Strawberries: Nutritional Content, Spoilage and Preservation Methods

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Fresh fruit and vegetables have existed on this planet for millions of years. They provide an excellent source of nutrients for a vast majority of organisms and their consumption has increased significantly over the past few decades. Reasons for this increase include; promotion of the products and increase awareness of their health benefits. There is also a better transport system allowing for a more versatile selection of fresh produce and in turn a rapid growth of healthy fresh cut, ready to eat produce (Barth et al., 2010).

Demands from today’s society expect fresh fruit and vegetables to be of the highest quality, retaining all of their health factors and freshness while keeping process to a minimum. Like other fresh fruit and vegetables, strawberries (Frageria x ananassa Duch) are an important part of a healthy diet. They are a false fruit and a member of the rose family (Harris and Mitcham, 2007).

The small yellow seeds embedded in their scarlet flesh are the true fruits and is what makes strawberries high in fibre. While they are low in calories, they are excellent sources of nutrients containing folate, potassium, manganese, pantothenic acid and are high in vitamin C. They provide small amounts of vitamins K, B1 & B6 (Murray et al, 2005). As well as beneficial phytochemical antioxidants such as anthocyanin and ellagitannins which are relevant to human biological activities (Giampieri et al, 2012).

Fresh strawberries are a popular choice of berry worldwide. The consumers’ perception of fresh strawberries can be related to intrinsic and extrinsic attributes. These attributes can be classified as “search attributes” meaning colour, appearance and price, “experience attributes” meaning taste, aroma, healing factors and flavour” and finally “credence attributes” meaning health benefits, calorie status and microbiological safety (Ragaert et al, 2004).

Spoilage

Like most other fresh fruit and vegetables, strawberries are subject to spoilage. Spoilage is the process in which food decays and renders it unacceptable for human consumption. Many fresh fruit and vegetables immediate beneficial conditions for microbiological survival and proliferation. Strawberries are very perishable lasting only a few days unless refrigerated. Microbial growth spoils fruit much quicker depending on the variety of intrinsic factors expressed by the food as well as extrinsic factors affecting the food.

Intrinsic factors of fresh strawberries include nutrient content, pH, water activity (Aw), redox potential (Eh), anti-microbial components and biological structures. All these factors contribute to its spoilage different ways.

· Microorganisms present on fresh strawberries are able to utilise nutrients such as vitamins, minerals and sugars that the strawberry provides. Low vitamin B levels compared to meat products mean that gram negative bacteria and moulds are able to survive and multiply on the fruit.

· Fresh strawberries are mainly acidic and produce a low pH value of about 3.3–3.6 (Harris and Mitcham, 2007). Although this pH value is too high to sustain some bacterial growth, filamentous fungi and acidic bacteria are to exist in these conditions and spoil it.

· Water is essential to for microorganisms to grow. It is responsible for many of their biochemical reactions and therefore plays a vital role in many of their biological functions. Like most fruits, strawberries have a high water content of about 0.90–0.99. This contributes to their spoilage as it allows for many microorganisms to mature and multiply.

· Redox potential means the ability within a chemical species to donate or accept an electron. Strawberries have a redox potential between 202–240mV. This state is very dependent on the ripeness of the strawberry. Overripe strawberries will have a lower antioxidant levels meaning that it will have a lower reducing state compared to ripening strawberries. This biochemical process makes strawberries very perishable and susceptible to going off (Dilbaghi and Sharma, 2007).

· Strawberries contain natural antioxidants and phenols. These are evolutionary antimicrobial components of the fruit that allow it to defend itself from microorganisms by inhibiting their microbiological activities. Strawberries have a soft interior due to their high water activity and a thin epidermis. This epidermis is surrounded by a waxy cuticle layer consisting of cutin polymers and acts as a physical barrier to pathogens. This epidermis is sensitive and physical damage can cause cracks and splints in their physical barrier. These lesions can grant microorganism’s access into the fruits interior thus leading to the fruits spoilage (Leque et al, 2003).

Extrinsic factors involved in the spoilage of fresh strawberries can include; application of fungicides, biocides and pesticides at pre-harvest, temperature, humidity and oxygen levels

· The application of aerial fungicides and biocides is an important process in ensuring that microbial pathogens are reduced from airborne spores and soil contaminants prior to harvesting. Pesticides are also crucial insects act as vectors for microorganism and cross contamination is a result. Chewing insects also cause physical damage to the fruit and in turn inoculate the wound sites with pathogens allowing further spoilage from microbial organisms (Lund, 1992).

· Strawberries are grown at temperate regions around the world. Microorganisms present on the fruit and the surrounding area will also grow at these temperatures. It is essential that fresh strawberries are kept refrigerated in order to slow down or stop proliferation of certain mesophiles. This will inevitably favour psychrophiles and psycotrophs to proliferate as they strive in temperatures between 0–15°C. Spoilage will occur but at a much slower rate than at room temperature (Ragaert et al, 2004).

· Relative humidity, water activity and temperature are all interlinking factors that contribute to spoilage of fresh produce. Strawberries have a high water activity, however humidity can transfer water from the gas phase and increase water content. This will in turn lead to spoilage as the associated flora will utilise the water provided. High temperatures will lead to low humidity due to evaporation. Condensation due to various storage temperatures can also accumulate on the strawberries. This will then allow moulds, yeasts and some bacteria to affect and spoil the fruit (Ragaert et al, 2004).

· Fresh strawberries are influenced by the amount of oxygen present and in turn affects the redox potential that the fruit expresses. Oxygen can allow moulds and gram negative to proliferate and spread. This can be controlled by the addition of CO2, which is a strong parameter for the inhibition of moulds and gram negative bacteria.

Pathogens Associated

Many microorganisms reside on fresh strawberries. They come in the form of bacterial pathogens such as Salmonella, E. coli 0157:H7, Shigella, Camptlobacter and Listeria manocytogenes (Lund, 1992; Mahovic et al, 2005). Viruses including Norwalk-like virus and Hepatitis A are also associated with strawberries and is common in many outbreaks of foodborne diseases. These pathogens are spread by certain vectors such as soil, irrigation water, or airborne spore’s or insects. Fertilizers used, the spread of infection from one fruit to another and poor sanitization during processing can also contribute to contamination (Niu, M. T., et al, 1992). Fermentation of trawverries can be caused by yeasts such as Saccharomyces, Torulopsis, Hansenula and Candida (Ragaert, P., et al, 2004)

Moulds account for two thirds of the spoilage of strawberries. Pathogens such as Cylindrocarpon spp., Fusarium spp., Pythium spp., Phoma spp., Rhizoctonia spp., Gnomonia fragariae, Pratylenchus penetrans and Meloidogyne hapla are all found associated with root rot complex or black root rot of strawberries. This causes stunted plant growth, yellow leaves, black root systems and poor production of small berries (LaMondia, 1999; Moročko, 2006).

One fungus in particular is responsible for the grey mould that accumulates on fresh strawberries. Botrytis cinerea is a common colonizer on strawberries and is a very selective and intelligent plant pathogen. It possesses many enzymes such as cutinases and lipases that can degrade the strawberries protective epidermal layers and survive its low pH levels (Van-Kan, 2006).

Methods of Preservation

Traditional methods for preserving strawberries include, drying, canning, freezing and making jams and jellies (Harris and Mitcham, 2007).

One traditional method for sustaining fresh strawberries is the use of Modified Atmosphere Packaging (MAP). Fresh strawberries are usually stored in closed containers with holes punctured into the lid. Once picked, fresh strawberries continue to respire within the trapped air within the containers. One method of MAP showed that a ratio of 2.5% oxygen (O2) and 16% carbon dioxide (CO2) is necessary to slow down and inhibit microbial growth and spoilage. However, as O2 levels decrease, CO2 inhibits the strawberries respiration causing acidity of the fruit and allows favourable growth of anaerobic bacteria due to the anaerobic conditions provided. This method of MAP treatment gives fresh strawberries a shelf life of 4–6 days in refrigeration temperatures (Zhang et al, 2003).

Another MAP method shows that elevated levels of O2, much greater than present in air (50% or more) prevents microbial spoilage by inhibiting lower organisms, reducing acidity and maintaining anti-oxidant status tor the first few days. These strawberries have been shown to be edible after 60 days (Stewart et al, 1999).

One novel method used to decontaminate the surface of fresh strawberries is the use of Ozone (O3). Ozone is a natural substance found in our atmosphere. It is an unstable gas and is characterized by its pungent odour. Due to its instability, the average life of ozone is 20 minutes and must be utilized on site as storage is not possible. It is applied to the fruit by electric field of at least 5000V, where O2 is passed through a corona discharge and joins with other O2 molecules to form ozone. Ozone has a high redox potential and acts as an oxidant to bacteria, viruses, fungi, protozoa and insects (Carletti et al, 2013). This oxidant attacks and penetrates the microorganism’s cell wall and causes cell damage by oxidizing certain essential components such as nucleic acids, proteins, lipids and enzymes. The microbial cell then begins to breakdown and cellular components begin to leak out resulting in death (Sun and Muthukumarappan, 2012).

This novel treatment is an effective method in reducing natural decay of the fruit from microbes but causes a temporary loss of strawberry aroma. The establishment and safety limit for ozone treatment is an important factor as it is toxic to humans at high levels. Ozone treatments at specific concentrations have been shown to increase shelf life for up to three weeks under refrigerated conditions (Nadas et al, 2003; Aday et al, 2013).

Conclusion

Strawberries are one of the worlds most consumed berries due to their health benefits and their exclusive taste. Like many other fruits and vegetables, they are easily spoiled by microbial contamination through various intrinsic and extrinsic factors. To meet the demands of consumers, growers must produce strawberries with minimum processing, retaining all of their benefits and freshness while having an acceptable shelf life. This limits the industry to the few methods of preservation. Traditional and novel methods have been proven to extend shelf life by inhibiting certain microflora that live on the fruit by various ways. These methods are able to meet customer requirements and in turn yield better turnover. However, there is a growing concern about the effects of these methods on the fruit itself which proves that more research is needed to ensure the safety and integrity of the these fresh products.

References

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