Effects of Chitosan-Coated Paperboard Trays on the Botrytis cinerea Formation and Sensory Quality of Bilberries (Vaccinium myrtillus L.) and Redcurrants (Ribes rubrum L.)

ABSTRACT

Comparison of a chitosan-coated food contact material (active packaging) against the standard is carried out in this study. The shelf life of bilberries and redcurrants has been assessed, using visual inspection. Berries passed a professional sensory assessment on organoleptic deterioration. The berries were stored on consumer/retail-orientated standards to achieve a valid assessment on shelf-life impact. An increased shelf life can be achieved between 2 and 4 days, using 2% chitosan-coated food contact material.

KEYWORDS:

• Active packaging
• berry
• Botrytis cinerea
• chitosan
• shelf life

Introduction

Bilberries (Vaccinium myrtillus L.) (also known as European blueberry) and redcurrants (Ribes rubrum L.) are berries that have been consumed by humankind since ancient times. Germany is a key grower/harvester for both berries; however, in recent years, there has been a decline of bilberry harvest due to the high amounts of manual labor required to pick the berries from the wild bushes, the introduction of highbush blueberries (Vaccinium corymbosum L.), which is also known as American blueberry, and competition through Austria, Poland, and the Balkan states. Regarding the production of redcurrants, Germany is the third largest producer after Russia and Poland (Franke et al., 2012).

Table 1. Results of the sensory scoring of the storage trial for bilberries and redcurrants.

		Sensory attribute
Berry type	Food contact material	Appearance	Odor	Aroma	Flavor
Bilberry	Control	6.7^a	7.8^a	8.0^a	7.4^a
Bilberry	Chitosan coated	6.6^a	7.7^a	7.8^a	7.2^a
Redcurrant	Control	8.2^a	6.3^a	7.8^a	8.4^a
Redcurrant	Chitosan coated	8.5^a	6.3^a	7.7^a	8.7^a

Means with superscript (^a) are not significantly different.

More and more research has confirmed the health benefits attributed to the consumption of berries, such as increased cognitive brain functions (Bowtell et al., 2017), decreased risks for depression (Khalid et al., 2017), a protective function against cardiovascular diseases and cancer (Neto, 2007), and many other health benefits. Berries are a rich source of important bioactive compounds (phenolic compounds like flavonoids and anthocyanins) and vitamins, minerals, and carbohydrates (Baltes and Matissek, 2011), and are often related to the group of so-called “superfoods” (Williams, 2016).

Limiting factors for the shelf life of berries are the high perishability through ripening, softening, water loss, inadequate storage temperatures, and mechanical injuries through harvesting and processing. Another major issue is the infection with spoilage-causing microbes and fungi. Most prominent examples are Rhizopus rot and Botrytis cinerea (Syn.: Botryotinia fuckeliana, often referred to as gray mold) (Nair and Allen, 1993). Standard food contact material for retail are polyethylene terephthalate (PET) plastic trays or paperboard trays. Active packaging describes materials that have compounds incorporated into their material matrix that have an active function (e.g., antimicrobial function), in addition to their passive containment and/or protection function. Such a material on a paperboard base incorporating chitosan has been developed previously (Lee et al., 2003). Chitosan is a biopolymer (linear, randomly distributed β-(1→4)-linked D-glucosamine and N-acetyl-D-glucosamine polysaccharide) that can be extracted from the cell walls of fungi or is made from chitin (natural material of the shells of crustaceans). Chitosan has proven bactericide and fungicide properties (Ravi Kumar, 2000).

The aim of this work is to investigate the impact of chitosan-coated paperboard on the shelf life of bilberries and redcurrants for the prevention of Botrytis cinerea formation.

Materials and methods

Materials

Paperboard trays for the intended use of berry storage (Smurfit Kappa GmbH, Germany) coated with 2% high molecular weight chitosan solution (w/v) (Sigma-Aldrich Chemie GmbH, Germany) were used in this study. The solution is prepared using 0.5% (v/v) glacial acetic acid (grade 99.85%) (Sigma-Aldrich Chemie GmbH, Germany) dispersed in purified water and adjusted to a final pH of 5.0 using 1 mol l⁻¹ sodium hydroxide solution. Uncoated paperboard trays were used for the control group.

Organic, ripe bilberries and redcurrants were purchased from a local farmer in Germany (Lower Saxony) and freshly supplied one day postharvest. The berries supplied had uniform, standard retail quality in terms of color appearance and firmness.

Shelf-life assessment

The bilberries and redcurrants were transferred postharvest into the paperboard trays (100 g per tray). Samples were stored in climate cabinets with set conditions of +5°C to represent home storage conditions. Ten replicate paperboard trays were used for each condition (untreated material and chitosan coated). Shelf-life assessment was performed in duplicate.

Spoilage with Botrytis cinerea was visually determined, using a stereo microscope (ZEISS Stemi 508 Stereo Microscope, Germany). The effectiveness of chitosan-coated paperboard trays regarding shelf life was examined by the incidence of berries infected with Botrytis cinerea per tray. Shelf life was assessed daily; however, full counts were taken on days 0 (day of delivery), 4, 8, and 14.

Sensory evaluation

Sensory assessment was performed by a trained panel consisting of nine panelists (age 30–45, six females, three males, all Europid). A 9-point hedonic scale was used, where 1 represents the lowest score (e.g., “very soft”) in terms of taste/texture and 9 was the highest score for these attributes (e.g., “very firm”). Questions asked covered taste, odor, aroma, color, and texture (firmness). The berries were stored at +5°C for 14 days. Samples (100 g), before presentation to the panel, were equilibrated for 15 min at +25°C, in order to intensify the flavor. Samples were issued in randomized order and only those samples fit for consumption (free from mold) were presented to each panelist. The individual sensory booths were equipped with artificial daylight and an odor-free environment (filtered air). The sensory assessment was performed in duplicate and in compliance with ethics requirements.

Data analysis

Collection of results was performed by Compusense software (Compusense Inc., Canada) and the results were analyzed by STATISTICA (Statistica, USA). Sample means are arithmetic means and the results were analyzed using Fisher LSD for multi-comparison (P ≤ 0.05). Graphics were generated with SigmaPlot software (Systat Software Inc., USA).

Results and discussion

Shelf-life assessment

Bilberries were stored postharvest at +5°C for a period of 14 days. Figure 1 shows the results of the storage test for the standard food contact material versus the material coated with chitosan.

Figure 1. Percentage of infected bilberries (Vaccinium myrtillus L.) with Botrytis cinerea over the storage time (14 days) at +5°C.

Significant differences (P ≤ 0.05) between bilberries stored in untreated and coated trays were found. The shelf-life assessment showed that fruits stored on chitosan-coated paperboard had an extended shelf life of 2 days based on the microscopic method.

The shelf-life assessment of redcurrants also provided significant differences (P ≤ 0.05) between both types of packaging. Here, an extension of shelf life of 3–4 days can be achieved, due to the lower microscopic infestation counts.

The differences between the Botrytis cinerea infestation and both packaging types are shown in Figure 2.

Figure 2. Percentage of infected redcurrants (Ribres rubrum L.) with Botrytis cinerea over the storage time (14 days) at +5°C.

The longer extension of shelf life for redcurrants can be explained in the composition of the fruits. Compared with bilberries, redcurrants have significantly higher contents of organic acids (citric acid, malic acid, tartaric acid, fumaric acid, shikimic acid) and bilberries have additionally a higher concentration of sugars (Mikulic-Petkovsek et al., 2012). A higher sugar concentration in bilberries favors the growth of Botrytis cinerea. Once the berry tissues are infected with Botrytis cinerea, the parasite perforates the berry skin, accesses leaking sugars and amino acids, and continues its growth (Kassemeyer and Berkelmann-Löhnertz, 2009; Staats, 2004).

Antimicrobial active packaging solutions can provide a significant contribution in the extension of shelf life, leading to a reduction in waste and hence lower costs for the farmers. Several research studies have proven the effectiveness of chitosan for a large number of yeasts and molds (El Ghaouth et al., 1992; Roller and Covill, 1999) and a fruit-coating of 2% chitosan also has proven its effectiveness on the inhibition of Botrytis cinerea formation on strawberries and raspberries (Zhang and Quantick, 1998).

Sensory evaluation

For all attributes, no significant differences (P ≤ 0.05) were detected between the standard food contact material and the chitosan-coated material. This means that no deterioration of the organoleptic characteristics was identified. This is important to meet the current EU legislation requirements (The European Parliament and the Council of the European Union, 2004), if intended to introduce such a material onto the market. Table 1 presents the results of the sensory panel. All samples have been rated between “acceptable” and “good” for flavor, odor, and aroma attributes. The firmness attribute was rated between “very soft” to “firm.”

Conclusion

Paperboard-based food contact material coated with 2% chitosan has a preventive effect on the growth of Botrytis cinerea, the most common mold for berries. This chitosan-coated paperboard material has also been described in its effectiveness in other fruits, such as strawberries (Vu et al., 2011) and mangoes (Chien et al., 2007). A plausible explanation might be that chitosan, with its fungicide properties, delays the lag-phase for the growth of Botrytis cinerea. This delay might benefit the product quality and help extend the shelf life of bilberries by about 2 days and for redcurrants by up to 4 days and could be subject of future research. Furthermore, future research should focus on the use of other soft tissue fruits, such as raspberries. In addition, a higher concentration of chitosan coating should be investigated to find out whether the berry shelf life can be increased further without any impact on the flavor perception. Higher concentrations may be limited due to chitosan’s astringent flavor, which has a proven negative impact on strawberry flavor perception (Han et al., 2006). However, it might not influence the flavor perception of already astringent fruits, such as bilberries, currants, blackthorn, and the common sea buckthorn.

Sensory evaluation has shown that the coating of chitosan in a concentration of 2% does not have a significant impact on the odor, berry appearance, aroma, or flavor. This makes the use of the tested chitosan-coated paperboard fit for use as a food contact material in the European Union, since it does not lead to the deterioration of the organoleptic properties.