The Effect of Storage Time and Temperature on Quality Changes in Freeze-Dried Snacks Obtained with Fruit Pomace and Pectin Powders as a Sustainable Approach for New Product Development

Abstract

The study aimed to evaluate the effect of storage temperature on quality changes in freeze-dried carrot–orange–ginger (COG) snacks obtained with 2% of apple (AP) or blackcurrant (BP) pomace powder or 1.5% of pectin (LMP). The material was stored at 4, 25 and 40 °C for 1, 3, 6 and 12 months in pouches impermeable to vapour, oxygen and light. Water content and activity, texture, colour, total polyphenol content and antioxidant activity were examined to evaluate the products’ quality. During storage, water content and activity fluctuations were noted, but no critical values were exceeded. Texture properties remained stable in snacks with pomace powders compared to those with pectin, the hardness of which significantly decreased when stored at ambient and elevated temperatures. However, the product with pectin was found to change colour the least over time. The results obtained for chemical properties were not clear, but they strongly indicated the occurrence of the transformation of some chemical compounds. Overall, a lowered temperature helped to maintain the quality of the freeze-dried snacks for the longest time. The presented research contributes to the general knowledge of the behaviour of innovative products with the addition of nonconventional but sustainable compounds, revealing the potential for quality and stability improvements.

1. Introduction

Dehydration is one of the most common methods being applied to prolong the shelf-life of easily spoiled foods. Its effectiveness results from lowering the moisture content, and thus the water activity of the material, below the level critical for biochemical reactions and microbial growth [1]. This rule suits traditional dried goods, e.g., fruits and vegetables, and innovative multicomponent products have been designed using drying methods not only as a tool to remove significant amounts of water but also to shape the product’s characteristics [2]. The freeze-dried snacks investigated in this research are a great example of those new products. They feature a porous structure that improves texture and elevates hygroscopicity [3] and, like most plant-based freeze-dried materials, they can lose their colour and sensory attributes faster than products obtained with different drying methods [4]. Although the shelf-life of freeze-dried snacks should be long due to the very low water activity, maintaining these products’ high quality over time requires the selection of packaging that provides a strong barrier against vapour, oxygen and light, specifically [5].

Some of the most important requirements a food product has to meet before its introduction to the market are safety and stability. To learn about the behaviour of food items during storage, it is suggested that their response to a wide range of conditions to which they may be subjected is tested, and the critical parameters that may be affected are analysed [6]. Prior to their presence on a store shelf, consumables are usually kept at lowered temperatures in order to reduce the possibility of any unfavourable changes occurring. Once on the store shelf and after being purchased by consumers, goods that do not require refrigerating are stored in ambient temperature surroundings. Moreover, the conditions that products face in a household can vary depending on time, climate, etc. Fluctuations in the environment can cause products to deteriorate more quickly [7]. Shelf-life testing takes a lot of time, so to predict changes that could occur over a prolonged storage time or due to exposure to critical conditions, such as elevated temperature, accelerated shelf-life tests are being conducted.

The objective of this study was to evaluate the effect of storage time and temperature on selected quality parameters of multicomponent freeze-dried snacks with fruit pomace powders or pectin.

2. Materials and Methods

2.1. Materials

The research was carried out on freeze-dried carrot–orange–ginger (COG) snacks derived from frozen carrot (Unifreeze sp. z o.o., Miesiączkowo, Poland), orange juice (Tymbark, Kraków, Poland), ginger (local market, Warsaw, Poland), dried apple pomace (AP) and blackcurrant pomace (BP) (Greenherb, Łańcut, Poland), low-methoxyl pectin (LMP) (Hortimex, Konin, Poland) and calcium lactate (Agnex, Białystok, Poland).

2.2. Sample Preparation and Storage Conditions

Prior to production, the ingredients were weighed according to the formulation presented in

Table 1. Formulation of the freeze-dried snacks.

Sample	COG-AP	COG-BP	COG-LMP
Compound	Formulation (%)
Carrot (C)		60
Orange juice (O)		30
Ginger (G)		0.4
Calcium lactate		0.1
Water	7.5	7.5	8
Apple pomace powder (AP)	2
Blackcurrant pomace powder (BP)		2
Low-methoxyl pectin (LMP)			1.5

and carrot cubes were thawed at room temperature for about 30 min. Water was heated to 85 °C, and then the calcium lactate salt and the additive (AP, BP or LMP) were added and mixed for 1 min. Prepared components were blended for 1 min using GRINDOMIX GM 200 (Retsch, Haan, Germany) at 4500 rpm. The mixture was poured into 1.5 × 1.5 × 1.5 cm silicone moulds, frozen at −40 °C for 4–5 h and freeze-dried using an Alpha 1–2 LD plus lyophilizer (Martin Christ GmbH, Osterode am Harz, Germany) at 30 °C and 63 Pa for 48 h. Directly after freeze-drying, the samples (30 cubes) were packed in double-sealed laminate (PET/AI/PE) packaging pouches impermeable to light, gas and vapour.

The prepared snacks were stored at three different temperatures: 4 ± 1 °C to test their behaviour under conditions commonly used for storing foods before exposure on a store shelf, 25 ± 3 °C to determine changes that could potentially occur during storage in a store or a household, and 40 ± 1 °C to perform accelerated shelf-life testing in order to evaluate the effect of a higher temperature and possible quality loss after an extended storage time. The material was stored for 1, 3, 6 and 12 months, assuming that 1 month was 30 days. One portion of the material was tested within 48 h after processing as a control. All the analyses were performed within 24 h after the package was opened.

2.3. Analytical Methods

Water content was determined using the oven method. The ground sample (1 g) was dried in an oven SUP 65W/G (WAMED, Warszawa, Poland) at 70 °C for 24 h. The water content was calculated as weight loss measured after drying over the initial weight of the sample. A water activity analysis was conducted utilizing HygroLab C1 meter (Rotronic, Bassersdorf, Switzerland) at 25 ± 1 °C. Both tests were conducted in triplicate.

The texture properties of the material were tested using TA.HD plus texture analyser (Stable Micro Systems, Godalming, UK). A compression test was performed using a 20 mm diameter platen probe, applying a test speed of 0.5 mm/s. The measurement was performed on 10 samples of the material (1.5 × 1.5 × 1.5 cm) until 50% deformation occurred. The results were expressed as hardness and compression curves.

Colour parameters L*, a*, b* were measured using reflectance mode on CR-5 Colorimeter (Konica Minolta, Tokyo, Japan). The diameter of the measuring hole was 8 mm. The measurements were performed on 10 different spots on the surface of the material. Total colour difference (ΔE) and chroma parameter (C) were calculated according to the following formulas:

$$∆E=\sqrt{{\left({∆L}^{*}\right)}^2+{\left({∆a}^{*}\right)}^2+{\left({∆b}^{*}\right)}^2}$$

(1)

$$C^{*}=\sqrt{{\left(a^{*}\right)}^2+{\left(b^{*}\right)}^2}$$

(2)

where ${∆L}^{*}$, ${∆a}^{*}$ and ${∆b}^{*}$ are the differences in lightness (L*), redness (a*) and yellowness (b*) between the stored and control material.

The extracts used for total polyphenols content and antioxidant activity determinations were prepared by extracting 0.3 g of the powdered material in 10 mL of 80% (v/v) aqueous ethanol solution overnight (around 18–20 h) at room temperature, continuously stirring using a laboratory shaker (Heidolph Instruments, Schwabach, Germany). After extraction, the extracts were centrifuged (2 min, 3000 rpm) utilizing a laboratory centrifuge (MegaStar 600, VWR, Leuven, Belgium), transferred into 0.2 mL PRC tubes and subjected to further analytical procedures, slightly modifying the methodology of Wiktor et al. [8]. Extraction was performed in duplicate for each sample.

The total polyphenol content (TPC) determination was conducted with Folin–Ciocalteau’s reagent using the spectrophotometric method. The extracts prepared as stated above were mixed (1:1 v/v) with 10 μL of distilled water in 96-well plates. Subsequently, 40 μL of 5-fold diluted Folin–Ciocalteau’s reagent was added, shaken and incubated in a dark place at room temperature for 3 min. Then, 250 μL of 7% sodium carbonate solution was added and the mixtures were incubated once more in a dark place for 60 min. For a blank test, an extract was replaced with the 80% aqueous ethanol solution. The absorbance was measured at a wavelength of 750 nm using Multiskan Sky plate reader (Thermo Electron Co., Waltham, MA, USA). The analysis was performed in triplicate for each extract.

Prior to the analysis, the stock DPPH^● solution was diluted with 80% (v/v) ethanol to obtain a working solution, the absorbance of which was in the range of 0.68–0.72, at a wavelength of 515 nm. A total of 10 μL of the analyte solution and 250 μL of the free radical solution were dispensed into a 96-well plate, shook and incubated at room temperature in a dark place for 6 min. A blank test was prepared using the extracting solution instead of the extract. The measurement of absorbance was performed at a wavelength of 515 nm using Multiskan Sky plate reader. The antioxidant activity was expressed as mg Trolox/g dry matter. The determination was conducted in triplicate for each extract.

The measurement of the antioxidant activity against ABTS^●+ was conducted following the procedure described for the DPPH^● assay, modifying the incubation time, which lasted for 30 min, and the absorbance was measured at a 734 nm wavelength.

The results are expressed on graphs as means with standard deviations. The effect of storage time and storage temperature was evaluated using the two-factorial ANOVA and a post-hoc Tukey’s test at p > 0.05. Each type of sample (COG-AP, COG-BP, COG-LMP) was analysed separately. The statistical analysis was carried out using STATISTICA 13.1 software (TIBCO, StatSoft Polska, Kraków, Poland).

3. Results and Discussion

Water-related properties are crucial for the stability of food. They determine the microbial growth, biochemical reactions and physical changes that are responsible for the quality and safety of a product. As is shown in Figure 1A, the water content in COG-AP samples fluctuated in the range from 0.011 to 0.022 g/g d.m.; in COG-BP, it fluctuated in the range 0.010–0.020 g/g d.m. and in COG-LMP, it fluctuated in the range 0.013–0.023 g/g d.m. Regardless of the type of additive used, the temperature tended to affect snacks in a congruous way. After the first month, the storage values of the parameter significantly dropped, and then they started to stabilize over time. When refrigerating, water content rapidly increased after 3 months and then continuously decreased until the end of the testing period. The accelerated shelf-life test showed that the first descent was followed by a progressive elevation of the water content. The most notable variation was observed during storage at an ambient temperature, which might be caused by the inconsistency of the surrounding temperature. By the end of the trials, the highest water content was recorded in samples stored at 40 °C and the lowest in the snacks that were refrigerated. The performed tests showed fluctuations in the water content of the snacks; however, the results indicate that the packaging used was sufficient to keep the products stable and separated from the humid environment. A previous investigation of freeze-dried mushrooms showed that when using “normal PE bags” and keeping samples at 25 and 37 °C and various RHs, the moisture content could increase from slightly over 4% to 8–10% within 25 days, which resulted in a microstructure transition and a drastic reduction in texture properties [9]. Other studies on extruded rice snacks proved that, due to the hygroscopicity of the products, moisture content significantly increases during storage regardless of the type of packaging [10].

Since the packaging used in the research was impermeable by vapour and double-sealed, the changes observed during storage are most likely a consequence of the migration of moisture present in the material, as well as interactions between the material and the atmosphere enclosed in the packet [11]. As was established before, the water content in the material is important, but food safety and stability rely mostly on water activity, which has been identified as the critical parameter affecting products’ quality during storage [12].

Figure 1B presents water activity fluctuations during 12 months of storage. Constant tendencies regarding the storage temperature can be seen. In the beginning, the water activity of samples stored at 4 and 25 °C slightly lowered and then progressively increased until it peaked after 6 months. After that, the values for the refrigerated material dropped to the initial grade, while maintaining snacks at an ambient temperature induced the water activity to remain constant. Different trends appeared while storing the samples at an elevated temperature. For the first six months, the water activity constantly increased, then it rapidly dropped. In this case, the samples obtained with fruit pomace powders behaved differently than those with LMP. The kinetic curves show that in the first month of storage, the increase in the water activity of COG-AP and COG-BP samples was more intense and then it slowed down during the next two months. Nevertheless, samples stored at the highest tested temperature reached the highest values of water activity, but it seems that they remained at this level only periodically, while storage at an ambient temperature caused the water activity to increase and stay that way for a longer time, which could potentially cause more damage to the sample. However, it is necessary to mention that even though the values of the parameter increased, they did not exceed any critical level for microbial growth or enzymatic and nonenzymatic reactions [12]. A low level of water activity may support lipids’ oxidation; however, as was established before, the tested samples do not contain a significant amount of fat [13]. This suggests that freeze-dried snacks remained stable and safe for consumption after 12 months of storage under various conditions.

The texture analysis resulted in hardness (Figure 2A) being obtained as the highest force recorded during compression of the samples, and compression curves (Figure 2B) show the variety of samples that break upon impact. As can be seen in Figure 2A, snacks obtained with fruit pomace powder showed only a slight deviation throughout storage. The hardness of COG-AP significantly rose after the first month by about 15%, regardless of the temperature. The lowest, not significant changes occurred during the low-temperature storage. The mechanical properties of snacks with blackcurrant pomace powders slightly differed at the beginning of storage, but overall hardness remained at the same level. The highest variation in and influence of the storing conditions was observed when analysing COG-LMP snacks. Refrigeration helped the material to remain unchanged for twelve months, but keeping it at ambient and elevated temperatures caused a significant reduction in hardness. A similar trend was established before, where the textural and rheological parameters of pectin gels and solutions were constant at 4 °C and decreased when they were kept at 25 and 40 °C. The authors indicated that the reason for this was the depolymerization of pectin over time [14]. The mentioned observations were also noticeable in the compression curves, examples of which are presented in Figure 2B. They depict the behaviour of a material reacting to an external force that causes deformation of the sample. When the material shows resilience to the impact, the compression force increases and the curve goes upwards, but when the material breaks, the compression force drops abruptly and a downward peak appears on the curve. As is observable in the graph, the curves of COG-AP and COG-BP samples were flatter and the visible disturbances in them were smaller compared to those recorded for COG-LMP. They were also determined within the range of 10 to 15 N, while the difference in the compression curves of samples with pectin varied in the scope of 30 N. This confirms the variation in the tendencies of mechanical properties observed during freeze-dried snacks’ storage and their dependence on the products’ formulation. Changes in texture are a common phenomenon in low-moisture food products. Mechanical attributes are connected to the internal structure, moisture content and chemical composition of the material [10]. The storage of freeze-dried orange snacks with various biopolymers caused a significant reduction in force peaks that indicated a loss of crispiness. The authors associated this with the water content in the examined products, which also affected their porosity and structure, and strongly recommended controlling the RH of the environment during storage, identifying this as a main factor responsible for unfavourable quality changes [15].

Colour is one of the first and most important factors in food products, attracting attention and determining consumers’ choice of food products. Although the materials used in the high-barrier packaging required for freeze-dried snacks are unlikely to be transparent and display the product, especially considering its sensitivity to light, consumers expect their purchase to meet their assumptions and for the graphic design of the packaging to be vivid and colourful [16]. In order to analyse colour transformation, lightness, colour saturation and total colour difference were determined. Lightness is presented in Figure 3A. As was expected, the L* parameter tended to significantly increase in all cases and the lowest shifts were observed in the lightness of samples stored in a refrigerator. However, despite the storage conditions, the variations noted for COG-AP, COG-BP and COG-LMP samples were within the range of 5, 4 and 3 unit points compared to the control sample. However, in the case of freeze-dried orange snacks stored at 20 °C, a decrease in L* parameter values was observed after only 6 months, while keeping samples in a refrigerator prevented significant lightness changes [15].

Figure 3A also presents the C* parameter, chroma. This represents the saturation and vividness of colour, which can also be affected during the storage of the material and can influence the overall perception of a product. The greatest and most significant drop in the saturation of the samples’ colour was noticed after the first month of storage; later on, the dynamics of the change were reduced and differences were not easy to distinguish regardless of the decreasing tendency. That suggests that the colourants contained in the material reacted with the atmosphere only at the beginning of their storage, and then the reaction was inhibited. This phenomenon may have resulted from the high availability and porous structure of the material; thus, the rapid usage of the reagents caused a degradation in their colour.

The general change in the colour of the samples was estimated as the total colour difference (Figure 3B). A clear difference can be seen in the course of changes that occurred in samples with the addition of pomace powders and low-methoxyl pectin. For the first three months of storage at 4 °C, the snacks’ colour transformed progressively compared to products kept at the ambient and elevated temperatures in which the ΔE values did not clearly differ over time. After that time, the changes evened out regardless of the storage conditions. In the case of COG-LMP samples, the time factor was not significant and the total colour change was constant from the first month of storage. The lowest difference was observed in refrigerated material, while the greatest was observed in samples kept at room temperature. The values of the ΔE parameter assessed for snacks with pectin were around 50% lower than those determined for snacks with the addition of pomace powders. However, it is worth mentioning that, regardless of the type of sample and storage conditions, in each examined case, the colour changed to the point at which even an inexperienced person would recognize the difference compared to the first period of storage, and in samples with fruit pomace powders it kept progressing. The obtained results suggest that, in terms of colour maintenance, using LMP allows the production of more stable products in comparison to fruit pomace powders, irrespective of their origin. This may result from the protective effect of the pure biopolymer addition. Previous findings suggested that natural biopolymers have the ability to encapsulate bioactive compounds and colourants and protect them during processing [17], so it is highly possible that this effect also works during storage. However, the susceptibility of the natural colourants contained in carrot and orange juice, such as carotenoids, to degradation resulting from oxidation or exposure to light has been established before. The metalized pouches were also found to be effective in terms of pigment retention and colour preservation during storage [18]. It was also found that, at ambient temperature and above a certain level of moisture content, colour changes may occur due to non-enzymatic browning [15].

Figure 4 shows the total polyphenols content in freeze-dried snacks stored at various conditions. As can be seen, the amount of polyphenols tended to increase over the storage period. The tendency was the most explicit during the accelerated shelf-life testing, while every time the examination was performed, the results were elevated compared to the previous values of up to 72, 35 and 116% for COG-AP, COG-BP and COG-LMP. A significant increase was also noticed in refrigerated samples. The lowered temperature did not prevent changes, but the ones that occurred were constant over time and stabilized at around 32, 10 and 35%, respectively, in the materials with AP, BP and LMP. An ambient temperature also caused a transformation in TPC results. They were rising for three months of storage, then dropped to the level observed after the first month of storage, or lower in the case of COG-BP. Considering the type of additive used in the snacks, the lowest changes were assessed in samples with the blackcurrant pomace addition. The obtained results were found to be opposite to the previous findings, in which the TPC content tended to significantly decrease over time [5,9]. However, a study on freeze-dried orange snacks stored at 4 and 20 °C also revealed increased TPC during 6 months of storage. The obtained results were associated with the transition of organic acids introduced via the orange puree into compounds of a phenolic nature [15]; therefore, in this research, the changes may be related to the concentrated orange juice in the formulation. On the other hand, an investigation of bioactive compounds in freeze-dried cherries showed that the packaging material used in this research allowed for a higher TPC to be retained during storage compared to other polyethylene materials. The importance of temperature and its effect on polyphenols and the loss of other compounds were also revealed, suggesting that refrigeration is the most beneficial condition [5].

The antioxidant activity against ABTS and DPPH is presented in Figure 5A,B. Evaluating these results, the most conspicuous was the opposite behaviour of the extracts reacting with different radicals. Regardless of the type of material that was examined, antioxidant activity against ABTS progressively increased over time, while activity against DPPH rose at the beginning of the storage, then significantly decreased. This, combined with the different tendencies observed in TPC results, suggests that, over time, the compounds contained in the material transformed into compounds reacting with ABTS and Folin–Ciocalteu’s reagent, but not compounds reacting with DPPH. However, in previous studies, antioxidant activity assessed with DPPH and FRAP assays remained constant while TPC increased, so the tendency observed with the two methods was the same [15]. This may be explained by the higher selectivity of DPPH radicals in comparison to ABTS; ABTS reacts with a wider range of chemical compounds, while DPPH is directed mostly to polyphenols [19]. During the storage of freeze-dried cherries, antioxidant activity decreased over time and the results correlated with a decrease in the contents of bioactive compounds, including TPC [5]. The course of the observed changes reveals that the highest antioxidant activity occurred in material stored at 40 °C, which may indicate that the elevated temperature accelerated the degradation of compounds present in the material, leading to the formation of compounds with antioxidant properties that did not necessarily have to be polyphenols. Therefore, an analysis and identification of the chemical composition would be highly recommended for future research to understand the exact character of the occurring changes.

4. Conclusions

Fluctuations in water content and water activity were observed, with the highest being recorded in samples stored at 40 °C, while refrigerated samples remained relatively stable. The packaging effectively preserved product stability, indicating its suitability for preventing moisture-related changes. The mechanical properties were influenced by the storage conditions. Snacks obtained with low-methoxyl pectin experienced the highest reduction in hardness, while samples with fruit pomace powder did not notably change within the storage time. Variations in the colour parameters indicated the significant effect of the storage condition on the general appearance of the snacks due to the degradation of pigments contained in the material. Total polyphenols content and antioxidant activity tended to increase over time, with fluctuations observed based on storage conditions and radical type. Antioxidant activity against ABTS increased steadily over time, while activity against DPPH initially rose before declining, with the highest activity levels being observed at 40 °C. An elevated temperature had the most influential effect on the quality of the snacks. Based on the obtained results, it can be stated that freeze-dried snacks containing both pectin and fruit pomace powders can be safely stored for 12 months if refrigerated. However, the observed quality changes should be compared to consumers’ perception of the said products to evaluate the optimal shelf-life. Moreover, a more detailed chemical analysis could be performed to obtain a better understanding of the origin and nature of the transformation of the bioactive compounds present during storage.