Abstract
Fresh blueberries, raspberries and blackberries are gaining popularity for their pleasant flavour and health benefits. However, their fresh supply, and the potential for market growth, are still limited by their short shelf life and seasonality. High respiration rates, delicate structures and high susceptibility to fungal decay are the main factors limiting the storability of these berry types. Current industrial practice for a longer shelf life relies heavily on cold chain and high humidity storage conditions. This typically results in a shelf life of 2–4 weeks for blueberries, and 2–5 days for raspberries and blackberries. This review discusses novel postharvest technologies from physico-chemical treatments (heat treatments, UV and edible coatings) to packaging-based solutions to improve the preservation of the freshness of blueberries, raspberries and blackberries through the supply chain. Sanitisation plays a crucial role in preventing fungal growth, while innovative packaging solutions act as complementary treatments to maintain quality attributes. The development and application of such technology combinations will increase berry shelf life, helping to satisfy the increasing global demand for these fresh berry products and improve consumer satisfaction.
Introduction
Berries are high-value crops that are not only seasonal but also highly perishable [1, 2]. Therefore, increasing their shelf life to enhance distribution options, and to extend availability outside of peak production periods has proven to be challenging [3, 4]. To date, the berry industry has predominantly relied on cold chain management (0–2°C) and high humidity (90–98%) for maintaining quality [5]. However, recent advances in technology, and the improved understanding of post-harvest berry physiology, could provide opportunities for improving the storability of berries.
The potential benefits of novel postharvest preservation techniques have been largely focused on strawberries [6–8], with fewer studies published for other commercialised higher value, but smaller production volume, berries including blueberries, raspberries and blackberries [5, 10]. This review aims to firstly summarise the current knowledge of the main factors that influence the shelf life of blueberries, raspberries and blackberries (Section 1) then evaluate a range of postharvest technologies that have been reported to extend the storability of these berries with focus on physico-chemical methods (e.g. heat, UV, sanitisation and edible coatings) (Section 2) and packaging-based solutions (Section 3). Additional technologies that have shown promise for shelf life extension of blueberries and raspberries are described in Section 4.
Causes of berry deterioration
Shelf life could be defined as the potential storage period of a product until it becomes unsuitable for human consumption or is rejected by customers [11]. Shelf life of fresh produce, therefore, is dynamic, depending on (i) the nature of the produce, (ii) preharvest and postharvest environmental conditions, and (iii) consumer expectation. The latter two factors lead to many variations in the definitions and assessments of shelf life of berries between studies. By their nature, shelf life of berries can vary considerably among cultivars, but in general, is still limited due to their high respiration rates (52–245 mg CO2 kg–1 h–1 at 20°C), fragile structures and high susceptibility to fungal decay [1, 12–14]. The shelf life of raspberries and blackberries can be as short as 2–5 days, even under optimum storage conditions (–0.5–0°C, 90–95% RH) [12, 14], but some blackberry cultivars such as ‘Navaho’ and ‘Shawnee’ were reported to last up to 21 days at 2°C [15]. The major species of blueberries grown commercially are highbush blueberry (Vaccinium corymbosum L.), rabbiteye blueberry (Vaccinium ashei Reade/Vaccinium virgatum Ait) and lowbush blueberry (Vaccinium angustifolium Ait) [16]. Depending on chilling requirements and winter hardiness, highbush cultivars are further classified as northern (800–1000 h of chilling and adapting to cold mid-winter temperatures < –20°C), southern (<550 h of chilling and not tolerating <0°C) and intermediate (400–800 h of chilling). Under current best commercial practice conditions (–0.5–0°C, above 90% RH, 10% CO2), northern highbush blueberries can last to 2 months, especially if the initial quality is high [4, 17]. In general, however, the shelf life is typically up to 2 weeks for lowbush, northern highbush and southern highbush cultivars [18, 19] and up to 4 weeks for rabbiteye cultivars [20]. The relatively longer shelf life of blueberries has been attributed to the presence of a protective layer of epicuticular wax comprising triterpenoids and β-diketone [21] that forms a natural barrier against moisture loss and pathogenic attacks [22].
Common symptoms of quality deterioration
Fungal decay
Mechanical damages can occur at any stage along the supply chain from the field to the packhouse or during the distribution and retail steps. It can markedly increase the susceptibility of berries to postharvest fungal infection resulting in decay. A visible decay incidence of 1–2% is considered enough to reduce the marketability of blueberries, raspberries and blackberries [23, 24]. For blueberries, the most common pathogens causing spoilage are Botrytis cinerea, Alternaria alternata and Alternaria tenuissim (Alternaria rot), and Colletotrichum gloeosporioides (anthracnose fruit rot) [25], while for raspberries and blackberries, they are B. cinerea, Cladosporium sp., Fusarium sp., Penicillium sp. and Rhizopus sp. [26]. As discussed below, management of fungal disease is hindered by a current lack of practical sanitisation treatments [27].
Changes in sensorial attributes
Sensorial indicators of the shelf life of berries include colour changes, dehydration and softening. Blueberries become darker with storage, turning from bright purplish blue to dark blue due to the loss of the waxy bloom [28], although the amounts of wax may vary with cultivars [21]. Raspberries darken quickly, turning from pink/light red to dark red after harvest, due to an increase in the anthocyanin content and the pH [29]. Picking raspberries at early maturity, i.e. pink and firm, was suggested to improve storability [29] but this may compromise the eating quality [30]. In contrast, blackberries should only be picked when fully black. Mottled blackberries (50% black) failed to darken even after 7 days at 2°C [31]. In addition, blackberries affected by red drupelet reversion disorder revert to red after suffering physical stresses [32].
Postharvest loss of moisture alters fruit appearance, texture and flavour, and reduces marketable weight [33]. Raspberries and blackberries are more prone than blueberries to dehydration due to the lack of epicuticular wax, with a maximum acceptable moisture loss of 6% [27]. In blueberries, moisture loss of >2–8% (depending on the cultivars) reduces the waxy bloom [33], causes loss of firmness and leads to shrivelling [34].
Softening also occurs in all berries owing to the solubilisation and depolymerisation of cell wall compounds due to enzymatic activities [35, 36]. An increase in water-soluble pectin and a decrease in sodium-carbonate-soluble pectin, hemicellulose and cellulose were observed during the softening of blueberry [35]. In raspberries, their quick postharvest softening was correlated to the activities of polygalacturonase and pectin methylesterase [37]. Meanwhile, softening of blackberries was caused by the solubilisation of cell wall pectins, rather than depolymerisation, as evidenced by a 50% increase of water-soluble uronic acids found during ripening [38].
Ethylene effects
Raspberries are non-climacteric [37], while the climacteric response of blackberries and blueberries varies depending on cultivars [16, 39], but all of them are low-ethylene producers (0.1–1.0 μL kg–1 h–1) [40]. Raspberries are highly sensitive to ethylene at pre-harvest [37], but the role of this plant hormone on postharvest quality and storability of raspberries, as well as of blackberries and blueberries, is still not well understood. In raspberries, ethylene increased the incidence of grey mould (B. cinerea) and darkened fruit colour from red to purple-red [41]. However, a recent study of five primocane raspberry genotypes (‘BP1’, ‘Crimson Treasure’, ‘Heritage’, ‘Nantahala’, and NY 10–24 – a breeding selection from the Cornell raspberry breeding program) found no correlation between ethylene production rates and fruit colour stability or anthocyanin content or shelf life [42]. Blackberry cultivars with higher ethylene production rates were linked with a shorter shelf life [43]. In blueberries, inhibition of ethylene action by its antagonist, 1-methylcyclopropene (1-MCP), had no effects on shelf life and quality of highbush cultivars [44], but accelerated loss of firmness in rabbiteye cultivars [45].
Other plant hormones that contribute to fruit ripening are abscisic acid (ABA) and auxins, but little has been done on their effects, particularly for blueberries, raspberries and blackberries. The activities of these phytohormones on a broader category of soft fruits have been reviewed [10].
Physico-chemical methods for shelf life extension of berries
Heat treatments
Heat treatments improve the shelf life of fresh produce by reducing physiological changes, eliminating insects of phytosanitary concern and controlling microorganisms and decay on produce surface [46]. While there has been no study applying heat treatments to raspberries and blackberries, possibly because of their delicate structures, beneficial effects were observed in blueberry treated by vapour heat and hot air (>30°C, RH > 90%) [47], and hot water (>40°C) dipping [48]. For example, incubation at 50°C for 30 min during storage reduced the respiration rate, malondialdehyde content and decay of southern highbush blueberries (‘Misty’, ‘O’Neal’ and ‘Sharpblue’) [47]. Similarly, dipping ‘Burlington’ blueberries in hot water (45–60°C) for 15–30 s lowered weight loss, shrivelling, fruit split, and decay caused by B. cinerea and Collectotrichum spp. after 4 weeks at 0°C and 2 days at 20°C [48]. However, the same study showed that the heat-treated fruits exhibited lower titratable acidity and total soluble solid contents and thinner wax bloom. Heat treatment might also trigger the production of stress-induced volatiles such as ethanol and ethyl acetate [48]. Such responses indicated that the benefits of heat treatment in blueberries would particularly require further optimisation of time-temperature combinations, as effective heat treatments are often near to the limits that the fruit can tolerate [49].
Ultraviolet (UV) irradiation
UV radiation refers to a broad band of wavelengths comprised of short-wave UV-C (200–280 nm), medium-wave UV-B (280–320 nm), and long-wave UV-A (320–400 nm). Although all wavelengths have microbicidal effects, UV-A is considered to have little practical value to shelf life extension of fresh produce because of the low absorption by living cells, while UV-C has stronger biocidal effects than UV-A and B, due to its high-energy state [50].
UV-C (short-wave)
Several studies have shown reductions in microbial load and improvements in produce shelf life treated with UV-C (1–8 kJ m–2). UV-C can alter microbial DNA or stimulate the production of photoproducts that suppress the germination of microbial spores [50, 51]. However, most of these studies examined blueberries whilst only one study was found for raspberries, and to the best of our knowledge, no studies were performed to date on blackberries. In blueberries inoculated with Escherichia coli O157:H7, 1 –10 min irradiation using UV-C at 200 J m–2 s–1 reduced the microbial counts on the calyx by 1.5–2.1 log CFU g–1 and on the skin by 3.1–5.5 log CFU g–1 [52]. Irradiation with UV-C at 1–4 kJ m–2 reduced ripe rot incidence caused by C. acutatum by 10% after storage for 7 days at 5°C followed by 2 days at 20°C in northern highbush ‘Bluecrop’ and ‘Collins’ cultivars [53]. Although showing some promise for blueberries, caution is required as higher doses (8 kJ m–2) of UV-C have been reported to increase decay incidence [53].
The effectiveness of UV-C in inactivating microorganisms has shown to be affected by the physical structure of the produce and the targeted microorganisms. The inactivation kinetics and E90 (the amount of energy required to kill 90% of the target microorganisms) of UV-C was influenced by surface roughness and spreading coefficients of the commodity being treated [54]. In that study, a 12-min treatment of UV-C at 10.5 kJ m–2 only reduced E. coli O157:H7 by 1.1 log CFU g–1 on inoculated raspberry. In addition, UV-C irradiation, although effective on bacteria, might not be effective against internal rot fungi due to its low penetration depth [55].
Reduced softening by UV-C was observed in boysenberries (Rubus ursinus×Rubus idaeus), primarily by disrupting cell-wall degrading enzymes [56], but there has been no study showing if the same effects could be achieved for raspberries and blackberries.
Additionally, the stress from the exposure to irradiation may induce the production of antioxidant compounds as part of the fruits’ natural defence mechanism [57]. At 2–4 kJ m−2, UV-C increased anthocyanins by 10% in ‘Bluecrop’ blueberries [53] and increased flavonoids by 10% in ‘Duke’ [57, 58], but not in ‘Collins’ cultivar [53]. Notably, the levels of antioxidants were particularly high immediately after the radiation treatment, but dropped sharply during storage [57, 58].
UV-B (medium-wave)
UV-B is of interest as an alternative to UV-C because it offers comparable efficacy while being less harmful to overall fruit quality [59]. However, this option might have limited commercial applications on berries. Although irradiation with UV-B at 6 kJ m–2 reduced weight loss, decay and delayed increase in the soluble solid – to – titratable acidity ratio in ‘Duke’ blueberries during 28 days of cold storage [58], observed changes in volatiles and phenolics in ‘Bluecrop’ blueberries irradiated by UV-B at different radiation intensities and durations could imply changes in sensory characteristics [59]. In ‘Navaho’ thornless blackberries, a daily 3-hour exposure to UV-B radiation (treatment not specified) significantly reduced fungal decay to 10% after 3 weeks and 40% after 4 weeks, compared to the control (47% after 3 weeks and 93% after 4 weeks) [60], but had adverse effects on fruit appearance when stored for more than 2 weeks.
Pulsed UV-light
Pulsed UV-light refers to the release of intense broad-spectrum electromagnetic radiation (100–1100 nm) energy in short bursts, which may allow greater decontamination potential than conventional UV-light radiation [20, 61]. In inoculated blueberries, treatment with 226 kJ m–2 reduced E. coli O157:H7 and Salmonella by 2.9 and 4.3 log CFU g–1, respectively [62]. In inoculated raspberries, pulsed UV-light at 720 kJ m–2 reduced E. coli O157:H7 by 3.9 log CFU g–1, and reduced Salmonella by 3.4 log CFU g–1 at 594 kJ m–2 [63]. Although pulsed UV-light affords good microbial control, its commercial viability may be discouraged by its effects on fruit sensory properties. High surface temperature due to the heat generated during treatments adversely affected blueberry appearance, including serious discolouration and loss of wax bloom [64]. A system voltage of 3800 kg m2 s–3 A–1 was noted for causing a cooked appearance and loss of integrity in blueberries [62]. Severe darkening and softening over 10 days of refrigerated storage was reported on raspberries treated at 282 kJ m–2 for 30 s [65].
A water-assisted pulsed UV-light system (wet pulsed light) immersing the produce in agitated water was proposed to minimise the temperature increase and allow fruit movement and rotation for better energy distribution [64]. Indeed, 60 s of water-assisted pulsed light reduced E. coli O157:H7 and a 4-strain cocktail of Salmonella by >5.8 log CFU g–1 in inoculated blueberries while fruit appearance remained unchanged [64]. Similarly, the same treatment reduced Salmonella by 3 log CFU g–1 in inoculated raspberries [66]. The feasibility of this technology, however, could be limited by the residual surface moisture left on berries, which can encourage microbial growth [27].
Sanitisation
In general, sanitisation by washing is not recommended for berries as it can promote mechanical damage and residual surface moisture, particularly in raspberries owing to their hollow structure [27]. The few studies that have reported the efficacy of sanitisers mostly examined blueberries with a focus on food safety rather than fruit quality and control of fungal decay. Washing with 50–100 mg L–1 chlorinated water failed to reduce microbial load in blueberries [67]. In contrast, sulphur dioxide (SO2) fumigation reduced fungal decay incidence of eight blueberry cultivars (‘Emerald’, ‘Jewel’, ‘Legacy’, ‘Misty’, ‘Reveille’, ‘Snow’, ‘South Moon’, and ‘Star’) over 28 and 35 days of cold storage at 1°C [68], and for six cultivars (‘Brigitta’, ‘O’Neal’, ‘Duke’, ‘Legacy’, ‘Elliott’, and ‘Aurora’) over 45 days at 0–1°C followed by 3 days at 20°C [69]. This was achieved without altering fruit quality even at an applied gas level as high as 194 nL L–1 s–1 [68]. The SO2-treated blueberries also had less decay when stored in ambient atmosphere compared to the untreated fruits stored in controlled atmosphere (3% O2, 3–12% CO2). A major obstacle of sulphite use is the consumer opposition to additives [70] and possible allergic reactions, particularly for asthmatics [71]. The use of SO2/sulphite must be declared on package labelling. Consumer concerns around possible health effects of sodium hypochlorite and sulphur dioxide have encouraged researchers to look for alternatives, with chlorine dioxide (ClO2) and ozone being the most studied for berries.
Chlorine dioxide (ClO2)
ClO2 is a powerful oxidiser, 2.5 times stronger than chlorine in oxidation capacity and is capable of penetrating microbial cell walls and altering cellular metabolism [72]. ClO2 can be used as an antimicrobial agent in both gaseous and aqueous forms. Aqueous ClO2 is stable at pH 6.0–10.0 [72], but less effective than its gaseous state which provides better penetration into small areas where water cannot penetrate due to surface tension [73]. In the USA, the highest level of aqueous ClO2 allowed for whole fresh produce is 3 mg L–1 and the treated produce must be subsequently washed with clean water [74].
Dipping blueberries in aqueous ClO2 (2 mg L–1, 2 min) maintained fruit firmness and resulted in significant but small reductions of decay incidence after an 8-day storage at 4±1°C (19% decay, vs. 22% in the untreated) [75]. Similarly, treatment with 1 and 3 mg L–1 for 10 s to 1 h reduced five foodborne pathogens (Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella sp., Staphylococcus aureus and Yersinia enterocolitica), yeasts and moulds in inoculated blueberries by <2 log CFU g–1 [76]. The effectiveness of aqueous ClO2 increased when higher concentrations and/or longer treatment times were applied. For example, increasing to 15 mg L–1 of ClO2 for 1 h reduced microbial load by up to 2.43–4.25 log CFU g–1 [76]. Likewise, dipping blueberries at a much higher concentration of ClO2 (100 mg L–1) for 10 min with agitation resulted in nearly 1–1.5 log CFU g–1 less total aerobic bacteria and fungi counts than wash water, without affecting sensory quality and overall acceptability over 12 days of storage at 4 and 20°C [77].
Gaseous ClO2 (4.1 mg L–1, 30 min at 23°C and 75–83% RH) reduced S. enterica and fungi by 0.52 log CFU g–1 and 3.02 log CFU g–1, respectively, in inoculated raspberries, and by up to 2.70 log CFU g–1 in blueberries [78]. Increasing treatment times to 60 and 120 min, and ClO2 concentrations to 6.2 and 8.0 mg L–1, however, provided no additional benefits. Treatment with ClO2 gas at 4 mg L–1, but over a longer duration i.e. 12 h was also effective in reducing microbial growth of blueberries inoculated with Listeria monocytogenes, Salmonella, E. coli O:157:H7 as well as yeasts and moulds [67]. Sensorial attributes were unaltered in both studies [67, 78]. ClO2 gas released from pads embedded in packaging was found to lower yeasts and mould counts of raspberries as well as reduce fruit weight loss and improve fruit redness intensity compared to the control after 8 days at 1°C [79].
Ozone
Ozone is a strong oxidising agent that has shown potential as a sanitiser for blueberries [80, 81], raspberries [82] and blackberries [83, 84]. It is typically applied in gaseous form. The ozone decomposes to O2 and is recognised as GRAS (Generally Regarded as Safe) by the US Food and Drug Administration [85].
Treatment of blueberries with 700 μL L–1 ozone for 2 or 4 days prior to controlled atmosphere storage (15% O2, 10% CO2) increased marketable yield by 4% and 7%, respectively, compared to the control [81]. Ozone storage also maintained firmness of ‘Ozark Blue’ blueberries after 10 days [86] and ‘Brigitta’ blueberries after 5 weeks, although this effect was not consistent across the three cultivars (‘Bluecrop’, ‘Coville’ and ‘Brigitta’) [80]. In ‘Chester’ blackberries, no fungal decay was observed following continuous exposure to ozone for 12 days at 0.1–0.3 μL L–1, compared to 20% for the untreated fruits [83]. In contrast, ozone storage (4 μL L–1 at 4°C and 2.5 μL L–1 at 12°C) was unable to control natural yeasts and moulds in ‘Ozark Blue’ blueberries over 10 days [86].
A further benefit of ozone is its ability to oxidise ethylene, thereby delaying ripening and senescence in ethylene-sensitive fruits including berries [87]. However, exposure to ozone can trigger a stress response altering the fruit metabolism. Respiration rates of ‘Coville’ blueberries increased immediately after 1–2 day(s) of constant treatments of 200 μL L–1 ozone [81]. In addition, the concentration of stress-indicating compounds such as methanol, ethanol and 2-nonanone (methyl heptyl ketone) increased markedly in ‘Coville’ blueberries stored in 700 μL L–1 ozone for 2–4 days, suggesting changes in flavours. Similarly, ‘Grandeur’ raspberries had better overall sensory quality than the control when stored for 9 and 15 days under continuous ozone flushing at either of 500 μL L–1 or a combination of 12 h of 200 μL L–1 + 12 h of 50 μL L–1. However, the higher concentrations resulted in lower scores for taste and flavours [82].
Industrial applications of gaseous ozone have been hindered by the risk of explosion and toxicity to the operators [88]. Alternatively, ozonated water, which is generated by passing ozone through water, could be a solution although residual moisture may restrict the practical feasibility of this option for berries. Ozonated water (0.6 mg L–1 for 3 min) was as effective as chlorine (200 mg L–1) in reducing yeast and moulds (0.6 and 0.87 log CFU g–1, respectively) in ‘Tupi’ blackberries without affecting the fruit colour, total phenolic content or antioxidant activity [84].
Edible coatings
Edible coatings are thin layers of edible biomaterials that can maintain fruit freshness and increase shelf life by acting as protective barriers preventing moisture loss and microbial growth, as well as limiting O2 and CO2 exchange (Table 1). However, the use of coatings can be incompatible with extension of shelf life and accelerate ripening resulting in higher productions of CO2, ethylene and fermentation-related volatiles as observed in blackberries coated with cassava starch and kefir [89] and starch-beeswax [90].
Recent studies of edible coatings on blueberries, blackberries and raspberries
Recent studies of edible coatings on blueberries, blackberries and raspberries
Findings from the literature indicate that chitosan-based edible coatings may be most promising at improving the shelf life of blueberries, raspberries and blackberries (Table 1). Chitosan (poly β-(1–4)-N-acetyl-D-glucosamine) is a polysaccharide derived from chitins that has been extensively studied to extend the shelf life of fresh produce [91]. In the context of berries, fruit weights were maintained, and large reductions of fungal decay were observed in various blueberry cultivars (5–30%) [92–95], ‘Tupy’ blackberries (45%) [96] and ‘Tullmeen’ raspberries (93%) [97]. However, while improving firmness of blueberries [92–95] and blackberries [96], chitosan coatings resulted in greater loss of firmness in ‘Tullmeen’ raspberries [52]. Additional technical concerns that need to be addressed before edible coatings have wide commercial applications include the maintenance of the natural wax bloom which affects the desirable perceived colour of blueberries [98, 99] and overcoming the difficulty in completely drying out any residual coating materials accumulating inside the fruit hollow of picked raspberries [97].
Berries sold in the fresh market are most commonly packed in clamshells with opening ratios of 3–10% [100]. This design is widely used in industry as it allows rapid and effective cooling of fresh produce [101], the escape of heat generated by produce respiration [102], as well as preventing ethylene accumulation [103] and moisture condensation. However, for berries with large surface area-to-volume ratios, the vents could make them more susceptible to freezing, chilling and drying damage [102]. For example, clamshells of raspberries packed in macro-perforated film with 200 holes of 3 mm-diameter per m2 (no modified atmosphere formed) exhibited 20% higher weight loss after 14 days at 7°C, compared to those packed in modified atmosphere films with low water vapour permeability [103]. The control was unsaleable by day 3 because of fungal growth. To address the limitations of clamshells, modified atmosphere packaging (MAP) and active packaging have been studied, both as alternative and complementary solutions. In addition, considering the high quantity of packaging needed versus berry weights, applications of eco-friendly packaging materials would make significant reductions in plastic waste and be key in meeting consumer concerns for sustainable packaging [104, 105]. An early study suggested that replacing polypropylene trays with cardboard was also beneficial in removing moisture and raspberry juice leakage [106].
Modified atmosphere packaging (MAP)
MAP has been used to suppress fungal decay [107, 108], reduce respiration rate [109], alter ethylene metabolism [110] and reduce weight loss (Table 2). Typical storage conditions recommended for blueberries range from 5–10% O2 and 10–12% CO2 at 0°C [1, 17], while for raspberries and blackberries, they range from 5–10% O2 and 10–20% CO2 [3]. Softening, decolouration and development of off-flavours particularly occurred in blueberries when CO2 was higher than 15% [111, 112]. An average loss of firmness of 24 N m–1 was also reported in nine raspberry cultivars stored under elevated CO2 atmospheres (12.5% CO2 plus 7.5% O2) for 4 weeks, compared to those kept in air [108]. In contrast, fermentation and formation of off-odours (acetaldehyde, ethanol and ethyl acetate) in raspberries were induced by low O2. An O2 level over 4% at 0°C, 6% at 10°C or 8% at 20°C was suggested as being needed to prevent fermentative induction [113].
Available literature on MAP applications for blueberries, blackberries and raspberries
Available literature on MAP applications for blueberries, blackberries and raspberries
aInitial atmo sphere inside packages was air (approx. 20% O2 and 0% CO2) if not mentioned. bResults were compared to fruits packed in vented clamshells unless stated otherwise.
The major concern for using MAP for berries is that inappropriate designs could lead to high concentrations of CO2 and depletion of O2 over time when subject to non-optimal temperature conditions during storage. This can result in atmospheres with less oxygen than those needed to maintain basal aerobic respiration. Most commercially available plastic films are unable to maintain appropriate ratios of O2 and CO2 permeability [6]. This explains why non-perforated films often resulted in softening in both blueberries [114] and raspberries [115–117]. In contrast, enclosed punnets to replace vented clamshells could allow CO2 accumulation during storage, but observed levels were insufficient to control fungal decay in blackberries [118]. In addition, the resistance of plastic packaging to water vapour permeation prevents water vapour due to produce transpiration escaping from the packaging and leads to moisture condensation that favours mould growth [27]. Perforated MAP could aid by accelerating gas exchange and increasing the permeability to water vapour. When LDPE films of the same thickness were used to pack 1.5 kg of blueberries, packages with two perforations each of 3 mm2, resulted in lower CO2 concentrations being accumulated (6.2% versus 9.2%) and greatly reduced the percentage of soft fruits (7.1% versus 27%), compared to the film without perforations [114]. However, commercial MAP films often have higher CO2 transmission rates than O2 transmission rates and therefore failed to create sufficient CO2 concentrations to control mould growth and delay quality losses [119]. For high-value products like berries, customised MAP bags based on produce respiration rates, produce weights, packaging properties and desirable atmospheres are currently commercially available. Industrial systems combining a device measuring the respiration rate, software to reliably calculate the numbers and sizes of microperforations, and an inline laser perforation system, are also available. Examples are PerfoTec® (PerfoTec B.V., The Netherlands) and StarMAP® (LaserMicro Rofin-BaaselLasertech, Germany) [120]. However, the high establishment and production costs associated with this MAP film technology may limit their commercial viability.
Trials on controlling ethylene during storage of blueberries, raspberries and blackberries have been conducted, although the roles of ethylene in quality and shelf life of these berry fruits are not fully understood. The most common ethylene scavenger is potassium permanganate (KMnO4), which is usually embedded in silica gel at a concentration of about 4–6%, and supplied in sealed, ethylene-permeable sachets [121] or it can also be added directly into polymeric films [122]. The ethylene action inhibitor 1-MCP can be released as a gas from sachets [123] and polymeric films when activated under high relative humidity conditions [124]. Alternatively, 1-MCP can be applied as a liquid formulation (SmartFreshtrademark, AgroFresh, Inc.).
There has been no work solely evaluating ethylene control for shelf life extension of raspberries and blackberries. The effectiveness of ethylene control on the postharvest storability of blueberries seems to depend on the cultivar and storage durations. In ‘Lanfeng’ blueberries, the addition of KMnO4 sachets reduced fruit weight loss and decay during both cold storage (60 days, 0°C) and on-shelf display periods (8 days, 20°C) [125]. Fruit firmness was also retained by suppressing the activity of cell wall enzymes (pectin methyl esterase, polygalacturonase, cellulase and β-galactosidase). Likewise, ‘Berkeley’ blueberries exposed to 1-MCP (1.0 μL L–1, 18 h) showed less softening (12.3%) than the control fruits during 8 days of storage at 4°C [126]. The treated fruits also showed a slower decrease in titratable acidity and soluble solid contents. Similar findings were reported for ‘Lateblue’ blueberries stored in air for 21 days after being treated with 1-MCP (0.3 μL L–1, 24 h) at 20°C, but the advantages were not found after 28 days, or among samples stored under controlled atmosphere (3% O2, 11% CO2) for 60 days [127]. 1-MCP up to 0.4 μL L–1 for 24 h at 20°C had no influence on the shelf life quality of two other highbush cultivars (‘Burlington’ and ‘Coville’) [44]. ‘Mistý’ and ‘Blue Cuinéx’ blueberries treated with 1-MCP (1.0 μL L–1, 12 h) had similar respiration rates and quality attributes as the control after 14-day storage at 4°C, except for the firmness of ‘Mistý’, which was about 1.2 times higher than the untreated fruits [128].
Combinations of ethylene control with other methods could be beneficial through synergistic effects. For example, a permeable packaging film with an ethylene transmission rate of 1.98 mL m–2 h–1 kPa–1 in combination with high-oxygen MA (95% O2, 5% N2) reduced ethylene accumulation inside raspberry packages to 37 μL L–1, compared to 60 μL L–1 in samples in high oxygen MA and high barrier film. This combination of ethylene permeable film and high oxygen MA limited mould growth after 14 days at 7°C with only a single mouldy fruit being found in a 150 g raspberry sample (approximately 40 fruits) [129]. Spraying ‘Blue Cuin
Additional approaches for shelf life extension of berries
As detailed in Table 3, a range of alternative approaches have been trialled and have shown some success for shelf life extension of blueberries and raspberries. These technologies include sanitisation using electrolysed water (EW) and hydrogen peroxide, and high O
Additional approaches for shelf life extension of berries
Additional approaches for shelf life extension of berries
Electrolysed water (EW) is generated from the electrolysis of NaCl solutions producing mainly hypochlorous acid and has shown to be a safe, effective and inexpensive sanitiser for a range of fruits and vegetables (reviewed by Rahman et al., 2016) [130]. Acidic electrolysed water (AEW, pH 2.3–2.7) has been shown to efficiently sanitise blueberries (Table 3), but the application of AEW to fresh sensitive produce is limited by its acidic property [131]. These concerns have shifted industry focus toward using neutral electrolysed water (pH 6–8), which has similar antimicrobial effects to AEW but with fewer associated risks to produce physiology [72, 131].
Hydrogen peroxide (H2O2) is also a strong oxidiser that can be used in either aqueous or vapour form at concentrations between 1–5%. H2O2 is safe as the compound quickly decomposes to water and oxygen [132]. Although only a few successes have been reported for the use of H2O2 as a postharvest sanitiser for fresh produce, positive results were obtained in blueberries [133] (Table 3). However, the oxidation of anthocyanins, and therefore, changes in fruit colour and decrease in antioxidant properties, could be a critical drawback of H2O2 technology in berries [134].
It is possible to extend the shelf life of blueberries, raspberries and blackberries by applying novel postharvest technologies together with good temperature and RH control. However, as highlighted in this review, there are still knowledge gaps and challenges that will need to be addressed. For raspberries and blackberries, the application of novel technologies is limited by their fragile drupelet structures and higher respiration rates. Potentially for these berries, gas-based sanitisation (particularly SO2 and ClO2) and UV-B irradiation treatments avoid the application of water and will offer the most benefits in terms of shelf life extension. In contrast, blueberries, characterised by their more robust structure and lower respiration rates, will benefit from additional options such as aqueous sanitisers, mild heat treatments and edible coatings. The latter two, however, are often limited by their adverse effects on appearance of blueberry wax blooms.
Applications of the reviewed technologies as combination hurdle treatments could increase their effectiveness for shelf life extension compared to individual technologies alone. For example, MAP pallet liners combined with sanitisation methods might reduce weight loss, retain sensorial quality and prevent fungal decay. As raspberries are ethylene sensitive, MAP combined with ethylene control could be considered to delay fruit senescence, as demonstrated in strawberries [135]. Other new technologies, such as packaging for minimising mechanical damage induced by vibrational effects, will arguably become more important as supply chains lengthen. While there are current packaging options that mitigate vibration damage, their efficacy for extending the shelf life of berries will need to be evaluated.
Lastly, while consumers’ purchasing behaviour at retail is strongly determined by visual appearance, it will be flavours and texture that will determine consumers’ acceptance and repeat purchasing behaviour [136]. Therefore, it will be necessary to investigate more explicitly how the reviewed technologies affect sensorial properties and consumer acceptability, which is currently lacking.
Conflict of interest
The authors have no conflict of interest to report.
Funding
The authors report no funding.
Footnotes
Acknowledgments
This work was supported by the Australian Research Council’s Industrial Transformation Training Centres scheme under Grant IC140100024. We thank Michele Buntain for valuable comments on an earlier draft of the manuscript.