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This is module 6, Vegetable and fruit microbiology.

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The first part of this section is the microbiology of vegetables including intrinsic properties of vegetables, pathogens and spoilage organisms.

The microbial growth environment of vegetables is considerably different from meats, poultry, or seafood. Vegetables have low proteins and lipid content, especially compared to meats. Vegetables are made up of mostly simple and complex carbohydrates.

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The carbohydrate content of vegetables is diverse. Carbohydrates include polysaccharides, oligosaccharides, monosaccharides, sugar alcohols, and esters. Microbial growth using these compounds requires the appropriate enzymes and transport proteins. Often microbial communities become commensal when extracellular enzymes are excreted by some genera breaking down complex polysaccharides making the simple sugars available to all.

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Most vegetables are in the neutral pH range from pH 6-7. The chart indicates the pH averages for many types of foods, including both vegetables and fruits.

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In general the biota of vegetables are reflective of the soil they are grown in, although exceptions occur. The actinomycetes are the most predominant microorganisms of soils, but they are rarely found on vegetables. The lactic acid bacteria are not common to soils, but they are one of the most predominant microbial biota of vegetables. Like most other foods, the surface of vegetables is highly contaminated with microorganisms, while the interior is sterile or mostly sterile. Vegetables also possess a tough outer skin. This may keep many miroorganisms from entering the interior.

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The lactic acid bacteria are a group of Gram positive bacteria, non-respiring (no ETS), non-spore forming, cocci or rods. They produce lactic acid as the major end product of carbohydrate fermentation. They can be found in rotting vegetation, but not usually in soil itself. Once they get access to a new plant they have the unique ability to adhere to plant surfaces. This provides them with a biota advantage over other microorganisms. The lactic acid bacteria may be 50% of the total vegetable biota.

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Coliform bacteria are also part of the vegetable biota. The genus Enterobacter is a common soil coliform. The remaining are generally fecal in origin, including E. coli. They find their way into soils from manure, both accidental and intentional. Note that the majority of E. coli are non-pathogenic.

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Soils are the natural reservoir for most spores from sporeformers. There, they survive in the latent form until conditions become conducive to growth.

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The most significant vegetable spoilage microorganisms are those that produce polysaccharide and oligosaccharide degradation enzymes. These include cellulase, pectinase, alpha and beta amylase, and similar enzymes. The polysaccharides provide the backbone structure to vegetables. Degrading that backbone leads to a loss of shape and texture.

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There is one major sporeformer pathogen that can contaminate vegetables from soils. Clostridium botulinum is gram positive and during growth can form the most potent neurotoxin on the planet. Consuming spores is a common daily event for humans, however the spores themselves are not toxic. C. botulinum spores can survive normal cooking temperatures and can start to grow once a cooked food cools. C. botulinum related to soils are different from those found in the ocean related to seafood. Bacillus cereus is a less common sporeformer, but it is commonly found in starch based plants. It can cause a vomiting and/or diarrhea illness when present in large numbers (approximately 10,000 or more per gram).

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Because of their handling methods spices are the greatest source of spores for any food products. Some companies sell gas sterilized spices where ethylene or propylene oxide is used to kill spores. However, with the benefit of spice sterility comes the question of toxic residues or byproducts of ethylene or propylene oxides. Incidently, propylene oxide is one of the pasteurization treatments being used on nuts and grains to kill Salmonella. Speaking of Salmonella…

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Salmonella does not grow at low water activities. However, recently it has been found to be the source of foodborne illnesses in low water activity and dried foods. It is found in cocoa (and then in chocolate), peanut butter, dried vegetable proteins, and in many dry grains.

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In 2006 a major E. coli O157:H7 outbreak occurred in baby spinach sold washed and raw. The exact cause is not known, but it has been linked to just one large spinach processor in California together with three spinach farms. The E. coli O157:H7 may have come from the irrigation water or from feral pigs that defecated in the fields. There were hundreds of illnesses and at least four deaths. The photo above is of a Utah boy who became ill from the spinach and suffered hemolytic uremic syndrome. He almost lost his life and he will certainly need kidney transplants when older.

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The second part of this section is fruit microbiology.

The microbial growth environment of fruits is similar to vegetables in that they also have low protein and lipid contents. The big difference is that fruits have an elevated carbohydrate content, especially that of monosaccharides.

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Most fruits have a pH below 4.6. This is important considering that the botulism microorganisms cannot grow at or below this pH level. There are some exceptions, including Asian pears, bananas, some mangos, mangosteen, figs, melon, papaya, and watermelon.

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In general the biota of fruits reflect the intrinsic properties of fruits. Essentially they are fermentative microorganisms that rapidly grow on simple sugars and those microorganisms that tolerate the lower pH levels of many fruits. These include yeasts and fermentative bacteria. Unlike vegetables, the lactic acid bacteria are not predominate microorganisms on fruits because they require external sources of B vitamins. Fruits do not contain significant levels of B vitamins.

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E. coli O157:H7 caused several major outbreaks in unpasteurized raw apple juice. Most likely orchards were spread with manure to encourage better tree growth. Apples were both picked and collected from the ground (drops). Many of these drops were contaminated with E. coli O157:H7. In response, all juice must be pasteurized or contain a safety warning. In addition, many orchards no longer use drops for juice that will not be pasteurized.

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Several outbreaks of waterborne pathogens have occurred in fruits such as strawberries. In 1997 a large outbreak in strawberries due to Cyclospora (a unicellular parasite) occurred in the USA. In 1997 strawberries were contaminated with Hepatitis A that resulted in many illnesses. Many of these outbreaks have been from imported fruits. The result is that now many large fruit companies have instituted good agricultural practices in countries that will export fruits to the USA. The use of potable water and worker hygiene are just two efforts that contribute to safer fruits (and vegetables).

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