Analysis of the Effect of Vegetable Broth Addition to a Gelatin Pork Edible Film and Coating Method on Select Physical Properties of Freeze-Dried Coated Vegetable Bars

Featured Application

The developed edible coatings and films can be used as an alternative to plastic packaging and additionally increase the attractiveness of the product. The use of an appropriate coating composition obtains high-quality freeze-dried vegetable bars.

Abstract

The aim of this study was to analyze the selected physical properties of vegetable bars coated with a coating based on pork gelatin (8% or 12%) with (25% or 50%) or without vegetable broth. The scope of work included developing the composition of edible coatings; preparing bars and coating freeze-dried vegetable bars; analyzing the water activity, dry matter content, the structure of vegetable bars based on microscopic analysis, and porosity; and then conducting a comparative analysis of the obtained results. The analyses show that the composition of the coating and the coating method influence the structure and selected physical properties of freeze-dried bars. Coating freeze-dried vegetable bars increases their water activity to the range of 0.215–0.389, and reduces their dry matter content to 93–96%. The porosity of the samples decreased (85–91%), but the use of coatings in the form of an edible film was more beneficial than immersing the dried material in a film-forming solution. The addition of vegetable broth to edible films improves the physical properties of the dried fruit and may additionally increase the taste of vegetable bars.

1. Introduction

Edible films and coatings are an emerging strategy for optimizing food quality. Their usefulness is based on their ability to maintain quality and extend the shelf life of products, and improve the economic efficiency of packaging materials [1]. The most common use of edible coatings occurs in the group of fruits and vegetables in fresh or preserved form [2]. In recent years, the importance of food coating technology has been growing, generating increased interest in the topic from both food producers and consumers [3]. However, research is still necessary to develop effective methods of producing food films and their potential applications. The pro-ecological approach, easy availability, and low price of raw materials provide additional motivation for scientists and producers to work on improving edible packaging based on proteins and polysaccharides [4]. Any type of material used for packaging food to extend its shelf life, whilst being suitable for consumption with food itself, is called an edible film or coating [5]. An edible coating is a thin layer of a food substance applied in the form of a liquid directly to the surface of various food products, mainly fruits and vegetables [6]. Edible film, on the other hand, is a sheet that is formed outside the product and used as packaging for a food product [7].

The choice of an appropriate coating method not only affects the preservative effect of the coating formed on food products, but also the production cost and process efficiency. Methods of depositing coatings on food products depend on the type of food to be coated, the surface properties, and the main purpose of using the coating. Procedures for applying an edible coating solution to food products are usually preceded by an adhesion process, which involves diffusion between the coating solution and the surface of the food product [8]. One of the most frequently used methods of applying edible coatings is dipping, which involves immersing the food product in a coating solution, then removing it and draining the excess solution covering the product. This process is widely used for fresh products [9]. The most commonly used laboratory technique for producing films is the casting technique, which involves pouring a slurry onto small plates, controlling the average thickness of the resulting films by measuring the desired amount of slurry deposited on the plate. This process is not suitable for forming foils larger than 20–30 cm due to difficult thickness control [10].

The use of edible coatings can significantly reduce the course of biochemical and physiological processes that have a strong impact on the commercial quality and consumption of food. The use of edible coatings to cover food extends their durability while maintaining quality [3]. Ogonek and Lenart [11] showed that coating frozen strawberries with a 4% pectin solution and a pectin–starch mixture resulted in reduced weight and water loss and increased dry matter weight gain. Film-forming materials include proteins, polysaccharides, and fats, as well as their two- or multi-component combinations [3]. Gelatin is a denatured form of collagen (occurring naturally in the skin, tendons, cartilage tissues, and bones of mammals, as well as in marine organisms and poultry) obtained by partial hydrolysis of this protein [12]. It is a tasteless, transparent, colorless, or pale yellow thickening substance [13]. In addition to basic physicochemical properties such as composition, solubility, transparency, color, taste, and smell, the main attributes that best define the overall commercial quality of gelatin are gel strength and thermal stability (gelation and melting temperature) [14]. Thanks to features such as foaming, stabilizing, thickening, gelling, emulsifying, and binding, it is widely used in the production of food, cosmetic, and pharmaceutical products [12]. The measure of gelation strength is the Bloom value. The Bloom values of various types of commercial gelatin available are in the range of 50–300 Bloom. High-Bloom gelatin typically has higher melting and gelation points, a shorter gelation time in the final product, and a lighter color and more neutral odor and taste. Stronger gel strength also means that less gelatin is needed to obtain the desired gel hardness in the finished product [15]. Gelatin is currently the most commonly used hydrocolloid in the world. Forecasts indicate that by 2025, the value of the gelatin market will exceed USD 5 billion, reaching a projected USD 6.7 billion by 2027 [16]. In the food industry, gelatin is used to formulate food as a gelling agent, stabilizing agent, and/or emulsifier, and to create packaging films and coatings to preserve food and extend its shelf life [17]. Plasticizers and additives are also added to coatings. Plasticizers are non-volatile compounds with low molecular weight, added directly to mixtures, forming coatings. This type of substance includes, for example, glycerol, sorbitol, and sucrose. Adding a plasticizer results in more flexible and stretchable coatings, which reduces coating cracking and shrinkage during handling and storage. In addition to their barrier properties against moisture and gases, food coatings can act as carriers of antimicrobial substances. Additionally, coatings may contain antioxidants, flavors, dyes, enzymes, and nutrients [3]. Such an additive may be bullion, which affects the properties of coatings and additionally increases the attractiveness of the product [18]. Coatings obtained based on beef gelatin with the addition of beef broth are characterized by good extensibility and lower solubility in water. The mere presence of broth in gelatin films changes the microstructure of the films and also causes them to become thinner.

Properly designed and produced food plays a key role in the everyday lives of consumers. The use of edible coatings based on gelatin with the addition of vegetable broth for packaging freeze-dried vegetable bars may be an innovative solution that increases the attractiveness of freeze-dried products because one of the interesting aspects in food innovation is the development of products that not only provide valuable nutrients, but also offer innovative and attractive forms of consumption. In this context, freeze-dried vegetable bars coated with edible coatings represent an excellent research area that combines aspects of healthy eating. Examining the influence of the addition of vegetable broth and the method of applying the coating on freeze-dried food will provide valuable information on the possibilities of packaging freeze-dried food in edible coatings. The aim of this study was to analyze select physical properties of vegetable bars coated with a coating based on pork gelatin with or without the addition of vegetable broth. The scope of work included developing the composition of edible coatings based on pork gelatin with or without vegetable broth and coating freeze-dried vegetable bars by dipping or coating them with edible foil. Then, selected physical properties of the coated vegetable bars were analyzed, based on water activity and dry substance content, porosity, and microscopic analysis.

2. Materials and Methods

2.1. Research Material

The research material included freeze-dried vegetable bars covered with pork-gelatin-based coatings with or without the addition of vegetable broth. The control sample consisted of vegetable bars not coated with any coatings. The bars were prepared based on a recipe developed by Ciurzyńska et al. [19] (

Table 1. Composition of vegetable bars.

Composition of Vegetable Bars	Percentage [%]
Water	58.4
Carrot	39.6
Sodium Alginate	1.5
Salt	0.4
Calcium Lactate	0.1

).

The compositions and types of coatings and edible films obtained based on pork gelatin with or without vegetable broth Culineo, (PRYMAT sp. z o.o., Jastrzębie Zdrój, Poland) are presented in

Table 2. The composition of coatings and edible films.

Rodzaj Produktu	Broth Percentage Solution by Volume (%)	Gelatin Percentage by Mass of the Solution (%)	Glycerol Mass Percentage of the Solution (%)
Coating 1	-	8	4
Coating 2	25	8	4
Coating 3	50	8	4
Coating 4	-	12	6
Coating 5	25	12	6
Edible film 1	-	8	4
Edible film 2	-	12	6
Edible film 3	25	12	6

. An attempt was made to obtain edible film with 8% gelatin and the addition of 25% and 50% broth, but it was impossible to create a uniform layer to cover the vegetable bars.

2.2. Preparation of Freeze-Dried Vegetable Bars

Carrots purchased in a hypermarket were cut into cubes and blanched for 1 min in boiling water, then washed with cold water. After this treatment, the raw material was ground with the addition of salt using a BOSCH MSM817180 blender (BSH Sprzęt Gospodarstwa Domowego Sp.zo.o., Warsaw, Poland) to a uniform mass. Then, a structure-forming substance was added—sodium alginate (Agnex, Białystok, Poland) and calcium lactate (Agnex, Białystok, Poland). The calcium lactate had previously been dissolved in a small amount of water. The gel obtained in this way was immediately poured into silicone molds with dimensions of 14 × 10 × 2.5 cm from Tescoma (TESCOMA POLSKA Sp. z o. o., Katowice, Poland). The semi-finished product thus created was cooled to room temperature and then frozen in an Irinox freezer (Irinox S.p.A., Treviso, Italy) at a temperature of approximately −40 °C. After this process, the vegetable gel was placed on the shelves of a Christ ALPHA 1-4 LSC plus freeze dryer (Martin Christ GmbH, Osterode am Harz, Germany). During freeze-drying (72 h), the pressure in the freeze dryer chamber was maintained at 63 Pa and the shelf temperature was 30 °C. A diagram of the process of obtaining freeze-dried vegetable bars is shown in Figure 1.

2.3. Preparation of Edible Coatings and Edible Films

When preparing coatings, the methodology described by Ciurzyńska et al. [18] was modified. Solutions intended for the production of edible coatings and films were obtained by dissolving pork gelatin with a Bloom index of 270 (Gelita AG, Eberbach, Germany) in water. The general scheme for producing foils and coatings is shown in Figure 2. The process began with precisely measuring the appropriate amount of pure gelatin (Table 2), which was poured into beakers. The beakers were filled with water to 800 mL. In the case of the coatings and foils to which vegetable broth was added, it was added together with water, where it was replaced by broth in amounts of 25% and 50%. Using RTC basic magnetic stirrers (IKA Poland Sp. Z o.o., Warsaw, Poland), which rotated at a speed of 400 rpm, the solutions were heated to a temperature of 60 °C and kept there for 30 min, stirring constantly. Glycerol (Avantor Performance Material Poland S.A., Gliwice, Poland) was then added to each beaker in an amount equivalent to half the gelatin concentration; the solutions were then cooled to 50 °C and mixed again for 30 min at constant speed with a rotating magnetic stirrer. Part of the film-forming solution was intended to form edible films. For this purpose, the film-forming solution was poured onto aluminum trays in a volume of 100 mL and dried at 30 °C for 24 h in the universal dryer using an SUP-65 WG electronic temperature controller (WAMED, Warsaw, Poland). After this time, films were removed from the dryer.

2.4. Coating Methods

Some of the freeze-dried bars were immersed in the film-forming solution and placed in an SUP-65 WG universal dryer (WAMED, Warsaw, Poland). After this period, the coated bars were ready for analysis. The rest of the freeze-dried products were wrapped in formed edible films, the edges of which were sealed using an FG.400HC laboratory sealer (FEIFER - kovovýroba, spol. s r.o., Holice, Czech Republic).

2.5. Analytical Methods

2.5.1. Water Activity

The determination was performed using the Rotronic Hydrolab C1 device (Bassersdorf, Switzerland), in accordance with the recommendations contained in the manufacturer’s instructions. For each type of coated vegetable bars, 4 measurement repetitions were performed, and the time of one determination was approximately 5 min.

2.5.2. Dry Matter Content

Approximately 2 g of the coated vegetable bars was weighed into weighing vessels of known mass and then placed in a WAMED SUP 65 W/G convection dryer (Wamed, Warsaw, Poland) at a temperature of 60 °C. The test duration was 24 h, after which the sample vessels were transferred to a desiccator for approximately 30 min; after cooling, they were weighed again to calculate the dry substance content. The determination procedure was performed in triplicate.

$$d.m.=\left[\frac{\left(m_3-m_1\right)}{\left(m_2-m_1\right)}\right]\times 100\%$$

where $d.m.$ is the dry matter content (%), $m_1$ is the mass of and empty vessel (g), $m_2$ is the mass of the vessel with the sample before drying (g), and $m_3$ is the mass of the vessel with the sample after drying (g).

2.5.3. Determination of Actual Density

Chia seeds and 2 cylinders of the same volume were used to determine the actual density: 250 cm³ of chia seeds was poured into a cylinder with a volume of 250 cm³, and a dried sample was placed into the second-tared cylinder and weighed. Vegetable bars were filled with seeds from the first cylinder until the upper meniscus was reached. The volume of grains remaining in the cylinder corresponded to the volume of a given freeze-dried sample, thanks to which the actual density was calculated from the following formula [20]:

$$\rho =\frac{m}{V_p}$$

where $\rho$ is the sample density (g/cm³), $m$ is the sample mass (g), and $V_p$ is the sample volume (cm³).

2.5.4. Porosity

Porosity was measured using a helium pycnometer from Quantachrome Company (USA), according to the manufacturer’s instructions. A sample that had a known mass but an unknown volume was placed in a chamber of known volume. The chamber was filled with helium, which penetrated all free spaces of the sample. The device measured the pressure value, and the obtained results were entered into Pycnometer software version 2.7 program, which allowed for the calculation of the density and apparent volume of the tested material [20].

$$\epsilon =\left(1-\frac{\rho d}{\rho s}\right)\times 100\%$$

where $\epsilon$ is the porosity (%), $\rho d$ is the real density (kg/m³), and $\rho s$ is the apparent density (kg/m³).

2.5.5. Microstructure

The structure was analyzed based on images obtained using a NIKON SMZ 1500 digital microscope (Nikon Instruments, Inc., Melville, NY, USA) and a TM-3000 scanning electron microscope from HITACHI High-Technologies Corporation, (Tokyo, Japan).

For the analysis of the internal structure using a TM-3000 HITACHI High-Technologies Corporation (Tokyo, Japan) electron microscope, samples were prepared by cutting 1–2 mm thick pieces, which were then coated with gold to prevent the image from glowing. Samples prepared in this way were placed in the measuring chamber of the microscope, where photos were taken at 50× magnification.

For surface structure analysis using a NIKON SMZ 1500 microscope (Nikon Instruments, Inc., Melville, NY, USA), samples were prepared by cutting the bar in half.

2.5.6. Statistical Methods

The results obtained in the research were subjected to statistical analysis using two-factor analysis of variance using the Stat-graphics statistical program.

3. Results

3.1. The Influence of the Coating Composition and Coating Method on Selected Properties of Freeze-Dried Vegetable Bars

Based on experimental research, it was possible to develop coatings based on 8% and 12% pork gelatin with or without the addition of vegetable broth at concentrations of 25% and 50%.

Table 3. Symbols and types of samples.

Sample Symbol	Sample Type
K	Control sample
C1	Coating with 8% of gelatin
C2	Coating with 8% of gelatin and 25% of both
C3	Coating with 8% of gelatin and 50% of both
C4	Coating with 12% of gelatin
C5	Coating with 12% of gelatin and 25% of both
EF1	Edible film with 8% of gelatin
EF2	Edible film with 12% of gelatin
EF3	Edible film with 12% of gelatin and 25% of broth

contains the codes of the obtained research samples; Figure 3 and Figure 4 present photos of the analyzed bars.

3.1.1. Water Activity

The water activity of bars covered with coatings based on 8% and 12% pork gelatin was in the range of 0.215–0.389, and the water activity of the control sample was 0.174 (Figure 5). Statistical analysis of the results presented in the form of bars determined significant differences in the water activities of vegetable bars depending on the composition of the coating and the use of the coating method. It was found that coating the bars statistically significantly increased the water activity of the samples compared to the control sample (K) (Figure 5 and Figure 6). Based on the results of samples coated with 8% pork gelatin (Figure 5), it was found that the addition of broth and increasing its amount from 25% (C2) to 50% (C3) caused a statistically significant increase in the water activity of the tested samples. It was shown that the coating method has a significant impact on the tested parameter. The bar covered with edible film (EF1) is characterized by statistically significant lower water activity than in the case of a sample covered with a coating of the same composition but by immersion in a coating-forming solution (C1).

For samples covered with a coating based on 12% gelatin (Figure 6), it was found that the addition of broth to the coating (C5) did not significantly increase the water activity of samples coated by immersion in the coating solution (C4). However, the addition of broth to edible film (EF3) significantly increased water activity. The coating method also changed the water activity of the bars. In the case of coatings and films based only on gelatin (C4 and EF2), a similar trend was found in the case of samples with 8% gelatin. The edible film coating reduced water activity relative to dip-coated samples, while the opposite trend was observed for broth coatings.

3.1.2. Dry Matter Content

Coating freeze-dried vegetable bars resulted in a statistically significant reduction in the dry matter content to the level of 93–96% compared to the control sample (K), in which dry matter content was 97% (Figure 7 and Figure 8). Analyzing the results obtained for the sample coated with 8% pork gelatin (Figure 7), it was found that the addition of broth to 8% gelatin in amounts of 25% (C2) and 50% (C3) did not statistically significantly change the dry substance content. However, it has been shown that the coating method significantly influenced the change in this parameter. Samples covered with a coating (C1) had a lower dry matter content compared to a bar covered with edible film (EF1) with the same composition.

However, for samples covered with a coating based on 12% pork gelatin (Figure 8), it was found that, as in the case of water activity testing, the addition of 25% broth (C5) did not significantly affect the change in the dry matter content of samples covered by immersion in the solution film-forming. However, the addition of broth to the edible film (EF3) significantly reduced the dry matter content compared to the edible film containing only gelatin (EF2). When comparing coatings and films based on 12% gelatin alone, it was found that coating with edible film (EF2) increased the dry matter content compared to dip-coated samples (C4), and in the case of coating (C5) and film with broth (EF3), differences were statistically insignificant.

3.1.3. Porosity

The porosity of bars covered with coatings and films based on 8% and 12% pork gelatin was in the range of 85–91%, with the control test result being 93%. Analyzing the results obtained for both samples covered with 8% and 12% gelatin (Figure 9 and Figure 10), it was found that covering vegetable bars with edible film does not significantly affect the change in porosity compared to the control sample (K). However, coating the bars with a coating in both cases shows a statistically significant reduction in porosity compared to the control sample.

In the case of vegetable bars coated with 8% gelatin, it was shown that the addition of 25% broth (C2) to the coating did not change the porosity of the dried fruit. However, increasing its amount to 50% (C3) significantly increased the tested feature to a value similar to the EF1 sample.

In the case of coatings based on pork gelatin with a concentration of 12%, a statistically significant reduction in porosity was found compared to the control sample (K) only in the case of using a coating based on gelatin (C4) (Figure 10). No statistically significant differences were found between the remaining samples.

3.1.4. Structure

After microscopic analysis, photos were obtained showing the structure of freeze-dried vegetable bars (Figure 11, Figure 12 and Figure 13). It was observed that all samples showed similar features, characterized by a delicate and porous structure, which is confirmed by the porosity results (Figure 9 and Figure 10). All samples showed a structure with irregular pores, which may be caused by the method of sample preparation (blending and mixing); therefore, measuring the size and number of pores is impossible. The vegetable bar without coating (K) has a delicate and porous structure, and open pores are visible on its entire surface (Figure 11). Microscopic photos indicate a delicate and porous structure throughout the sample.

Immersing the vegetable bar in a film-forming solution based on gelatin 8% (Figure 12) and 12% (Figure 13) resulted in the formation of a coating on the surface of the sample, which penetrated the surface layer, partially closed the pores, and obtained a compact surface structure (C1, C2, C3, C4, and C5). The addition of broth to the solution or increasing its amount from 25% (C3) to 50% (C4) did not cause any significant changes in the structure of the freeze-dried vegetable bars (Figure 12).

Similar results were obtained in the case of coatings based on 12% gelatin (Figure 13). Immersing the bars in the film-forming solutions caused changes in the surface layer of the samples, which partially collapsed due to the absorption of the film-forming solution and became more compact. Vegetable bars in edible film (EF1, EF2, and EF3) have a coating clearly visible in the microscopic photo, creating a barrier in the form of a “pocket” that does not adhere tightly to the surface of the bar (Figure 12 and Figure 13). This layer is extremely thin and delicate; it breaks when pressure is applied strongly; and in the case of film with broth, it is flexible and tensile. No changes were observed in the surface layer of samples covered with edible film because it does not adhere directly to the surface of the bars.

4. Discussion

Water present in food affects the chemical, physical, and biological transformations of the ingredients of a given product, including consistency, appearance, taste, and smell [21]. The state of water in food is constantly changing and is expressed by water activity—a thermodynamic indicator of the chemical potential of water in food. Water activity in moist food ranges from 1.00 to 0.90; in food with a medium water content it ranges from 0.90 to 0.55; and in food with a low water content it ranges from 0.55 to 0.00 [22]. It was found that coating the bars statistically significantly increased the water activity of the samples compared to the control sample. This is most likely because coating the bars by immersion in a film-forming solution increased the water activity of the dried material, and even drying the material together with the coating in a convection dryer was not able to reduce the water activity level to that of the control sample. In the case of packaging the bars in formed edible films, the film layer also contained a certain amount of water and humidity, which increased the water activity of the bar sample. The coating method has a significant impact on the water activity of freeze-dried vegetable bars. Bars covered with edible film were characterized by significantly lower water activity than samples covered with a coating of the same composition but by immersion in a coating-forming solution. Only in the case of samples covered with a coating based on 12% gelatin did the addition of 25% broth to the coating not significantly increase the water activity of samples coated by immersion in a film-forming solution. However, the addition of broth to the edible film significantly increased water activity.

Invichian [23] investigated the effects of trehalose, alginate, and calcium chloride composite films on freeze-dried apples and showed that the water activity of composite film-coated samples was statistically significantly lower than the uncoated samples. The opposite effect from in the case of coated with gelatin coatings and edible films vegetable bars may result from the fact that the composite trehalose coating was applied to the apples before freeze-drying; freeze-drying removes water to a greater extent than convective drying of the coatings. The obtained results are typical for freeze-dried products. Ciurzyńska et al. [19] tested the water activity of freeze-dried vegetable bars made from vegetable waste and showed that it ranges from 0.036 to 0.045. Karwacka et al. [24] researched snacks obtained from frozen plant by-products and apple pomace. They also obtained very low water activity values ranging from 0.009 to 0.017. The results of these tests are much lower compared to the samples analyzed in this study, which may be influenced by the presence of the coatings themselves and their composition, although with the water activity of the tested samples not exceeding 0.37. This value ensures the microbiological safety of the dried fruit because it is considered that microorganisms are unable to grow in food that has a value of a_w < 0.6. Chemical reactions have a significant impact on the durability and quality of food. Non-enzymatic browning reaches a maximum in the water activity range from 0.3 to 0.7, depending on the type of product. As activity increases, the oxidation of compounds such as vitamin C, heme pigments, chlorophylls, and anthocyanins increases. The greatest stability of food products is achieved when the water activity is in the range of 0.07 to 0.35, with a water content of 2% to 15% in the monolayer [22].

The water content in food products is a key criterion determining their quality, nutritional value, and storage possibilities. As the percentage of water in a given product increases, the amounts of valuable nutrients such as proteins, fats, and carbohydrates decrease. Additionally, increasing the amount of water promotes the development of microorganisms in food products, which reduces their suitability for long-term storage without appropriate technological processing. The range of water content in food products varies greatly, ranging from a low proportion to over 90%. This value is subject to changes as a result of technological processes and during storage. Although some raw materials, such as milk, some fruits and vegetables, or certain parts of animal carcasses, have a practically constant amount of water, most products are subject to significant fluctuations in their water content. The water content in a food product is defined as the amount of water that can be accurately determined by using appropriate measurement methods appropriate for a given product. However, the dry matter content of a given food product is the amount remaining after subtracting its water content [25].

Coating freeze-dried vegetable bars reduces their dry matter content. This is most likely because immersion in the coating solution reduces the dry matter content in the dried product. After all, the layer that was applied to the bar was damp, and even drying the material together with the coating in a convection dryer cannot eliminate it; the proof was the water activity results. Additionally, coating vegetable bars with molded edible film also leads to a reduction in dry matter content, and as with coatings, the moisture content of the edible film may have been a major factor influencing this effect. It was found that the addition of broth to 8% and 12% gelatin did not statistically significantly change the dry substance content of the bars compared to samples covered with a gelatin coating. However, it has been shown that the coating method significantly influences the change in this parameter. Samples covered with the coating had a lower dry matter content compared to a bar covered with edible film of the same composition. The results of water activity were confirmed, which indicated that immersing the bars in the film-forming solution caused the coating to penetrate the layer of freeze-dried products, and thus increased its humidity. However, the addition of broth to edible film with a gelatin concentration of 12% significantly reduced the dry matter content of the samples compared to food film containing gelatin only, which confirms the results of Ciurzyńska et al.’s [18] research on edible film derived from pork gelatin and beef broth. Ciurzyńska et al. [19] obtained a dry matter content in the range of 96.72–98.54% for freeze-dried vegetable bars with the addition of hydrocolloids. Kanarek [26] examined the influence of the coating method on the properties of freeze-dried fruit bars and obtained results in the range of 95–99%, which are similar to those analyzed in this study. It follows that the freeze-drying process is an effective method of removing water from the product to ensure the long-term shelf life of the product. Slight differences may result from the use of different methods of coating freeze-dried products or coating compositions.

Leeks or vesicles are elements found in various food products. To design an effective preservation process, and to determine various properties and product quality, it is important to predict the formation of pores in food during processing. In processes involving heat and mass exchange, such as drying, food changes volume through contraction, resulting in a loss of moisture, and expansion, due to the release of gases or the formation of pores. The variety of porosity, average pore size, and their arrangement have a significant impact on the textural characteristics of dried food. Therefore, it is important to predict the process of pore formation in food during drying [27]. It was found that the use of edible coatings and films to cover freeze-dried vegetable bars did not cause changes in the porosity of the dried fruit in most cases compared to the control sample. The addition of broth to the coatings and edible film was also statistically insignificant in most cases, as was the coating method. Only in the case of vegetable bars coated with 8% gelatin did increasing the addition of vegetable broth to 50% significantly increase the tested feature. The results obtained are typical for freeze-dried samples. Sundaram and Durance [28] examined the porosity of gels from carob gum, the average value of which was 93%. Wu et al. [29] determined the porosity of gelatin gels at the level of 95–99%, which allows us to conclude that the porosity of gels depends on the type and amount of the structure-forming substance, the characteristics of the remaining ingredients, and the parameters of the freezing and sublimation process.

Microscopic analysis of the structure of freeze-dried vegetable bars covered with coatings confirmed the results of other analyses of the physical properties of dried samples. All samples are characterized by a delicate and porous structure, which confirms the porosity results. Similar results were obtained in the studies by Karwacka et al. [24], who examined freeze-dried snacks obtained from frozen vegetable by-products and apple pomace, and Ciurzyńska et al. [19], who examined the structure of freeze-dried vegetablebars. Immersing the vegetable bar in a film-forming solution based on 8% and 12% gelatin caused a partial collapse of the structure of the surface layer due to the penetration of the film-forming solution into the porous surface of the freeze-dried products. This confirmed the results of porosity, which decreased in the case of dip-coated samples compared to the control sample. The addition of broth did not cause any significant changes in the internal structure of the bars. The use of a coating in the form of edible films resulted in the formation of a thin layer on the surface of the dried fruit in the form of a “pocket” that does not adhere tightly to the surface of the bar, thanks to which the structure of the vegetable bar itself did not change compared to the control sample. The foils with broth were flexible and tensile, which is consistent with the observations of the foils themselves from the research by Ciurzyńska et al. [18].

5. Conclusions

The research conducted showed that the composition of the coatings and the coating method influence changes in the physical properties of freeze-dried vegetable bars. All samples are characterized by a delicate and porous structure, which was confirmed by microscopic image analysis and porosity measurement. Covering the freeze-dried bars with an edible coating by immersing them in a film-forming solution caused the solution to penetrate the surface layer of the dried bars. The pores were partially collapsed, which was confirmed by porosity testing. The addition of vegetable broth did not significantly change the porosity in most cases. The use of edible film did not cause any changes in the internal structure of the dried fruit because the created barrier in the form of a “pocket” is not closely connected to the surface of the bar. Edible films with the addition of vegetable broth have greater elasticity than those made only on the basis of gelatin.

Coating vegetable bars increases their water activity compared to the control sample. In the case of coatings based on 8% pork gelatin, the addition of broth and increasing its amount to 50% increased the water activity of the freeze-dried products. However, in the case of coatings based on 12% pork gelatin, the addition of broth did not increase the tested feature. The use of edible film also increases this parameter, and in the case of edible film based on 12% gelatin, the addition of broth to the film additionally increased the water activity of the dried material, while all samples achieved water activity below 0.37. It has been shown that coating freeze-dried vegetable bars reduces their dry substance content compared to the uncoated sample. The addition of broth to the coatings with 8% and 12% gelatin did not change this parameter. However, the use of edible film with 8% and 12% gelatin increased the dry substance content of the bars compared to dip-coated samples. The addition of 12% broth to the edible film reduced this parameter.