Does a Freeze-thaw Pretreatment Enhance the Quality of Dried Foods? A Meta-Analysis

Based on the results of several studies


Introduction
Drying is a technique used to reduce water content and activity to inhibit degradation (Onwude et al., 2017).This method extends food shelf life and boosts nutritional and economic value (Vallespir et al., 2019).However, the heat process during drying also reduces the quality of the final product.

jTEP JURNAL KETEKNIKAN PERTANIAN,
The question of whether freeze-thaw pretreatment is beneficial for drying raises a contradiction.
To address this question, a statistical technique called meta-analysis can be used to determine the impact of freeze-thaw pretreatment on the quality of dried products.Meta-analysis involves summarizing the results of independent studies to conclude the significance of the treatment effect on the control (Červenka et al., 2018).By synthesizing the outcomes of multiple studies and determining their effect sizes, the primary conclusion may be drawn regarding the significance of the treatment effect on the control (Borenstein et al., 2009).In this case, a meta-analysis was conducted on drying, as reported by Červenka et al. (2018) and Kurniasari et al. (2022).Červenka et al. (2018) investigated the effect of drying temperature on the concentrations of ascorbic acid, flavonoids, and phenolics, while Kurniasari et al. (2022) studied how the drying method influences the bioactivity of ginger.The results showed that the drying temperature had a significant impact on the value of ascorbic acid but not on the levels of phenolics and flavonoids (Červenka et al., 2018), while the drying method had different effects on phenolics, flavonoids, 6-gingerol, and antioxidant content (Kurniasari et al., 2022).
In this study, the impact of freeze-thaw pretreatment on the quality of dried products will be examined by employing meta-analysis.This research aimed to determine the effects of freeze-thaw pretreatment on the quality of dried plant-based foods.

Data Source and Selection Criteria
During June 2023, a thorough search was carried out on the Scopus database (https://www.scopus.com) to find research studies related to the impact of freeze-thaw pretreatment on the quality of dehydrated plant-based foods.The search criteria were narrowed down to Englishwritten peer-reviewed journal articles and specifically targeted the keywords drying, pretreatment, and freezing.The selection of the relevant literature was based on the PICO protocol (Ogbuewu & Mbajiorgu, 2023), an acronym for Population (referring to plant-based food), Intervention (referring to freeze-thaw pretreatment), Comparison (referring to freeze-thaw pretreatment and without pretreatment), and Outcomes (referring to ΔE, TFC, TPC, shrinkage, and hardness).Preferred reporting items for systematic review and meta-analysis (PRISMA) protocol were also used to reduce bias and ensure meta-analysis quality (Liberati et al., 2009).This study shows that using freeze-thaw pretreatment has a significant impact (P < 0.05) on ΔE, TFC, and TPC of dried plant-based foods compared to the control.However, it does not significantly affect (P > 0.05) shrinkage and hardness.Freeze-thaw pretreatment hurts discoloration by increasing ΔE of dried foods compared to the control.This is consistent with the findings of Vallespir et al. (2018), who reported that frozen beetroots, apples, and eggplants had a higher ΔE than the control.The destruction of cellular structural integrity results in the enzymatic browning reaction of enzymes and polyphenolic compounds, which causes a color change (Vallespir et al., 2019).
The total flavonoid content of dried plant-based foods improved by freeze-thaw pretreatment compared to the control.Cell damage due to freeze-thaw pretreatment promotes the extraction of total flavonoids, leading to a higher TFC (Xu et al., 2021).In contrast, freeze-thaw pretreatment has an unfavourable effect on the TPC of dried plant-based foods compared to the control.The reduction in phenolic chemicals may be due to their interactions with other substances, such as proteins, or changes in their chemical structure brought on by drying (Piroozi et al., 2023).Ice crystal formation may also cause cellular wall rupture, leading to bioactive chemicals' loss and/or oxidation (Vallespir et al., 2019).
Due to drip loss and oxidation reactions, the TPC of the frozen potato was substantially lower than the control (Zhu et al., 2020).The freeze-thaw process does not influence (P > 0.05) shrinkage and hardness of dried plant-based foods compared to the control, but their values tended to decrease.The drying temperature and drying time play the largest roles in the shrinkage (Noshad & Ghasemi, 2020).Differences in food structure and drying techniques could be responsible for some of the variation in reported shrinkage results (Bassey et al., 2023).Freeze-thaw enhanced the product's porosity, resulting in low hardness (Zhang et al., 2022).Conversely, the surface removes water faster than it migrates from the inside, forming a hard coating of previously dissolved solutes (Quispe-fuentes et al., 2023).Some caramelization and Maillard reactions have been attributed to increased hardness (Khiari et al., 2021).This contradicting could be the origin of variations in hardness levels, resulting in meta-analysis calculations that are not significantly different.

Subgroup analysis:
The effect of moderator variable on the quality of dried plant-based foods

Freezing temperature
Table 3 displays a breakdown of the freezing temperature sub-group analysis.The results indicate that a temperature of -18 °C does not have a significant impact (P > 0.05) on TFC, TPC, and hardness of dried foods compared to the control.However, at -20 °C, there is a significant effect on ΔE, TFC, and hardness (P < 0.05), while TPC and shrinkage remain unaffected significantly (P > 0.05) compared to the control.Freezing at -80 °C does not considerably influence (P > 0.05) ΔE, TFC, and shrinkage, but it significantly reduces (P < 0.05) TPC, and hardness compared to the control.On the other hand, a temperature of -196 °C has a significant effect (P < 0.05), which increases TFC and shrinkage while lowering hardness.However, it does not significantly impact (P > 0.05) ΔE and TPC.It is important to note that higher freezing temperatures result in longer freezing times, increased ice crystal formation, and more tissue cell damage (Chung et al., 2013;Ergün et al., 2021).
Based on the results of the data analysis, it seems that temperatures of -20 and -196 °C are both feasible for freezing.Zhang et al. (2022) also came to the same conclusion: -20 °C is the optimum freezing temperature out of -40, -60, and -80 °C.On the other hand, freezing in liquid nitrogen (-196 °C) may have minimized structural damage and preserved quality due to the rapid freezing rate and small crystals formed during the process (Vallespir et al., 2019).-2.048 -3.347 -0.749 0.663 0.002 ΔE = total color difference; TFC = total flavonoid content; TPC = total phenolic content; SMD = standardized mean difference; CI = confidence interval; SE = standard error.

Drying method
The findings of the sub-group analysis of the effect of the drying method are presented in Table 4.
This study revealed that drying with hot air (HA) has a significant impact (P < 0.05) on both the ΔE and TPC of dried foods while having no significant influence (P > 0.05) on shrinkage and hardness when compared to the control.In contrast, infrared-convective (I-C) drying significantly impacts (P < 0.05) shrinkage, ΔE, TPC, and hardness, though TFC remains unaffected significantly (P > 0.05).Midinfrared (MIR) drying has no significant effect (P > 0.05) on the observed qualities.Mid-infrared (MIR) drying has no significant effect (P > 0.05) on all qualities observed.Near-infrared drying significantly affects (P < 0.05) shrinkage and TFC, but not ΔE, TPC, or hardness.Hot air-microwave vacuum (HA-MV) drying only significantly affects (P < 0.05) TFC, but not TPC, shrinkage, or hardness.In general, it can be concluded that numerous drying methods have a favorable effect on dried product quality, notably I-C (shrinkage and hardness), NIR (TFC), and HA-MV (TFC).Convective air at high temperatures will accelerate drying (Červenka et al., 2018).Transfers energy on infrared drying quickly from heat emitter to foodstuffs without heating the environment, preserving product quality (Adak et al., 2017).Vacuum drying increases heat transmission between frozen cells and rapid water evaporation from ice crystals (Shyu & Hwang, 2001).In the final drying stage, replacing hot air with microwaves substantially increased TFC (Zhou et al., 2021).

Commodity
Freeze-thaw pretreatment has been employed for drying various types of commodities.A subgroup analysis was conducted to investigate the impact of freeze-thaw pretreatment on the quality of dried food in various commodities (Table 5).A statistically significant increase (P < 0.05) in the ΔE is observed in dried lotus roots and grapes compared to the control.In contrast, applying freeze-thaw pretreatment to red dragons does not have a statistically significant impact on ΔE.The cranberries and red dragons significantly rise (P < 0.05) in TFC compared to the control.For lotus roots, the freeze-thaw pretreatment has no significant impact (P > 0.05).Compared to controls, total phenolic content is reduced in the presence of freezethaw pretreatment on cranberries, lotus roots, red dragons, and grapes.However, only on cranberries is the effect insignificant (P > 0.05).Compared to the control, lotus roots experienced significantly lower shrinkage (P < 0.05), whereas grapes experienced substantially more significant shrinkage (P < 0.05).In contrast, the freeze-thaw pretreatment has an insignificant impact (P > 0.05) on the shrinkage

Figure 1 .
Figure 1.Flow chart of literature selection procedure based on PRISMA protocol

Figure 1
represents the literature selection procedure based on PRISMA that summarizes the details of the article search.
Full-text articles assessed for eligibility (n=48) Full-text articles excluded (n=34): irrelevant quality parameters (n=5); not pure freezethaw (n=23); lack of information for effect size calculations difference; TFC = total flavonoid content; TPC = total phenolic content; n s = number of studies; n c = number of comparisons; SMD = standardized mean difference; CI = confidence interval; SE = standard error; QM = coefficient of moderators; DF = degree of freedom; I 2 = Inconsistency index.

Table 2 .
Pooled results freeze-thaw pretreatment and control on quality plant-based foods

Table 3 .
Sub-group analysis of the effect of freezing temperature on the quality of dried plant-based foods

Table 4 .
Sub-group analysis of the effect of the drying method on the quality of dried plant-based foods

Table 5 .
Sub-group analysis of the effect of the commodity on the quality of dried plant-based foods ΔE = total color difference; TFC = total flavonoid content; TPC = total phenolic content; SMD = standardized mean difference; CI = confidence interval; SE = standard error.