STRAWBERRY QUALITY MANAGEMENT STRATEGIES
STRAWBERRY, FRAGARIA VESCA VAR. HORTENSIS / ROSACEAE
Postharvest Atmosphere Management
The quality marks for strawberries are linked to their appearance (colour, type, shape, free of any defect) and also to firmness, taste, scent and nutritional value. The optimal levels of moisture in storage range from 90 to 95%. The maturation criteria are based on the colour of the surface of the berry. At least 1/2 or 3/4 of the surface of the berry must be red or pink, depending on the maturity stage.
Pre-cooling is an essential process to maintain the quality of strawberries. The only case in which it is allowed to elude this technique is when they are consumed in 24 hours time after the harvesting.
The strawberries’ adaptability to conservation is scarce and they can only be stored from 5 to 7 days. For longer conservation, the temperature must be maintained at 0ºC and the relative moisture between 90-95% or even higher, both in storage as in transport. The deterioration is very fast once they are removed from the cold storage room.
Controlled atmosphere with a concentration of carbon dioxide around 10-15% reduces the growth of Botrytis cinerea and the strawberries’ respiration rate, thus increasing their post-harvest life. The application of this gas is used in the United States for transport over great distances.
Distribution
Relative moisture during transport must be of 90-95% or even higher; temperature must be around 0 ºC. During transport, the use of small packing improves conservation.
Postharvest Problems
The physiological damages are not a serious problem for strawberries, maybe because they are sold very fast, due to their short period of life. However, damages caused by pathogensrepresent a real problem. Diseases are the main cause of losses in strawberries. Fungicides are not used in post-harvest. Therefore, fast cooling at 0ºC prevents from damages in the fruit. Shipping under high levels of carbon dioxide is one of the best methods to control them. The ideal thing would be to maintain injured strawberries away from the healthy ones.
The two most important types of rot affecting strawberries are those produced by Botrytis cinerea and the fungus Rhyzopus stolonifer. Botrytis cinerea is the most important disease affecting strawberries. This kind of rot is known as grey rot. The fruits are covered by a cottony mycelium and the tissues soften. It cause many post-harvest losses. This fungus continuous its growth over 0ºC, although it is very slow.
In the case of the second type of fungus, Rhyzopus stolonifer, its spores are usually circulating in the air and it is easily propagated. It causes a soft rot and the tissues loose their juice, that drips from the packages. This fungus does not usually grow at temperatures below 5ºC.
How to influence strawberry quality
Growing a crop with a high quality is important for maximum profitability. Agronomically, there is a lot that a grower can do to improve strawberry quality and proper nutrition is central to this. Balanced crop nutrition is needed to achieve good quality strawberries. All nutrients are needed by plants but potassium, calcium and boron have particularly important roles in achieving high quality.
Strawberry fruit consists of approximately 90% water and 10% total soluble solids. They are a good source of folate and potassium, as well as dietary fiber, manganese and antioxidants. The fruit is high in vitamin C and consumption of 10 fruit per day virtually meets all of the recommended dietary requirements for this vitamin. The main soluble sugar components are glucose and fructose. The primary acid is citric acid. Strawberry flavor is a key characteristic and is a complex mix of the sweetness, acidity and aroma of the fruit. The most intensely flavored fruit have a high TSS and also acidity.
Balanced crop nutrition will improve quality
Good crop nutrition will ensure the production of fruit that handles well and has a longer shelf life with the right balance of sugars and acidity plus a good aroma and taste.
All nutrients are needed by plants but potassium, nitrogen, calcium and boron have particularly important roles in achieving high quality.
Potassium plays a key role in increasing fruit sugars, acidity and improving its taste
Nitrogen – particularly nitrate forms – used at adequate rates during flowering and fruiting – will maintain taste and acidity without encouraging rots
Calcium – is essential to maintain fruit integrity, health and a longer shelf-life, with reduced damage when handling
Boron – also helps to maintain good fruit strength
Boron deficiency causes malformed fruit
Boron plays a key role in fruit quality with poor supply leading to smaller, malformed fruit.
Effect of foliar boron and calcium on yield and quality
Potassium improves key quality characteristics
Potassium is particularly important in terms of berry quality providing a high sugar and acid content, and a good taste to the fruit.
Effect of potassium on fruit acidity and sugar content
Nitrogen excess leads to softer fruit
Excess nitrogen during fruit growth and development has an adverse effect on fruit quality. It increases disease susceptibility and the softening of fruit. This leads to a shorter shelf life and fruit that is quicker to rot. Maturity can also be delayed and fruit malformed, resulting in reduced yields.
Effect of nitrogen on anthracnose
Excess nitrogen also encourages diseases such as anthracnose crown rot. Fertigation can help ensure that applied nitrogen is better utilized by the plant and not available to encourage rots. Although, even under more controlled fertigation systems, high nitrogen rates may still result in greater disease severity.
Other factors influencing strawberry quality
Variety selection is particularly important and producers should select resistant cultivars with the quality characteristics that most suit their intended market.
Use of appropriate mulches or growing systems to minimize soil contamination is important to physical quality.
Good hygiene, sanitation and appropriate in season fungicides and pesticides will help to provide fruit that is less at risk of pest and disease damage.
Maximizing growth through appropriate irrigation will help ensure good water and nutrient flow to the developing fruit.
Refrigerated storage and transport, utilizing controlled carbon dioxide environments will help to maximize the shelf life of the fruit that is picked.
Quality Evaluation of Strawberry
The strawberry is a kind of very popular fruit with its attractive appearance, unique flavor, and nutritional compositions. Computer vision is a necessary useful and effective technology for analyzing and evaluating the external and internal qualities of strawberries. Computer vision was used for grading different quality levels based on the analysis of size, shape, and volume. Meanwhile, computer vision has been developed for the nondestructive measurement of internal qualities such as internal bruising, firmness, and sweetness using imaging analysis in narrow bands of wavelengths. Thus, based on the computer image analysis, the computer vision technology can be developed into hyperspectral and multispectral imaging techniques for strawberry fruit quality evaluation and control.
STRAWBERRY QUALITY control APP
Strawberry quality is a combination of appearance and flavor. According to grade standards, the berries should be of one variety or similar varietal characteristics with the cap (calyx) attached, which are firm, not overripe or undeveloped, and which are free from mold or decay and free from damage caused by dirt, moisture, foreign matter, disease, insects, or mechanical or other means.
Fruit should be free from: (1) Attached stems (2) Mold (3) Decay; (4) Insects or when there is visible evidence of the presence of insects; (5) Mummified berries; and, (6) Clusters, and free from damage caused by (1) Shriveling; (2) Broken skins; 3) Scars; and (4) Green berries.
Quality characteristics of interest to the grower include plant vigor, yield potential, ease of harvest and disease resistance.
Flavor top
Strawberry flavor is related to degree of ripeness. Fruit that is harvested at full ripeness will have the highest sugar content and flavor. However, fruit is often harvested prior to full ripeness so that it is firm enough to be shipped. New varieties have improved firmness for shipping. Additionally, improvements in post harvest handling and shipping conditions have allowed growers to ship fruit that is more ripe.
Texture top
Strawberry fruit should be firm but not crunchy. Excessively ripe fruit can be too soft.
Color top
Strawberry varieties vary in color from deep red to red-orange. For a given variety, fruit should be fully colored, without white or green tips. Calyx color is also important. The calyx should remain green and healthy.
Strawberries are bright colored at harvest with healthy green calyxes. Water loss will cause the fruit to become wilted and dull.
Size top
Strawberry fruit range is size by variety and as the season progresses. Many new varieties produce very large fruit that are over 2 inches long and more than an inch in diameter. There are grade standards for strawberries related to diameter. U.S. No. 1 fruit must be at least 3/4 inch in diameter and U.S. No. 2 fruit must be at least 5/8 inch in diameter.
Shape top
Strawberry fruit should be well developed and normally shaped. Varieties vary slightly in shape, but most are conical. Occasionally fruit is misshapen due to lack of seed development, which results in differential growth of fruit flesh. Grade standards state that undeveloped means that the berry has not attained a normal shape and development due to frost injury, lack of pollination, insect injury, or other causes. "Button'' berries are the most common type of this condition.
Strawberries grown under normal conditions will have fully developed fruit. Frost injury is a common cause of undeveloped fruit. Frost injury is avoided through site selection and overhead irrigation during frost events. Undeveloped fruit can also be caused by tarnished plant bug feeding on developing fruit and flowers. Damage can be avoided by scouting for insect pests and well-timed insecticide applications. Control recommendations can be found in the Midwest Small Fruit Pest Management Handbook and Midwest Small Fruit and Grape Spray Guide for Commercial Growers, or through your local Cooperative Extension Service. Lack of pollination can also lead to undeveloped fruit but it is a less common cause. Installation of beehives can improve pollination.
Defects and Disease top
Defects
Soft fruit, wet stem scars, attached stems, and green berries are common defects of blueberries. These must be graded out during packing.
Disease
The most common diseases affecting fruit is gray mold (Botrytis). Control recommendations can be found in the Midwest Small Fruit Pest Management Handbook and Midwest Small Fruit and Grape Spray Guide for Commercial Growers, or through your local Cooperative Extension Service.
Gray Mold (Botrytis cinerea) can occur in the field or in storage. Gray mold is most likely to develop when there is a full canopy of leaves creating a micro-climate with high humidity. Infection occurs primarily during bloom, so control measures are timed accordingly.
Insects
Tarnished plant bug is the most common pest that affects fruit quality. This pest feeds on developing flowers and fruit causing seed abortion and subsequent lack of fruit development. Other insect pests such as spittle bug and root worms affect plant vigor.
Shelf Life top
Decreased quality during postharvest handling is most often associated with water loss and decay.
It is important to remove field heat as soon as possible to prevent water loss. Most shippers will use forced air cooling to achieve temperatures of 32-34˚F. Berries freeze at 31˚F. A shelf life of 5-7 days can be expected if these temperatures are maintained at 90% to 95% humidity.
STRAWBERRY QUALITY ENHANCEMENT
Impact of Beneficial Microorganisms on Strawberry Growth, Fruit Production, Nutritional Quality, and Volatilome
Valeria Todeschini1*, Nassima AitLahmidi2, Eleonora Mazzucco3, Francesco Marsano3, Fabio Gosetti3, Elisa Robotti3, Elisa Bona1, Nadia Massa3, Laurent Bonneau2, Emilio Marengo3, Daniel Wipf2, Graziella Berta3 and Guido Lingua3
1Dipartimento di Scienze ed Innovazione Tecnologica, Università del Piemonte Orientale, Vercelli, Italy
2Agroécologie, AgroSup Dijon, CNRS, INRA, Univ. Bourgogne Franche-Comté, Dijon, France
3Dipartimento di Scienze ed Innovazione Tecnologica, Università del Piemonte Orientale, Alessandria, Italy
Arbuscular mycorrhizal fungi (AMF) colonize the roots of most terrestrial plant species, improving plant growth, nutrient uptake and biotic/abiotic stress resistance and tolerance. Similarly, plant growth promoting bacteria (PGPB) enhance plant fitness and production. In this study, three different AMF (Funneliformis mosseae, Septoglomus viscosum, and Rhizophagus irregularis) were used in combination with three different strains of Pseudomonas sp. (19Fv1t, 5Vm1K and Pf4) to inoculate plantlets of Fragaria × ananassa var. Eliana F1. The effects of the different fungus/bacterium combinations were assessed on plant growth parameters, fruit production and quality, including health-promoting compounds. Inoculated and uninoculated plants were maintained in a greenhouse for 4 months and irrigated with a nutrient solution at two different phosphate levels. The number of flowers and fruits were recorded weekly. At harvest, fresh and dry weights of roots and shoots, mycorrhizal colonization and concentration of leaf photosynthetic pigments were measured in each plant. The following fruit parameters were recorded: pH, titratable acids, concentration of organic acids, soluble sugars, ascorbic acids, and anthocyanidins; volatile and elemental composition were also evaluated. Data were statistically analyzed by ANOVA and PCA/PCA-DA. Mycorrhizal colonization was higher in plants inoculated with R. irregularis, followed by F. mosseae and S. viscosum. In general, AMF mostly affected the parameters associated with the vegetative portion of the plant, while PGPB were especially relevant for fruit yield and quality. The plant physiological status was differentially affected by inoculations, resulting in enhanced root and shoot biomass. Inoculation with Pf4 bacterial strain increased flower and fruit production per plant and malic acid content in fruits, while decreased the pH value, regardless of the used fungus. Inoculations affected fruit nutritional quality, increasing sugar and anthocyanin concentrations, and modulated pH, malic acid, volatile compounds and elements. In the present study, we show for the first time that strawberry fruit concentration of some elements and/or volatiles can be affected by the presence of specific beneficial soil microorganisms. In addition, our results indicated that it is possible to select the best plant-microorganism combination for field applications, and improving fruit production and quality, also in terms of health promoting properties.
Introduction
Plants interact with a huge variety of beneficial microorganisms such as arbuscular mycorrhizal fungi (AMF) and plant growth-promoting bacteria (PGPB), which can improve both plant fitness and production.
AMF belong to the Glomeromycotina subphylum (Spatafora et al., 2016) and are symbiotically associated to the roots of the majority of land plants, including the main crop species. They play an ecologically important role and provide various ecosystem services such as the improvement of nutrient uptake, soil aggregation, and protection against biotic and abiotic stress (Lingua et al., 2008; Gianinazzi et al., 2010; Smith et al., 2010; Antunes et al., 2012; Boyer et al., 2015).
PGPB comprise different functional and taxonomic groups (Ghosh et al., 2003), among which Pseudomonas fluorescens is one of the most extensively studied species (Duijff et al., 1997; Vazquez et al., 2000). They directly enhance plant growth by a variety of mechanisms, such as mobilization of soil nutrients, atmospheric nitrogen fixation, phosphorus solubilization, and phytohormone synthesis, especially IAA (Indole-3-Acetic Acid) (Glick, 1995). PGPB can also act indirectly, suppressing phytopathogens (Benizri et al., 2001). Some fluorescent pseudomonads were shown to improve mycorrhizal root colonization (Gamalero et al., 2008, 2010), extraradical hyphal growth (Mugnier and Mosse, 1987) and AMF spore germination (Frey-Klett et al., 2007), functioning as mycorrhiza helper bacteria (MHB). Conversely, AMF can influence the chemical composition of root exudates, which are a major nutrient source for the bacteria in the rhizosphere (Hegde et al., 1999; Artursson et al., 2006).
The strawberry, Fragaria × ananassa Duch., belonging to the family Rosaceae, is one of the most cultivated berry crops in Europe and all around the world (Akhatou and Fernandez-Recamales, 2014a; Prat et al., 2014). The large size and the deep red color of its false fruit, besides its characteristic aroma (Ulrich et al., 2007), are the results of the crossing between the North American strawberry Fragaria virginiana Mill. with the Chilean strawberry Fragaria chiloensis (L.) Mill. (Darrow and Wallace, 1966). The consumption of this fruit, fresh, frozen, or processed (juices, jams, yogurts, etc.), in the daily diet is an important source of healthy compounds (Tulipani et al., 2009) such as fibers, vitamins (in particular C and B9), minerals (mainly potassium and magnesium), and antioxidants (flavonoids, phenolic acids, and ellagitannins) that can prevent or reduce certain types of cancer, cardiovascular diseases, obesity, type II diabetes and cellular damage induced by reactive oxygen species (ROS) (Olsson et al., 2004; Wang and Lewers, 2007; He and Giusti, 2010; Giampieri et al., 2012). Therefore, the strawberry nutritional values and beneficial properties of its bioactive components (a heterogeneous group of biologically active non-nutrients, mainly represented by polyphenols), together with a complex blend of volatile organic compounds, make it a highly appreciated fruit (Diamanti et al., 2012). Among polyphenols, flavonoids, especially anthocyanins in the form of pelargonidin and cyanidin derivatives, are responsible for the strawberry bright red color. These two characteristics are markers of ripening, a process in which anthocyanin accumulation in the fruit flesh and/or skin is highest (da Silva Pinto et al., 2008; Giampieri et al., 2012; Jaakola, 2013). Moreover, the key role of these molecules, both in fruit tolerance to the environmental stresses and in ameliorating post-harvest quality and shelf life, is well-known (Xu et al., 2014). The biosynthesis and accumulation of anthocyanins in fruits are regulated by different factors: genetic (e.g., MYB genes), environmental (light exposure, temperature, nutrient availability such as nitrogen and calcium) and developmental (sugars, in particular sucrose, and hormones including abscisic acid, jasmonate, and ethylene) (Carbone et al., 2009; Jaakola, 2013).
Texture (firmness, juiciness, and crispness), color, sweetness and aroma are the most important quality indicators for strawberry consumers (Christensen, 1983; Colquhon et al., 2012; Negri et al., 2015). Sugars and organic acids are the principal soluble component in ripe fruit (Perez et al., 1992; Moing et al., 2001). The ratio between sugars and acids strongly affects fruit aroma and flavor (Montero et al., 1996; Kallio et al., 2000). For this reason it is considered an index of fruit quality and its variations depend on plant genotype, ripening stage, climatic factors, and soil type (Kallio et al., 2000; Akhatou and Fernandez-Recamales, 2014b; Valentinuzzi et al., 2014). Fructose, glucose, and sucrose, the most abundant soluble solids in strawberries, are responsible for fruit sweetness (Perez et al., 1997). Sugars are involved in many biological processes: in particular, sucrose is involved in fruit ripeness (Jia et al., 2011) and anthocyanin synthesis (Gollop et al., 2002), while fructose is a precursor of the volatile compounds furanones (Sanz et al., 1997; Forney et al., 2000), that contribute to the caramel aroma of the fruit (Prat et al., 2014). Organic acids are important for the maintenance of nutritional values and fruit quality (Mikulic-Petkovsek et al., 2012), besides their involvement in the gelling process of pectin (Cordenunsi et al., 2002). The most abundant organic acid in strawberry is citric acid, whose concentration is responsible for about 92% of total acidity (Cordenunsi et al., 2002; Mahmood et al., 2012), while malic, tartaric, shikimic, quinic, and fumaric acids are present in very small amounts.