregnating substances into its pores.
The results confirmed that dried fruit obtained by the freeze method was characterized by higher water activity, density, VGI, and better color and shape retention compared to the vacuum method. Therefore, this method is recommended for the production of health-promoting foods.
The obtained results of selected physicochemical properties, i.e., color, water activity, shrinkage, density, VOCs, dry weight, and VGI, are important in the context of producing new functional foods. Broccoli and courgette are low-calorie products, so they can successfully provide a healthy and light snack, even for people with a healthy lifestyle. The use of organic beet juice as an impregnant allowed the introduction and identification of 56 VOCs, including 2-(E)-hexen-1-ol, 2-(Z)-hexen-1-ol, or acetophenone. This research has successfully provided valuable information on the first such courgette and broccoli snacks containing beet juice. However, further research is needed that takes into account the content of bioactive ingredients.