Food and Industrial Microbiology

Module 2. Microorganisms and food materials

Lesson 7
MICROBIAL SPOILAGE OF FRUITS AND FRUIT JUICES

7.1 Introduction

Fruits are natural sources of minerals, vitamins besides carbohydrates and other essential substances. Naturally fresh fruits and juices made out of them contain high amount of water thereby making them highly prone to attack by microorganisms. While most of the fruits are naturally provided with coatings and coverings in the form of skins, but these are fragile enough to be easily disturbed by various biological and mechanical factors. Like vegetables, fruits being produce of plants get contaminated through different sources by a variety of microorganisms which may play significant role in their spoilage. These are soil, water, diseased plant, harvesting and processing equipments, handlers, packaging and packing material and contact with spoiled fruits.

7.2 Microorganisms Associated with Spoilage in Fruits and Juices

The microorganisms associated with fruits depend on the structure of fruit. The fruits contain different organic acids in varying amounts.The types of acids which are predominately found are citric acid, malic acid and tartaric acid. The low pH of fruits restricts the proliferation of various types of organisms. The pH and type of acids found in different fruits is given in Table 7.1.

Due to the low pH, a large number of microorganisms are restricted to grow on fruits. Fungi are most dominating organisms to grow on fruits because of the ability of yeasts and molds to grow under acidic conditions. A small number of bacteria which are aciduric (ability to resist acidic conditions) also grow. Also the dry conditions prevailing on the skin and surface do not allow the growth of certain microorganisms. Besides these plants also produce certain antimicrobial components too.

Table 7.1 Type of acid associated with fruits and their pH

Despite the high water activity of most fruits, the low pH leads to their spoilage being dominated by fungi, both yeasts and molds but especially the latter.

7.2.1 Yeasts

Yeasts are unicellular fungi which normally reproduce by budding. Of the 215 species important in foods, about 32 genera are associated with fruits and fruit products . Only a few species of yeasts are pathogenic for man and other animals. None of the pathogenic species are common contaminants of fruits and fruit products. Fruit that has been damaged by birds, insects, or pathogenic fungi usually contain very high yeast populations. The yeasts are introduced into the exposed tissue, often via insects, and are able to use the sugars and other nutrients to support their growth. Types of yeasts growing in fruits depend upon the nature of the fruit and the strain of yeast. Growth of a strongly fermentative type such as certain strains of Saccharomyces cerevisiae may produce sufficient CO₂ (90 lb/in. or more) to burst the container,. Growth of some species in a clear fruit juice may produce only slight haze and sediment. While carbon dioxide and ethanol are the predominant metabolic products of yeasts, other products such as glycerol, acetaldehyde, pyruvic acid, and a -ketoglutaric acid are also formed. Oxidative yeasts such as species of Brettanomyces produce acetic acid in wines and other fruit products. Although yeasts produce hydrolytic enzymes which degrade pectins, starch, and certain proteins, enzymatic activity is usually much less than that exhibited by other aciduric microorganisms, molds in particular.

7.2.2 Molds

These are filamentous fungi which are important group of microflora of fruit products due to following reasons

1. Some of the members are xerophilic, thereby having potential to spoil foods of low water activity such as dried fruits and fruit juice concentrates.
2. Some of the species have heat resistant spores such as ascospores which can survive the commercial pasteurization treatments that are given to most fruit products.
3. Growth of molds on processing equipment such as wooden tanks can result in the generation of off-flavors in wines, juices, and other fruit products.
4. Mold-infected raw fruit may become soft after processing because pectinases were not inactivated by the thermal treatment.
5. The metabolic products of many molds are toxic to humans. Of these toxins, mycotoxins are important components.

Molds are aerobic microorganisms, but many of them are very efficient scavengers of oxygen. Due to this property of molds, processed fruits, including those hermetically sealed in cans or glass, are susceptible to spoilage. In case of limited vegetative growth, evidence of spoilage may be the changes produced by fungal enzymes such as the breakdown of starch or pectins while in case of heavy growth, colonies develop in the headspace or as strands throughout a beverage or similar product. Some types of spoilage by fungi are shown in the Figure 7.1 to 7.10.

Penicillium italicum (blue mold) and Penicillium digitatum (green mold) seen in oranges, lemons and citrus fruits (Figure 7.1).

Fig. 7.1 Green and blue mold by Penicillium growth on oranges and lemons

Fig. 7.2 Soft rot of apples- Penicillium expansum (blue mold)

Fig. 7.3 Grey mold Botrytis cinerea

Fig. 7.4 Another mold in fruits- Botryotinia fuckeliana

Fig. 7.5 Rhizopus soft rot

• R. stolonifer cause soft and mushy food ,cottony growth of mold.

• Anthracnose -Colletotrichum lindemuthianum, cause spotting of leaves and fruits and seedpods

Fig. 7.6 Alternaria rot- Alternaria tenuis, area becomes greenish brown and then black spots

Fig. 7.7 Blue mold rot- Penicillium digitatum, bluish green color

Downy mildew:Initially, the lesions tend tobe small and confined to the upper surface of wrapper leaves. As the areas enlarge, they turn from light green or yellowish to brown and become soft. It is caused by Phytophthora

Fig. 7.8 Downy mildew- Phytophthora: wooly masses

Watery soft rot. This rot occurs on the lower part of heads. The tissue is water soaked and light or pinkish brown.A white, cottony mold spreads over the decayed tissue , and the head eventually becomes a watery mass .

Fig. 7.9 Watery soft rot: Sclerotina sclerotiorum

Fig. 7.10 Black rot- Alternaria mainly Physalopora and others

7.2.3 Bacteria

Various groups of bacteria have ability to grow on fruits and its juices. These bacteria by virtue of their diversity in metabolism grow on fruits and produce different types of compounds. The major group of bacteria which are involved are:

• Lactic acid bacteria
• Acetic acid bacteria
• Spore formers

7.2.3.1 Lactic acid bacteria

The lactic acid bacteria are Gram-positive, catalase negative organisms which can grow under anaerobic conditions. These are rod-shaped (lactobacilli), or cocci (pediococci and leuconostocs) . The homofermentative species produce mainly lactic acid from hexose sugars; the heterofermenters produce one molecule of lactic acid, one molecule of carbon dioxide, and a two-carbon compound, which is usually acetic acid or ethanol or a combination of the two.

Growth of lactic acid bacteria in juices and other fruit products cause the formation of haze, gas, acid, and a number of other changes. Certain heterofermentative lactobacilli lead to slime in cider. The lactobacilli and leuconostocs that are present in citrus juices generate acetylmethylcarbinol and diacetyl, compounds that give the juices an undesirable, buttermilk-like flavor. Some strains, being extremely tolerant to ethanol grow in wines. Lactobacillus fructivorans can grow in appetizer and dessert wines containing as much as 20% ethanol. Lactic acid bacteria have the ability to decarboxylate malic acid to lactic acid. This malolactic fermentation is often desirable in high-acid wines because the acidity is reduced and desirable flavors are produced. Oenococcus oenos is the most acid and alcohol-tolerant species and often is isolated from wines that are undergoing a malo-lactic fermentation.

7.2.3.2 Acetic acid bacteria

These are Gram negative, aerobic rods having two genera, viz. Acetobacter and Gluconobacter . Both of these species oxidize ethanol to acetic acid under acidic condition, Acetobacter species can oxidize acetic acid to carbon dioxide thus, the genus is called as over oxidizer. Because the bacteria are obligate aerobes, juices, wines, and cider are most susceptible to spoilage while held in tanks prior to bottling. Some strains of Acetobacter pasteurianus and Gluconobacter oxydans produce microfibrils composed of cellulose, which leads to formation of flocs in different fruit juice beverages.

7.2.3.3 Spore formers

Spores are heat resistant, so role of organisms producing spores is important in heat treated juices and beverages. Variuos spore formers such as Bacillus coagulans, B. subtilis, B. macerans, B. pumilis, B. sphaericus , and B. pantothenticus have been found to grow in different types of wines. Some of these organisms have also been involved in canned fruits. Spore-forming bacilli that actually prefer a low pH have been responsible for spoilage of apple juice and a blend of fruit juices.