*3.3. Changes in Quality Parameters during Storage*

Firmness is one of the most important quality criteria for traders and consumers alike [12]. Fruit starts to soften when still on the tree, about 4 weeks prior to the optimum harvest date [44]. After harvest, fruit continues to soften to finally reach edible firmness, and the softening rate depends on such factors as fertilization, harvest date and cultivar, but it is mostly influenced by the length and conditions of storage. The softening rate of late-maturing pear cultivars, such as 'Conference', was found to depend on the synthesis of two enzymes: acetyl-CoA synthase (ACS) and 1-aminocyclopropane carboxylic acid (ACC) [6]. In this study, both the softening rate and the total loss of firmness were clearly dependent on storage conditions (Tables 5–7). The initial firmness measured at harvest depends of many factors, which was visible in our study because it varied little between years, with the lowest value observed in 2012 and the highest in 2011. In addition, there were virtually no differences between years in the final firmness after 180 days of NA storage, and the total loss of firmness ranged from 34 to 40%. No clear regularity was revealed as regards firmness loss per month in individual years, but the firmness showed a tendency to decrease, as the greatest loss of firmness was noted in the last (sixth) month of NA storage in two years and in the next-to-last (fifth) month in one year of the study.

1-MCP application significantly reduces the softening rate of climacteric fruit and vegetables [24]. This study revealed that 1-MCP application visibly slows down the softening rate, particularly in the first months of storage—it reduced the total firmness loss by about 1/3 compared to the untreated fruit after six months of NA storage. Additionally, CA storage with the concentration of O2 of 1.5–3% and CO2 of 0.5–1.0% for 'Conference' slows down the softening rate [45]. Such CA storage conditions were applied in our study, and the CA-stored fruit softened significantly more slowly than the NA-stored fruit, but at an approximately equal rate as the NA-stored + 1-MCP-treated fruit. No differences were

observed after 180 days of storage in two years, whereas there was a significant difference in one year of the study, but it did not exceed 10%.

CA-stored + 1-MCP-treated pears had, by far, the highest firmness. Their average firmness loss within the three years of the study was only about 15% of the initial value, and such a difference does not affect consumer preferences [46]. During the first months of storage, the firmness loss was considerably higher in the non-1-MCP-treated fruit. 1-MCP application had a stronger influence on the softening rate than the gaseous composition of storage atmosphere during the first two months of storage, but the reverse was true (storage atmosphere more strongly reduced the softening rate than 1-MCP) at the end of the storage period. This could be explained by the way in which 1-MCP works: it slows down respiration by blocking ethylene receptors, but new non-blocked receptors are formed on the fruit skin with time [24], and the treated fruit becomes similar to untreated fruit.

**Table 5.** Effect of storage technology on the quality parameters of pears in 2011.


<sup>1</sup> One-way analyses of variance; data in the same column marked with the same letter are not significantly different within a year at α = 0.05 (Duncan's test). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.



<sup>1</sup> One-way analyses of variance; data in the same column marked with the same letter are not significantly different within a year at α = 0.05 (Duncan's test). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.

**Table 7.** Effect of storage technology on the quality parameters of pears in 2013.



**Table 7.** *Cont.*

<sup>1</sup> One-way analyses of variance; data in the same column marked with the same letter are not significantly different within a year at α = 0.05 (Duncan's test). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.

#### 3.3.1. Total Soluble Solids, Total Acidity and TSS/TA Ratio

Two very important qualitative criterions are: TSS, which is the content of solids, notably sugars, in a liquid, and TA, which is the content of acids and is assessed as the sum of acids converted into malic acid [7,12]. The TSS value usually grows in the initial storage period, which is caused by the degradation of polysaccharides into monosaccharides, but may decrease later as the fruit uses stored energy for respiration [47]. The speed of changes depends on the storage time and storage conditions [11]. The TSS value at harvest was similar and ranged between 12.1 and 12.6% in all years of the study. The TSS value changed most rapidly in the NA-stored fruit: it increased relatively quickly to reach the maximum after four or five months. After six months of storage, the TSS content dropped but was still significantly higher than at harvest. In NA-stored 1-MCP-treated pears, the TSS content grew more slowly and reached its peak in the last 2–3 months of storage. CA storage considerably reduced the ripening speed expressed by TSS, regardless of whether 1-MCP was applied or not. 1-MCP application may have an inconsistent effect, both in CA and NA storage [48]. In this study, TSS in the CA-stored pears gradually grew and rose to the maximum value after the entire storage period. This suggests that after 180 days of CA storage the 'Conference' pears did not start yet the excessive consumption of sugars in the respiration process.

Total acidity in the stored fruit changed according to a clearer pattern than TSS. Monthly measurements showed a steady decrease in TA at a rate dependent on the storage conditions and 1-MCP treatment. This corresponds with the findings by Hedges et al. [23], who additionally pointed to the harvest date as a factor affecting TA as a harvest delay resulted in a visible decrease in the initial TA value. What deserves mentioning as regards

our study is the weather conditions prevailing during the growing season because the initial TA values measured at the optimal harvest date varied between years. The differences were not large, but were consistent with the findings from a study on apples harvested in the same year in three European countries characterized by different weather conditions [49]. In this study, TA declined during storage irrespective of storage conditions, but it reached the lowest values in the NA-stored fruit and amounted to about 1/3 of the initial value after storage in each year. The 1-MCP-treated pears had a significantly higher TA, which never fell below 50% of the initial value. CA storage slowed down the degradation of acids still further and the combination of CA and 1-MCP prevented TA from dropping below 20% of the initial value after six months. Such conditions were shown to be highly effective not only for 'Conference' pears, but also for another popular European pear cultivar, 'Alexander Lucas' [23].

Consumers' perception of fruit sweetness and acidity depends not only on the absolute TSS or TA values, but also on the TSS/TA ratio [50]. In our study, the TSS/TA ratio changed very rapidly in each of the three years and was strongly influenced by storage conditions. For example, whereas the TSS of NA-stored pears did not vary by more than 20% in any of the years, their TSS/TA ratio varied by at least 300%. This was the only quality parameter that was different in virtually every month, regardless of storage conditions and 1-MCP application.

#### 3.3.2. Oxygen Radical Absorbance Capacity (ORAC)

Free radical scavenging activity is highly dependent on the species, cultivar, climatic conditions and harvest date [11]. During storage, it can remain unchanged, as was the case for 'Rocha' pears in Portugal [28], or it can grow, as shown by [51] for 'Golden Smoothee' apples. In our research, ORAC varied depending on storage conditions. Antioxidant capacity dropped considerably after NA storage, whereas the decrease was significantly smaller in the 1-MCP-treated sample. No changes were observed after CA storage, the ORAC value even rose after 1-MCP application. These differences can be explained by the differences in the ripening processes induced by increased respiration. Larrigaudiere et al. [52] found out that ripening may involve a noticeable decline in the content of ascorbic acid, which is one of the substances making up the antioxidant potential of fruit.

#### *3.4. Revenue Differences Related to Differences in Storage Technology*

Pome fruits, the most popular of which are apples and pears, are characterized by high storability [12]. As apples and pears can endure long-term storage, they can be supplied on the market all year round. The price of stored fruit is often similar to, and in some years even higher than, that of freshly harvested fruit [53]. In addition to market dependencies, including mainly the supply of fruit over a given period, the most important factor affecting the price is fruit quality [54]. For a grower who prepares fruit for sale it is important that it meets the parameters allowing its classification as top-class fruit for fresh consumption [55]. Fruit discarded after grading generates either no or very low revenue. This study compared the values of fruit stored in different conditions, taking into account the losses which arose during storage. Two types of loss were considered that could be measured based on the study results. Fruit mass loss caused by transpiration depends on relative humidity and respiration [12]. Assuming that the storage humidity was equal for all fruit, fruit mass loss resulted mainly from the rate of respiration. It has been shown in Section 3.2 that CA storage and 1-MCP application have a significant impact on respiration. The figures in Table 8 demonstrate that this translated directly in to the grower's revenue.


**Table 8.** Differences in revenues from the sale of the average pear yield in the EU at market prices in each year of the study, depending on storage conditions.

<sup>1</sup> Average pear yield per 1 hectare in the EU countries where production exceeds 1000 ha. <sup>2</sup> Price of pears obtained by growers in the period. <sup>3</sup> Average pear yield per ha in the EU countries in which pear production exceeds 1000 ha x average pear price for growers. <sup>4</sup> Standard deviation. NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.

> The difference between the highest and the lowest value of the yield that could be obtained per 1 ha did not exceed EUR 100 after the first month of storage (NA, NA + 1MCP, CA, CA + 1MCP), but it grew with each month. Even in the 2011/2012 season, in which the market prices were the most stable, this difference amounted to over EUR 350 after six-month storage. In the following season, during which the market prices rose much faster and the average yield was 2.5 t h−<sup>1</sup> lower, the difference was as much as about EUR 500. With the previous season's yield, the potential difference between revenues from the yield subjected to the simplest treatment (NA) and the most advanced treatment (CA + 1MCP) would have exceeded EUR 1500. In the last season, the difference decreased, but only to EUR 400, due to a high average yield.

The least advanced storage technologies give low protection against increased transpiration and the incidence of physiological disorders and fungal diseases [12,53]. The total losses caused by fungal diseases varied among individual years, because many of them originate in the orchard. The biggest difference was observed in the last year of the study—the revenues from the sale of the average pear yield in the EU after CA + 1-MCP treatment were over EUR 2300 higher than those calculated for the same yield after NA storage. CA + 1MCP-treated pears will be certainly easier to sell because also their other parameters are superior to those of the NA-stored pears. Since the costs of building a controlled atmosphere storage room, if properly designed, can be only about 5% higher than the costs of building a cold storage room [56], the financial advantages that can be achieved each year will surely more than make up for higher capital expenditures.

#### *3.5. Incidence of Diseases and Disorders*

Postharvest diseases of pome fruit result in substantial economic losses during storage worldwide every year [9]. In this study, the occurrence of fungal diseases, physiological disorders and visible physical damage of fruit flesh changed every year but always depended on treatment (Figures 1 and 2). Fungal diseases occurred more often than physiological disorders—this tendency is stronger in pears than in apples [53]. The biggest total losses of about 30%—were observed after six-month storage in 2013 (Figure 1C). The 1-MCP treatment of NA-stored pears reduced the total losses and visibly curbed the incidence of fungal diseases despite an increased incidence of internal browning. CA storage reduced the total losses by about half and the CA+ 1-MCP combination cut the incidence of diseases and disorders significantly. The smallest losses caused by diseases and disorders were noted in the first year of the study (Figure 1A), but 1-MCP application reduced the incidence of diseases and disorders only in the NA-stored fruit. In the second year, model results were obtained—starting from NA storage, each further treatment caused a drop in the total losses caused by diseases and disorder after six-month storage (Figure 1B). Other studies report different outcomes, though, which certainly depended on weather conditions in the orchard [23,57–59].

Fungal pathogens are the main source of losses during the storage and sale of pears [53]. This finding is confirmed by the results of this study except the CA-stored + 1MCP-treated pears. In 2011–2012, the share of fruit with physiological disorders was similar to that of those with fungal diseases (Figure 2). It seems that it was because the treatments very strongly reduced the incidence of fungal diseases. The most important fungal pathogens causing losses due to rotting are *Penicillium expansum*, *Botrytis cinerea*, and *Mucor piriformis* [58]. Other etiological factors include *Phialophora malorum*, *Alternaria* spp., *Cladosporium herbarum*, and *Neofabrea* spp. [60]. Their spores are ubiquitous in the orchard and infect also other tree parts. Fungal diseases were found to be the main loss factor in each year of the study. In 2013, the year of increased incidence of fungal diseases during storage, the percentage of fruit infected with fungal diseases in NA storage was about 10 times higher than the percentage of fruit showing symptoms of physiological disorders. Pears suffer from physiological disorders more rarely than apples [12], which was also clearly apparent in the two other years of the study. In 2011 and 2013, no differences in the incidence of fungal diseases were found between the NA-stored + 1-MCP-treated pears and the CA-stored pears, but the combination of CA and 1-MCP treatment allowed a reduction in the incidence of fungal diseases by half (2013) or by one-third (2011). In 2012, the incidence of fungal diseases decreased gradually and significantly between samples from the level observed after NA storage to the level noted after CA +1-MCP treatment.

Even though physiological disorders occur more rarely compared to fungal diseases, they are more difficult to contain by changing the composition of the atmosphere or applying 1-MCP [61–63]. The share of fruit affected by physiological disorders varied considerably between treatments, but it did not exceed 7.5% in any of the samples, which shows that physiological disorders are a minor cause of losses during storage.

The share of individual diseases and disorders causing losses during storage varied between years (Figure 3). Blue mold led to the biggest losses in all years of the study. It is caused by *Penicillium epansum*, a fungus commonly found in orchards [64]. The study clearly shows that the best way to control the disease is to improve the storage conditions. Every method to enhance the storage conditions significantly reduced the infection rate so that the incidence of blue mold in the CA-stored + 1-MCP-treated fruit constituted 20–30% of that found in the NA-stored fruit. Blue mold control has not only economic relevance for growers, but it also makes it possible to limit the development and spread of strains that produce patulin, a mycotoxin that affects humans [64].

Grey mold, which is caused by *Botrytis cinerea*, was another fungal disease that was observed to infect the fruit in each year of the study. The lowest grey mold incidence rate was noted in 2011 and it is probably due to the low number of infected pears that the differences between the treatments were small and generally insignificant. In the two subsequent years, the differences were bigger and CA storage had a noticeable limiting effect on the disease. A controlled atmosphere is recommended for the storage of grapes that are very susceptible to grey mold for fresh consumption [65].

**Figure 1.** Total fruit losses caused by fungal diseases and physiological disorders. Numbers with different letters were significantly different at *p* = 0.05 according to Duncan's test. The data are expressed as mean ± SD (n = 10). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP controlled atmosphere + 1-methylcyclopropene.

**Figure 2.** Fungal diseases and physiological disorders after storage in different conditions. Numbers with different letters were significantly different at *p* = 0.05 according to Duncan's test. The data are expressed as mean ± SD (n = 10). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.

Postharvest pathogens with economic importance for stored fruit also include bitter rot caused by *Colletotrichum* spp. [9]. In this study, the application of 1-MCP was much more effective in controlling this disease than the modification of storage atmosphere. Such a positive outcome of 1-MCP treatment was observed during the storage of apples [66]. Brown rot occurred in less fruit compared to the other fungal diseases. The differences in the incidence of bitter rot between treatments were small, although significant in some cases. In 2011, the highest number of infected pears was found in the samples stored in NA for six months. The other samples showed no differences. In 2012, 1-MCP had a beneficial effect on both NA- and CA-stored fruit, whereas, in 2013, a difference was noted only between the NA- and CA-stored samples, regardless of 1-MCP treatment.

**Figure 3.** *Cont*.

**Figure 3.** Impact of storage technology on the incidence of postharvest diseases and disorders in 2011–2013. Numbers with different letters were significantly different at *p* = 0.05 according to Duncan's test. The data are expressed as mean ± SD (n = 10). NA—normal atmosphere; NA + 1-MCP—normal atmosphere + 1-methylcyclopropene; CA—controlled atmosphere; CA + 1-MCP—controlled atmosphere + 1-methylcyclopropene.

Physiological disorders caused much smaller losses than fungal diseases, which confirms the findings usually reported after the storage of pears [7,11]. The differences between individual treatments appeared to be random and there was even a growth in the number of fruit showing the symptoms of physiological diseases after the application of 1-MCP. 'Conference' pears are prone to internal browning, the incidence of which may increase during CA storage [67,68]. This study only partially supported those findings and the 1-MCP treatment seemed to more strongly promote the development of internal browning, which agreed with the observations already made by Hendges et al. [63] during and after the storage of 'Alexander Lucas' pears. The possible reason for this is the loss of the antioxidant capacity and/or energy deficit caused by a reduction in the respiratory activity of fruit stored under CA. It has also been suggested that the inhibition of ethylene production may induce a stress response and thereby cause cell damage [69]. In our study, the significant increase in the incidence of internal browning was noted in 2011 and 2013 in both NA- and CA-stored fruit.

Senescent scald was a second physiological disorder observed in our study. It manifests itself with skin decolourization [12]. 1-MCP effectively reduces senescent scald in apples, but it is not clear how successfully it helps to control this disorder in pears [70]. Our study did not yield a clear result either; however, much more often than not, 1-MCP limited the occurrence of senescent scald. In 2011, the positive impact of 1-MCP was found after both NA and CA storage. In the following years, the difference was significant for NA-stored fruit. This suggests that the period of 6 months is too long for the storage of 'Conference' pears under NA conditions. The same conclusions were presented in an Italian study assessing the influence of 1-MCP on 'Abbé Fétel' stored in normal atmosphere [57].

#### **4. Conclusions**

A three-year study showed that the rootstock type, storage atmosphere, and 1-MCP application affected the storability of 'Conference' pears. This is the first study that presents a simultaneous assessment of the influence of the above factors on the quality parameters, the losses caused by diseases and disorders, the antioxidant capacity, and of the economic profitability of long-term storage of an important European pear cultivar.

Rootstock had the weakest influence on storability, and its effects were identified only when determining the fruit mass loss caused by transpiration and respiration.

Antioxidant capacity, just like various other quality parameters, was strongly affected by storage conditions. It grew during six-month CA storage after applying 1-MCP, whereas it stayed at the same level or declined in other storage conditions. This is an important fact that may enable the promotion of the consumption of 'Conference' pear long after harvest.

Most of the results obtained in the study on how six-month storage affects fruit quality and proceeds from its sale show that 'Conference' pears should not be stored in NA for so long. The high incidence of fungal diseases and physiological disorders after such a long storage period and the resulting losses cannot be compensated by the benefits of long-term storage. The economic analysis has revealed that it pays off much more to sell the fruit directly after harvest than after six months of NA storage. The application of 1-MCP alleviates the above-mentioned drawbacks, but does not fully make up for the expenditures. The best solution is to keep the fruit under CA and to additionally apply 1-MCP. This technology is recommended as it allows the preservation of firmness, an appropriate proportion between sugars and acids, and a high content of antioxidant substances.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/agriculture11060545/s1, Table S1–S3: Firmness (N) of untreated (control) and 1-MCP treated 'Conference' pears analysis after storage in normal (NA) and controlled (CA) atmospheres at 2011– 2013 year, Table S4–S6: Soluble solid content (%) of untreated (control) and 1-MCP treated 'Conference' pears analysis after storage in normal (NA) and controlled (CA) atmospheres at 2011–2013 year, Table S7–S9: Acidity of untreated (control) and 1-MCP treated 'Conference' pears analysis after storage in normal (NA) and controlled (CA) atmospheres at 2011–2013 year. Table S10–S12: Impact of storage technology on the incidence of postharvest diseases and disorders in 2011–2013.

**Author Contributions:** Conceptualization, G.P.Ł.; methodology, G.P.Ł.; software, K.R.; validation, G.P.Ł., K.R. and D.W.-T.; formal analysis, K.R.; investigation, G.P.Ł. and D.W.-T.; resources, G.P.Ł., K.R. and D.W.-T.; data curation, K.R.; writing—original draft preparation, G.P.Ł. and K.R.; writing—review

and editing, G.P.Ł.; visualization, K.R.; supervision, G.P.Ł.; project administration, G.P.Ł.; funding acquisition, K.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The publication was co-financed within the framework of the "Regional Initiative Excellence" programme implemented at the initiative of the Polish Ministry of Science and Higher Education in 2019–2022 (No. 005/RID/2018/19)', financing amount: PLN 12,000,000.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available on request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

