It is a global issue that fresh fruits and vegetables will go bad after they are harvested. Reports show that about 10-20% fresh fruits and vegetables turn rotten because of post-harvest plant diseases in the developed countries, while this number reaches 40-50% in the developing countries. Chinese featured fruits that are easy to rot add up to 116 million tons. The total loss after harvest ranges from 25% to 30%, equal to 70 billion RMB or so. There are three causes that lead to the decay of fruits and vegetables: (1) physiological dysfunction or aging of their tissues; (2) being infected by the pathogenic microbes; (3) mechanical injuries during the picking process. These three factors influence within each other, of which the pathogenic infection mainly accounts for the decay of fruits and vegetables.
Food preservatives play an important role in the control of post-harvest decay. Present food preservatives actually used in China mainly consist of benzoic acid, sorbic acid and its salt form, and esters of p-hydroxybenzoic acid. These preservatives are made by artificial synthesis, which is harmful more or less to human health. As the concern with the food security is growing, plant-derived preservatives and fresh-keeping materials become popular among the public in that they are free of chemical toxicity, remains and side effects. China abounds in natural herbs and plant resources. According to the statistics, there are over 5,000 herbs containing bacteriostatic components in China. After compared and screened by the antibacterial experiments, most widely used herbs for preserving fruits and vegetables are Syzygium aromaticum, liquorice, mugwort leaf, Houttuynia cordata, Rhizoma atractylodis, Agastache rugosus, Rhizoma kaempferiae, citronella, galangal, Rheum officinale, Radix scutellariae, Folium isatidis, Phellodendron amurense, Scrophularia ningpoensis, Forsythia suspensa, Anemarrhena asphodeloides, vervain, smoked plum, Chinese pulsatilla, capillary Artemisia, dandelion, and wintergreen. Therefore, it is promising to study and develop natural preservatives using herbal extracts.
Most of the small organic molecules in the Chinese herbs are hydrophobic and even lytic to microbial cell membranes, by which the microorganisms are inhibited or killed. Thereby the principle for the herbal extracts to keep fruits and vegetables fresh lies in the fact that the active substance in the herbal extracts is able to inhibit the microbes on the surface of fruits and vegetables, reduce the enzymatic activity of these plant food. In this way, fruits and vegetables are survived from microbial damages and their physiological actions are weakened.
The natural anti-bacterial matters in the plants are classified into 4 groups: phytoalexins, phenols, organic acids and essential oils. Phytoalexins are synthesized by their plant hosts. They are chemicals of low molecular weight and broad antibacterial spectrum. Phytoalexins can act on phytopathogenic fungi and some of them even act on bacteria. For example, isoflavones are one of the most important phytoalexins. Phenols in the natural plants are the main active ingredients in food preservatives. They also have broad spectrum of antibacterial activity. For example, capsaicins extracted from spices can keep bacterial spores from germination. Fruits and vegetables have organic acids such as citric acids, malic acids, tartaric acid, etc. to combat bacteria. These organic acids take effects against the synthetic systems of cell walls, cell membranes, metabolic enzymes and proteins, as well as genetic factors. Essential oils extracted from plants are a kind of natural fresh-keeping component with broad spectrum of killing bacteria and inhibiting microbes. Hammer et al. led a study on the anti-bacterial activity of 52 essential oils. They found that vetiver oil, coriander, Oenothera biennis, apricot oil, majoram and clove oil have great inhibition against microbes like Staphylococcus aureus, Salmonella typhimurium, Eschericia coli, Aspergillus flavus, Bacillus subtilis, and Pseudomonas aeruginosa.
Most fruits and vegetables are acidic, so a majority of post-harvest pathogens are composed of fungi, including Botrytis cinerea, Penicillium species, Aspergillus species, Alternaria sepcies, etc. In addition, fruits and vegetables usually carry human pathogens like Escherichia coli and Listeriosis during post-harvest logistics.
Scholars all over the world have studied some herbs and plant extracts with very good anti-bacterial activities. Ates found pimpinellin extracts have a strong antibiotic effect against Micrococcus and Branch aureus when they analyzed several anti-bacterial extracts of medicinal plants. Moreover, extracts of licorice root and cassia bark show inhibitory actions towards bacillus and Staphylococcus aureus. Wilson et al. studied on the inhibition of 7 essence oils or ingredients against Botrytis cinerea, including tarragon, ajowan, allspice, thyme red, hinokitiol, etc. The result indicated that ajowan, allspice and thyme red have the best effect among all, with 0.1% (V/V) to completely inhibit the germination of B. cinerea and to reduce the growth of E. coli. Yu et al. determined the anti-bacterial activities of the ethanol extracts of 60 plants which can be used as food and medicines. Among all the tested samples, clove, pepper, galangal have excellent antibiotic action on Aspergillus flavus and Aspergillus niger. Yujing Gong discovered some herbal medicines with high inhibitory effects on one or several fungi, such as moutan bark, Rhizoma anemarrhenae, galangal, Coptis chinensis, clove, cinnamon, golden cypress, Radix scutellariae, calamus, gallnut, nutmeg, Stephania terandra, costustoot, and Salvia miltiorrhiza. Kong et al. discovered the extracts of cloves and cinnamons are able to inhibit Aspergillus niger and Penicillium expansum from forming spores. It was reported that liquorice extracts can inhibit preferably against A. niger, Aspergillus oryzae, mucor and Paecilomyces varioti. Further investigations will make sure whether the key antifungal ingredient belongs to licorice flavonoids by using thin-layer chromatography. The in vitro experiments of wintergreen confirmed that wintergreen extracts strongly inhibit Staphylococcus aureus, Typhoid bacillus and Pseudomonas aeruginosa.
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