Use of xanthan gum and calcium ascor­ bate to prolong cv. Butirra pear slices shelf life during storage

: After cut, pear fruit ( Pyrus communis L.) during shelf life can be sub­ jected to color and ﬂavor changes. To maintain ﬂesh colour and ﬁrmness, dif­ ferent technologies could be employed during shelf life, such as chemical, phys­ ical and edible coating treatments. In the present study, the eﬀects of two edi­ ble coating formulations containing xanthan gum and calcium ascorbate on fresh­cut pear fruit were investigated. After harvest, 200 fruits were cut and coated with Xanthan Gum (XAN) and distilled water or Xanthan gum + Calcium Ascorbate (ASC), respectively, while control (CTR) pear fruit slices were soaked in distilled water and lastly packed in polyethylene terephthalate (PET) pack­ ages sealed with a composite ﬁlm (PP­PET). All samples were stored at 5 ± 0.5°C with RH 90% for 10 days. Measurements were carried out at 3, 5, 7 and 10 days of storage evaluating visual quality score, browning index, color, total solid sol­ uble content (TSS), ﬂavor, ascorbic acid content and total phenols content. The results showed that ASC treatment was the most eﬃcient treatment in terms of color changes, ascorbic acid content, visual quality score and browning index, until the 7 th day of storage. Moreover, ASC treatment reported lower mean values in terms of taste and ﬂavor score if compared to CTR and XAN treatments. Untreated pear slices (CTR) kept good values concerning ﬂavor score until the 3 rd day


Introduction
'Butirra' pear fruit is cultivated in southwestern Sicily and belongs to sicilian MIPAAF PATs (traditional agricultural products).After harvest, usually occurring between the second decade of July and the first decade of August, they must be consumed as they are easily rotten and are sub jected to fast decay.It's easy to understand how freshcut 'Butirra' poor shelflife is a key barrier to its commercialization especially for the fast weakening of tissues and surface's browning that happens after cut due to the action of polyphenol oxidase (PPO) (Amiot et al., 1995;Hodges and Toivonen, 2008).Edible coatings are widely employed since they prevent the loss of quality acting like a selective barrier to gas exchanges between food and external environment.Xanthan gum enhances all these properties and also controls the rheology of the final food product exhibiting pseudo plastic properties in solutions (Palaniraj and Jayaraman, 2011).As a generally recognized as safe (GRAS) molecule, xanthan gum is an exopoly saccharide produced by the fermentation of a carbo hydrate by cultures of Xanthomonas campestris.It is then refined by extraction with ethanol or 2 propanol, dried, and powdered (FDA).Calcium ascor bate is the calcium salt of ascorbic acid that is widely used as an antioxidant whose reducing action against quinones and diphenols prevents browning of unprocessed fruit as it only produces colorless deriv atives; it is a reducing agent, capable of promoting the chemical reduction of the pigment precursors responsible for browning, acting by reducing oben zoquinone or dihydroxyphenol or irreversibly inacti vating PPO, promotes the regeneration of antioxi dants and acts synergistically with complexing agents (Araújo, 2004).Furthermore, this cation can maintain cell wall structure by binding to pectins and forming calcium pectate (Vilas Boas et al., 2009).Calcium sta bilizes the membranes and cell walls, preserving their integrity and functionality and protecting them from being cleaved by hydrolytic enzymes that cause fruit softening (Poovaiah, 1986;Vilas Boas et al., 2009).Xanthan gum combined with antioxidant agents had positive effects on the reduction of weight loss and browning, preventing the loss of firmness, and the growth of psychotropic microorganisms, molds and yeasts in minimally processed apples and pears (Sharma and Rao, 2015;Allegra et al., 2022).The aim of the present study was to evaluate the effective ness of edible coating based on xanthan gum and xanthan gum enriched with calcium ascorbate on freshcut cv 'Butirra' pear fruit stored in passive atmosphere.

Materials and Methods
The experiment was carried out in 2021.'Butirra' pear fruit (Quince BA29 rootstock and intermediate 'Butirra Hardy' graft) were harvested during the sec ond week of august in a commercial orchard located in Zafferana Etnea (Catania, Italy), Italy, (730 m above sea level).The soil is a sandy clay loam (63% sand, 19% silt, 18% clay), with pH 6.9 and active carbonates lower than 5%, trees were trained as a free palmette.Fruits were handpicked at an optimal ripening stage tested with Lugol solution.All trees received the same conventional cultural cares from planting until the end of the current experiment.After harvest, fruit were cold stored and transported at University of Palermo and stored at 5±1°C in cold room the night before the analysis.

Experimental design
Two hundred fruit were harvested from 20 trees and brought to the laboratory where they were dipped in chlorinated water (100 ppm of free chlo rine) for 360 s to sanitize them.Defective fruit were discarded and the remaining were sorted by firmness (4.1±1 kg/cm 2 ) and average weight (120± 20.2 g).Quality indexes were calculated the first day of analy sis, particularly, color (CIELab), flesh compactness and total solid soluble content (TSS).
Fruits were selected for weight, maturation index, caliber and absence or presence of defects and sani tized with OxVirin 200 ppm and H 2 O by soaking for 30 minutes.Then, they were peeled and cut.Edible coatings were applied by dipping and solutions were formulated as follows: i) Control (CTR): fruits were dipped in distilled water and used as control; ii) XAN: the solution was made by mixing 3% of xan than gum in distilled water using a magnetic stir rer; iii) ASC: the solution was made by mixing 3% of xan than gum and 2% of calcium ascorbate in distilled water using a magnetic stirrer.After treatments, fruit were packed in PET boxes, sealed with a composite PPPET film and stored at 5±1°C with 95% relative humidity (RH) for 10 days.Trials were carried out at 3, 5, 7 and 10 days of stor age evaluating visual quality score, browning index, color, total solid soluble content (TSS), sensorial analysis, ascorbic acid content and total phenols con tent.

Weight loss
The following formula was adopted to determine weight loss during storage Weight loss (%) = [(W1W2)/(W1)] x 100 Where W1 and W2 represent initial weight (T0) and measured weight at 3, 5, 7 and 10 days of storage with a precision balance (Gibertini, Italy), respec tively.At the beginning of trial period all boxes had homogeneous weight (100 ± 2.1 g).

Color
Flesh color was measured throughout the experi ment on the first day of analysis (0) and on the 3 rd , 5 th , 7 th and 10 th day of storage.Color was measured through a portable colorimeter (Chroma Meter CR 400, Konica Minolta Sensing Inc., Tokyo, Japan) equipped with an 8 mm measuring head and a C illu minant (6774 K).The white standard plate of the manufacturer was used for calibration.Chromatic dif ference (DE) was calculated using the following for mula to express the magnitude of difference between the nonaged pulp and stored samples: All trials were carried out in triplicate and data were reported as ± mean standard error (SE n=3).

Firmness
Fruit firmness was tested with a texture analyzer equipped with a 2.5 cm flattip (Instron 5564, MA, USA).The maximal force was expressed in kg/cm 2 and slices were compressed with a speed of 5 mm/s to a depth of 4 mm.

Total solid soluble
Total solid soluble content was determined on pear fruit juice extracted from samples at each stor age time using a handheld refractometer (ATAGO Palette PR32).

Total phenols content
Total phenols content was quantified according to Sortino et al. (2022).30 grams of fresh tissue for each replication was homogenized with methanol on 1:10 ratio and then filtered through a Whatman grade N.1 filter, the application of reduced pressure allowed the concentration of methanolic extracts and the residue was then suspended in 50% aqueous methanol and used for phenolic content quantifica tion.Phenols content was determined through a spectrophotometrical analysis at the wave lenght of 700 nm and results were expressed in gallic acid equivalent (mg kg 1 fresh weight).

Ascorbic acid
Ascorbic acid content was analyzed at each sam pling date with the Megazyme kit (Bray Business Park, Bray, Co., Wicklow, Ireland) as reported by Allegra et al. (2015).

Visual quality score
Edible coatings effect on 'Butirra' freshcut slices was evaluated at each storage time on six slices used as single replicates, for each treatment.Six trained judges used a list of descriptors wrote down in pre liminary meetings.Descriptors involved the medium value of color, integrity and appearance on pear fruit slices as reported by Allegra et al. (2022).Descriptors were quantified using a 5 points hedonic scale where 5= very good, 4= good, 3= sufficient (limit of mar ketability), 2= poor (limit of usability) and 1= very poor (inedible).

CO 2 and O 2 inside packaging
O 2 and CO 2 content inside packages was analyzed at each sampling date using a PBI Dansensor Checkpoint O 2 CO 2 analyzer (Ametek Mocon, MS, USA) equipped with infrared detectors.

Statistical analysis
The experimental design consisted in two treat ments and one control, observed at 0, 3, 5, 7 and 10 days at 5°C after treatment.Nine slices were used as single replicates and analyzed at each sampling date.Analysis of variance was applied (Systat 13.0 for Windows was used as statistical software) and the significance of data (p≤0.05)was evaluated with Tukey's test.

Total solid soluble content
Our results showed a general increase in TSS dur ing storage; control slices content increased about 29.03% during the 10 days of storage while XAN and ASC treatments scored an increase of 16.30% and 15.45%, respectively.CTR slices increase is probably due to the greater water loss of the untreated sam ples, which also results in higher percentages of weight loss (Fig. 1).XAN and ASC treatments were more efficient than CTR in limiting the increase in TSS content, this phenomenon is due to ripening process es that result in the hydrolysis of starch into mono and disaccharides (Mahajan et al., 2004) and in the activation of respiration processes where sugars are the main substrate used (Dong et al., 2004).Significant differences occurred between treatments on 5 th and 7 th day while on 10 th day XAN and ASC treatment recorded the same value.closely linked, as previously discussed, with TSS con tent data.

Color changes and browning index
Color monitoring during cold storage showed a change in L*, a* and b* values in 'Butirra' in all treat ments (Table 1).A low decrease in L* values occurred in XAN treatment.Color changes are more limited in ASC treated samples.
At the end of cold storage period (T10), ΔE reached its highest in CTR samples and its lowest in ASC treated samples.Untreated 'Butirra' fruit slices recorded the highest browning values at each sam pling date, and it began to increase sharply from day five, while XAN and ASC treatments recorded the lowest values.Significant differences occurred between ASC and XAN treatments from 5 th day of storage while no significant differences were record

Weight loss and firmness
Weight loss during cold storage showed an increase in all sample slices (Fig. 2).CTR samples showed a higher percentage of weight loss if com pared to other treatments.ASC treatment showed lower weight loss than XAN and control pear slices.The effectiveness of ascorbate calciumcan be attrib uted to the ability of calcium to preserve the com pactness of cell structures by limiting the action of pectolytic enzymes.In other work on freshcut pear the use of calcium ascorbate could be responsible of the maintenance of cellular wall structure since calci um maintains glycosidic bindings stable avoiding the collapse of cellular wall and the subsequent loss of liquids (Akhtar et al., 2010).Grant et al. (1973), on the other hand, showed that the maintenance of firmness in calciumtreated fruits may be due to its accumulation in cell walls, which facilitates the cross ing of pectic polymers by increasing wall strength and cell cohesion.Obtained results for weight loss are ed at 10 th day between the two edible coatings for mulations (Fig. 3).A similar trend occurred in Sharma and Rao (2015) on freshcut pear treated with Xanthan gum for 8 days of cold storage.

Sensorial analysis and visual quality score
Sensorial analysis was carried out at each storage time and at harvest time high values were recorded for all descriptors except for bitterness, sourness, herbaceous and floral odor and flavor.A slower decrease in treated pear slices was recorded.All treatments recorded higher values than CTR one at each sampling time and no offflavors or negative descriptors were found in edible coating treated sam ples until 10 th day of storage.In fact, 'Butirra' pear slices treated with ASC and XAN recorded positive val ues for descriptors concerning sweetness, compact ness, external color uniformity, juiciness and pear fla vor during all storage time while, on the contrary, CTR samples recorded increasing values of negative descriptors such as bitterness and sourness as early as the 5 th day of analysis.Overall rating was quite posi tive in treated samples until 10 th day (Fig. 4).
Visual quality score test enhanced that a decrease in mean values occurred in all treatments registering significant differences since the 3 rd day of analysis (Fig. 5).Since day 3, CTR samples recorded mean val ues under 3 (limit of marketability).Instead, the judges evaluated ASC samples on 10 th day at the limit of marketability (score=3).Xanthan coating had a positive effect on visual score of freshcut pear (Sharma and Rao, 2015) extending their marketability until the 10 th day.

Total phenols and ascorbic acid content
No significant differences were showed between treatments on pear slices ascorbic acid content.During storage, total phenol content increases slowly in both CTR and XAN treatment.After 7 days a sharp increase was observed in treated and untreated sam ples and significant differences occurred between  ASC and other treatments (Fig. 6).An increase of total phenol content is possible in stress conditions after cutting or in low temperature (Amodio et al., 2014).
CO 2 and O 2 inside packaging A limited O 2 consumption and CO 2 production occurred during storage in both XAN and ASC treat ments while higher values were registered in CTR samples (Fig. 7).Significant differences occurred from the 3 rd day between treated and untreated samples, particularly, CTR samples registered a value of 1.5 kPa for O 2 and a value of 21 kPa for CO 2 at 10 th day of storage.Both XAN and ASC kept the two para meters more stable.Similar trends were observed on freshcut peach treated with calcium lactate and ascorbic acid and on breba fig fruit stored in passive atmosphere (Allegra and Colelli, 2015;Allegra et al., 2015).

Conclusions
The two different formulations based on calcium ascorbate and Xanthan gum preserved pear slices of 'Butirra' during the 10day storage at 5°C.Positive effects were observed on browning, weight loss and firmness up to the 10 th day, furthermore, the two edible coating formulations preserved the sensory attributes of fresh cut 'Butirra'.Our results showed that Xanthan gum with calcium ascorbate treatment improved the retention of firmness, browining and weight loss than control slices.This result was con firmed by the sensorial analysis in which positive descriptors showed positive values until the 10 th day of storage while untreated samples began to develop offflavor and offcolor since day 3.

Fig. 1
Fig. 1 Total solid soluble content of untreated (CTR) and treat ed with Xanthan gum and Xanthan gum + calcium ascor bate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after for 3, 5, 7, 10 days of storage at 5°C.At each sampling date, different letters indicate sig nificative differences between treatments.p≤0.05 was used in the Tukey's test.The data are provided as the mean ± SE (n = 3).

Fig. 2
Fig. 2 Weight loss (%) and firmness (kg/cm 2 ) of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calci um ascorbate (XAN and ASC) Pyrus communis L. cv.'Butirra' fruit slices at 0 time and after for 3, 5, 7, 10 days of storage at 5°C.At each sampling date, different letters indicate significative differences between treatments.p ≤ 0.05 was used in the Tukey's test.The data are provided as the mean ± SE (n= 3).

Fig. 3
Fig. 3 Browning index of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calcium ascorbate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after for 3, 5, 7, 10 days of storage at 5°C.At each sampling date, different letters indicate significative differences between treatments.p≤0.05 was used in the Tukey's test.The data are provided as the mean ± SE (n = 3).

Fig. 5
Fig. 5 Visual quality score of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calcium ascorbate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after for 3, 5, 7, 10 days of storage at 5 °C.At each sampling date, different letters indicate significative differ ences between treatments.p≤0.05 was used in the Tukey's test.The data are provided as the mean ± SE (n = 3).

Fig. 6
Fig. 6 Total phenols content (mg kg 1 fw) of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calci um ascorbate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after storage for 3, 5, 7, 10 days at 5°C.At each sampling date, different letters indicate significative differences between treatments.p≤0.05 was used in the Tukey's test.The data are provid ed as the mean ± SE (n = 3).

Fig. 7
Fig. 7 O 2 and CO 2 inside packaging of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calcium ascorbate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after for 3, 5, 7, 10 days of stor age at 5°C.At each sampling date, different letters indi cate significative differences between treatments.p≤0.05 was used in the Tukey's test.The data are provid ed as the mean ± SE (n = 3).

Table 1
Color slices (CIELab index) and color variation (ΔE) of untreated (CTR) and treated with Xanthan gum and Xanthan gum + calci um ascorbate (XAN and ASC) Pyrus communis L. cv.Butirra fruit slices at 0 time and after for 3, 5, 7, 10 days of storage at 5°C At each sampling date, different letters indicate significative differences between treatments.NS = not significant.p ≤ 0.05 was used in the Tukey's test.The data are provided as the mean ± SE (n= 3).