**Abstract**

Nutritional security can be achieved only with the proper intake of fruits and vegetables. However, on an average 30% of the fruit produce are lost between harvest and consumption due to post-harvest spoilage. About 30–40% of total fruits production is lost after harvest. Main causes of postharvest loss include lack of temperature management, rough handling, poor packaging material, and lack of education about the need to maintain quality. There are many ways in which the post-harvest spoilage is managed. Use of chemicals in post-harvest management has direct effect on the consumers and there is a need for alternative strategies. Use of microbial biological control agents have been successfully adopted for soil borne diseases. Registration and biosafety issues make it difficult to use them against postharvest diseases. Use of volatile organic compounds (VOCs) from bioagents for the post-harvest management provides an opportunity to explore the use of bioagents without having contact with fruits. Many classes of chemicals are produced as volatiles by microbial agents. This chapter describes the potential of VOCs in managing post-harvest diseases, their characterization and identification, biosynthesis, volatiles reported from bacterial, fungal and yeast bioagents, success stories of their use as potential bioagents.

**Keywords:** volatile organic compounds, bacteria, yeasts, fungi, fruits, fruit rot, post-harvest spoilage

## **1. Introduction**

Fruit farming is one of the most important and long-standing traditions throughout the world. The cultivation of fruit crops has a significant impact on the overall well-being of humans and the state of the nation. Fruit crop production can be viewed as an open and complex system in which factors including the sowing or

planting method, environmental conditions, soil type, crop management, and their interactions affect the growth, development, and future yield [1].

Fruits and vegetable consumption provides nutritional security and ensures that malnutrition is addressed efficiently. However, the entire product produced do not reach the table of consumers. Due to delicate nature, the post-harvest loss interrupts the reach of fruits to the consumers. Post-harvest loss (PHL) is defined as a measurable quantitative and qualitative loss of an edible food product from harvest to consumption. Increasing post-harvest loss has been highlighted as a global problem in food industries across nations. Food production would need to expand by 70% from present levels by 2050, according to projections, and the situation is significantly more dire in developing nations with poor productivity [2, 3]. As per FAO reports, approximately 14% of produced food was lost after harvest during storage in 2019 with a global net worth of approximately one trillion US dollars but at present up to 30% postharvest loss has been observed in vegetables and fruits.

"An apple a day keeps the doctor away" is a popular saying that emphasizes the importance of fruit crops for the human diet. A diet rich in fruits and vegetables and low in saturated fats is healthy and protective against cardiovascular diseases and certain cancers [4–6]. The World Health Organization (WHO) recommends a daily intake of at least 400 g of fruits and vegetables per person [7].

Huge pre– and postharvest losses in fruits are caused by various diseases and unfavorable environment leading to the total failure of the crops. It has been estimated that phytopathogenic fungi cause more than 50 per cent of total post-harvest losses. Fruits are prone to number of fungal, bacterial and viral diseases which significantly affect its quality and production. However, fungal diseases inflict huge losses to the crop. Fruits are highly susceptible to postharvest spoilage because of high perishable nature. During the storage conditions numbers of fungi are known to cause spoilage of fruits. Under storage conditions considerable losses occur due to the rots caused by different species belonging to the genera viz., *Alternaria*, *Aspergillus*, *Bipolaris*, *Botryodiplodia*, *Botrytis*, *Colletotrichum*, *Curvularia*, *Fusarium*, *Penicillium*, *Rhizopus*, *etc*. [8]. Among these pathogens, green and blue molds, caused by *Penicillium digitatum* (Pers.: Fr.) Sacc. and *P. italicum* Wehmer, are the most economically important postharvest diseases of citrus in all production areas [9]. The losses due to penicillium decay are variable and depend upon climatic conditions, orchard factors, citrus cultivar and the extent of physical injury to the fruit during harvest and subsequent handling [10]. It has been estimated that fruit rots caused by *Penicillium* spp. accounted for 55–80% of total postharvest decay observed in oranges and mandarins during the entire commercialization season, and for 30–55% of decay observed in storage rooms in citrus packing houses [11]. Synthetic fungicides are primarily used for the control of postharvest diseases of fruits and vegetables [12]. However, the trend around the world is shifting towards reduced use of fungicides on produce and thus, there is a strong public and scientific interest in safer and eco-friendly alternatives to reduce the high loss due to decay of harvested commodities. Furthermore, the growing awareness about health hazards and environmental deterioration due to chemical use has necessitated the switching to new nonchemical strategies for the control of postharvest diseases of fruits and vegetables [13].

Among the different biological approaches, use of the microbial antagonists are quite promising and gaining popularity. Microbial bioagents can be used in management of postharvest diseases of fruits and vegetables in two ways. They are use of microorganisms which already exist on the produce itself, which can be promoted and managed or those that can be artificially introduced against postharvest pathogens [12]. Several modes of action have been suggested to explain the biocontrol

*Volatile Organic Compounds Produced by Microbes in the Management of Postharvest Diseases… DOI: http://dx.doi.org/10.5772/intechopen.110493*

activity of microbial antagonists, that include competition [14–18] antibiosis, mycoparasitism, cell wall degrading enzymes, and induced resistance [19–21].

Antibiotics are microbial toxins which at low concentrations can kill other microbes. Bacteria produced volatile antibiotics viz. hydrogen cyanide, aldehydes, alcohols, ketones, and sulphides and non-volatile antibiotics: polyketides (diacetyl phloroglucinol and mupirocin), heterocyclic nitrogenous compounds (phenazine derivatives: pyocyanin, phenazine-1-carboxylic acid, and hydroxy phenazines) and phenylpyrrole antibiotic (pyrrolnitrin) [22].

There is growing importance for the non-chemical control methods to reduce postharvest decay throughout the world. Volatile organic compounds released by microbial antagonists have shown greater potential and it substitutes to synthetic fungicides for the control of postharvest decay of fruit [23]. Therefore, bioagents have gained the considerable attention and emerge to be a promising as well as a viable alternative to chemical management practices.

#### **2. Volatile organic compounds (VOCs)**

Microorganisms have potential of synthesizing numerous volatile substances called as microbial volatile organic compounds (VOCs) with low boiling points, small molecular masses (on average 300 Da) that quickly evaporate at normal temperature and pressure [24]. Both plants and microorganisms produce VOCs that enable them to communicate intra- and inter-specifically. By emitting VOCs, plants defend themselves against herbivores and pathogens, compete with other plants, and/or feed microbial populations. Microorganisms emit VOCs to communicate or attack each other [25]. Production of VOCs with antimicrobial activity has been described in filamentous fungi [26, 27], bacteria [28, 29], yeasts [30, 31], *Streptomyces* spp. [32, 33], and higher plants [34].

The majority of microbial VOCs have distinct smells [35]. Some of them, which are found in wine, beer, and other fermented foods, have pleasant flavors preferred by people. On the other hand, VOCs are connected to wastelands, deterioration, sewerage facilities, dirty socks, and water-damaged structures also. Hundreds of distinct volatiles, including mixtures of alcohols, ketones, esters, tiny alkenes, thiols, monoterpenes, and sesquiterpenes, can be released simultaneously by any type of microbe [36].

The most interesting developments encompassing about volatile organic compounds come from the study of endophytes, i.e., the microbe that colonize inside the plant tissues without causing any negative effects. Bacterial, fungal and yeast endophytes constitute the plant microbiome. Among them, *Muscodor albus* which is a nonsporulating, filamentous and VOC-emitting, endophytic, "stinky white" fungus isolated from the spice tree *Cinnamomum zeylanicum* and was antagonistic to pathogenic bacteria and fungi. More than 28 VOCs were identified from the laboratory cultures of *M. albus*, comprising acids, alcohols, esters, ketones, and lipids [37]. Therefore, application of these antimicrobial properties of cocktail of VOCs to control microbial contamination has been termed Mycofumigation [38].

#### **3. Identification of VOCs**

The identification and quantification of VOCs have to overcome many technical challenges. As, VOCs are highly evaporative, during sampling, handling and assay procedures the chances of occurrence of compound loss is more [39]. In addition to

that the VOCs synthesis at low concentrations in complex mixtures. VOCs identification, collection and quantification depends on either the compounds of interest, the required sensitivity, the intended application, the cost and the ease of use. In classical work, the extraction, separation, and identification steps were separate. Earlier days, the fungal VOCs were identified using the steam distillation method followed by liquid extraction and concentration [40, 41]. Present-day use of Solid phase microextraction (SPME) based gas chromatography (GC) combined with a mass spectrometer (GC–MS) [42], Solvent extraction/liquid–liquid extraction (LLE), stir bar sorptive extraction (SBSE), dynamic headspace (DHS) approaches are used for separation and identification of compounds [43].

Besides these methods, electronic nose or artificial nose combined with multisensory array, an information-processing unit, pattern recognition software, and reference library databases are also used in the identification of VOCs. These resultant electronic fingerprints express unique aroma that helps to detect odor profiles without separation of the mixture into its components [44].

### **4. Biosynthesis of VOCs**

Biosynthesis of VOCs mainly depends on the availability of carbon, nitrogen, sulfur and energy received from the primary metabolism. Therefore, the primary metabolite are considered as a building block of secondary metabolite and exhibits major impact on the concentration of any secondary metabolite, including VOCs. This demonstrates the high degree of connectivity between primary and secondary metabolism. VOCS are divided into different classes based on the biosynthetic origin. The important classes include terpenoids phenylpropanoids/ benzenoids, fatty acid derivatives and amino acid derivatives in addition to a few species−/genus-specific compounds which may not be represented in those major classes. Hence, precursors for VOCs basically originate from the primary metabolism (glycolysis, tricarboxylic acid and pentose phosphate pathway) which helps in the synthesis of VOCs. The four major VOC biosynthetic pathways are the mevalonic acid (MVA), shikimate/phenylalanine, lipoxygenase (LOX) and the methylerythritol phosphate (MEP) pathways lead to the emission of benzenoids/ phenylpropanoids, sesquiterpenes, monoterpenes, hemiterpenes, diterpenes, volatile carotenoid derivatives and methyl jasmonate/green leaf volatiles [45].

Though VOCs are considered as secondary metabolites, some researchers argue that they are degradation products of fatty acids, biotransformation products of amino acids, or incidental breakdown products of fungal extracellular enzymes acting on exogenous substrates. The biosynthetic pathways for geosmin which is a vital odor present in soils produced by many streptomycetes, cyanobacteria and fungi [46] and 1-octen-3-ol which is a breakdown product of linoleic acid, also known as mushroom alcohol [34] have been elucidated. Less is known about the biosynthetic pathways that fungi use to produce VOCs [38].

How enzymes are putatively involved in VOCs synthesis has been reported utilizing bioinformatic methods using either terpene or alcohol or ester synthases as an example. The combination of volatilome data collected at various developmental stages with transcriptional data of selected genes makes an effective method for locating enzymes which are likely to be involved in fungal VOC production . Representative pathways for different VOCs are given in **Figure 1**.

The selected references on the identification of VOCs for post-harvest pathogens of fruits are listed in **Table 1**.

*Volatile Organic Compounds Produced by Microbes in the Management of Postharvest Diseases… DOI: http://dx.doi.org/10.5772/intechopen.110493*

#### **Figure 1.**

*Three main pathways required for the production of volatile organic compounds (VOCs). (yellow boxes are the volatile organic compounds and blue colored boxes indicate the name of the pathway (Modified figure based on Dudareva et al. [47] and Kaddes et al. [45]).*



#### **Table 1.**

*Effect of MVOCs against postharvest pathogens.*
