Fruits and fruit products

6 Fruits and fruit products

I Introduction

A Definitions Fruits are defined in general terms as “the portions of plants which bear seeds”. Such a definition

includes true fruits such as citrus, false fruits such as apples and pears, and compound fruits such as berries. The definition includes tomatoes, olives, chilies, capsicum, eggplant, okra, peas, beans, squash, and cucurbits such as cucumbers and melons although, for culinary purposes, a number of these fruits are classified as vegetables. For the purpose of the current chapter, tomatoes, olives, cucumbers, and melons will be considered fruits whereas egg plant, okra, peas, beans, squash, chilies, and capsicum will be considered as either vegetables or spices.

B Important properties Most fruits are high in organic acids, and hence have a low pH (Table 6.1). However, melons and some

tropical fruits such as durian (Durio spp.) have a pH near neutrality. The principal acid in citrus fruits and berries is citric acid; in pome and stone fruits, malic acid; and in grapes and carambola, tartaric and malic acids. Care must be exercised in interpreting the pH values cited for most fruits. The pH values for fruits are typically determined by homogenizing an intact fruit and determining the pH of the expressed juice or pulp. However, this is not the microenvironment that a microorganism experiences when invading an intact fruit. For example, in an intact orange, the acidic juice is maintained within juice sacs, leaving the surrounding tissue with pH values closer to neutrality. The traditional interpretation of the acidity of many fruits, is being modified as recent research with apples, tomatoes, and oranges, has demonstrated the growth of pathogenic enteric bacteria within intact or wounded fruit (Asplund and Nurmi, 1991; Wei et al., 1995; Janisiewicz et al., 1999a; Dingman, 2000; Liao and Sapers, 2000).

Fruits are divided into two categories, climacteric and non-climacteric, based on their respiration pattern during ripening. Climacteric fruits are those exhibiting moderate respiration that peaks during climacteric stage. They have a short-life after ripening commences (1 week) with moderate total post- harvest life (a few days to a few weeks). They have fairly critical harvest maturity, and are sensitive to damage and microbial infection after climacteric peak in respiration. Examples are strawberries, grapes, and litchi. Non-climacteric fruits exhibit low to moderate respiration rates with a short to medium storage life. If harvested mature, but unripe, they have the potential for a long shelf-life if ripening can be delayed. Examples are bananas, papayas, and avocado (ASEAN-COFAF, 1984).

Fruits are of substantial nutritional value as a major source of vitamin C. Some provide useful levels of potassium, calcium, magnesium, and contain significant levels of vitamin A, thiamine and niacin (Holland et al., 1992).

C Methods of processing Due to their low pH most fruits are more susceptible to damage from fungi (and yeast) rather than

from bacteria. This low pH also means that most fruit-based products require only pasteurization to be

327 Table 6.1 Representative pH values for fresh fruit a

FRUITS AND FRUIT PRODUCTS

Fruit

pH range

Fruit

pH range

a Adapted from Beuchat (1978), Splittstoesser (1987), CRC (1990) and Brackett (1997).

microbiologically stable. Examples of exceptions include olives, cucumbers, melons, and some varieties of tomatoes. Pasteurization alone may not be sufficient for fruit based products prepared under tropical conditions.

Fruits may be processed by canning, freezing, sun-drying, or dehydration by reducing their water activity through concentration–removal of water or the addition of salt and/or sugar. The pH of tomatoes is reduced to below 4.5 by adding acids during processing, while olives, chilies, cucumbers, and durian are often pickled or fermented with lactic acid bacteria to produce microbiologically stable products that no longer need a low–acid canning process to retard spoilage.

D Types of final products Fresh fruits are commonly sold after minimal processing and packaging treatments and may be chilled

or refrigerated. Common processing steps for fresh fruits may include washing, dipping, waxing, or wrapping in paper impregnated with preservatives against mold. In some countries, certain fruits such as apples may be held for several months under refrigeration or controlled atmosphere storage, and then sold as fresh produce. Fresh fruits that have been pre-peeled, pre-sliced or otherwise prepared and packaged for convenience are increasingly being sold at retail.

Fruits are also frequently sold as canned, frozen or dried products. Moistened dried-fruit packaged with an added preservative have become increasingly popular. Dried fruits are also used in a variety of other products, e.g., confectionery bars, biscuits, chocolates, breads, mueslis, and other cereal-based products. The microbiology of these products usually differs little from that of unprocessed dried fruit and will not be considered further here.

Chopped fresh, frozen, or canned fruit may be sold in fruit salads and related products, or incorporated in dairy products such as yoghurts. Tomatoes are canned as juice, as whole, peeled or diced fruits with or without added juice, or as concentrated purees, pastes or soups; dried as whole or halved fruit, or as a powder; or formulated into products such as tomato sauce (catsup or ketchup) preserved with vinegar, or in the form of chilli sauce. Canned Salsa products consisting of chopped tomatoes and various vegetables are also common.

Stone fruits (e.g. peaches) are sometimes infused with glucose syrups and partially dried to produce glac´e confectionery.

MICROORGANISMS IN FOODS 6

II Initial microflora (fresh fruits)

The initial microflora of fruits comes from the field, and from harvesting and transportation equipment. Field sources include soil, insects, air, birds, animals, and fruit exudates. Soil is the primary source of heat resistant fungal ascospores, especially Byssochlamys species. Insects carry a variety of microbes, and species that puncture or otherwise injure a fruit are important carriers of spoilage microorganisms. For example, piercing insects are responsible for inoculation of figs with yeasts and other fungi including Aspergillus flavus. Insects are a potential means by which human enteric pathogens could be transmitted to fruit (Janisiewicz et al., 1999a,b). Plant pathogens such as fire blight can be disseminated among pear and apple trees by rainwater runoff (van der Zwet and van Buskirk, 1984). Irrigation water can be an important source of microorganisms including enteric pathogens such as Salmonella. Fruit exudates provide nutrients for yeasts, especially pigmented Basidiomycetous yeasts such as Rhodotorula species. Some fungal pathogens of fruit, including Lasiodiplodia theobromae, may be present systemically in trees and invade developing fruit through the stems (Johnson et al., 1991, 1992). Plant pathogens that cause crop losses or “market diseases” have developed a variety of means for invading plants. For example, Pseudomonas syringae and Ps. solanacearum, two species that infect tomato, gain entry via the stromata and roots, respectively (Getz et al., 1983; Vasse et al., 1995).

While microorganisms are largely restricted to the surface of intact, healthy fruit, low levels of largely Gram-negative bacteria can be routinely isolated from the interior of fruit, particularly species of Pseudomonas, Xanthomonas, Enterobacter, and Corynebacterium. The frequency and location of the bacteria varies with the fruit and the stage of maturity, with frequency of internal bacteria being high in tomatoes and cucumbers, occasional for melons and bananas, and rare for citrus fruit, grapes, peaches, and olives (Samish et al, 1961, 1962; Samish and Etinger-Tulezynska, 1963).

Tropical fruits may contain high numbers of microorganisms. Mangoes, collected from the field, supermarkets and wet markets were reported to contain total plate counts of 10 4 –10 6 cfu/g and yeasts and mold of 10 3 –10 4 cfu/g (Anonymous, 1999a).

III Primary processing

A Effects of processing on microorganisms The processes of harvesting, cleaning, sorting, packing, and initial storage of fruit usually have little

effect on the initial microflora. Examples of long used techniques designed to reduce fungal load and infection include waxing, dipping in warm (40 ◦ C–50 ◦

C) water which may contain fungicides such as benomyl, thiabendazole, or sodium o-phenylphenate (SOPP), and washing with 200 ppm chlorine. After dipping, fruit may be individually wrapped in paper impregnated with fungicides such as biphenyl, or packed in trays that ensure separation of individual fruit. With the increase in fresh produce microbial food safety concerns, there has been increased interest in the identification of dry or wet cleaning processes for the reduction of microbial populations. This includes evaluation of a number of different sanitizers such as trisodium phosphate, hydrogen peroxide, peroxyacetic acid, chlorine dioxide, acidified sodium chlorite (Zhuang and Beuchat, 1996; Pao and Brown, 1998; Buchanan et al., 1999; Park and Beuchat, 1999; Sapers et al., 1999; Liao and Sapers, 2000; Pao et al., 2000; Wisniewsky et al., 2000; Du et al., 2002) and the evaluation of new antimicrobial coatings (Zhuang et al., 1996; McGuire and Hagenmaier, 2001). Such treatments have limited effectiveness, with microbial reduction generally limited to the range of 1–3 log cycles.

Such primary processes may effectively delay spoilage, but may also cause damage to some fruit and hence hasten infection and ultimate spoilage. The impact of immersion of fruit in water, during operations such as fluming, washing, and hydrocooling, on the internalization of microorganisms

329 has been studied extensively both in regard to spoilage and transmission of human pathogens. The

FRUITS AND FRUIT PRODUCTS

immersion of warm fruit in cold water, results in a pressure differential that results in the uptake of water.

A similar pressure differential can occur, when fruits are immersed in water of sufficient depth. This has been observed experimentally with a number of fruits. This inward movement of water has been experimentally associated with the uptake of spoilage bacteria and fungi in apples, pears, and tomatoes (Segall et al., 1977; Bartz and Showalter, 1981; Bartz, 1982; Sugar and Spotts, 1993; Bartz, 1999), and identified as a contributing factor for core rot in apples. Infiltration of enteric pathogenic bacteria into intact fruit has also been demonstrated experimentally with apples, tomatoes, oranges, grapefruit, mangoes, and cantaloupes (Buchanan et al., 1999; Burnett et al., 2000; Walderhaug et al., 2000).

B Spoilage Bacteria, yeasts and molds account for up to 15% of post harvest spoilage of fresh produce. Consequently,

microbial spoilage represents significant economic loss throughout the fruit distribution chain (Brackett, 1994). Spoilage of fruit is often classified according to where or when it becomes evident. Pre-harvest or field spoilage refers to spoilage occurring before the fruits are harvested, while spoilage that expressed itself after harvest, is often termed post-harvest spoilage. However, some microorganisms can cause spoilage to occur both pre- and post-harvest, and some microbiological problems that become evident after harvest often begin before harvest (Wiley, 1994). Spoilage conditions within are often referred to as market diseases and are typically named after the overt characteristics of the conditions and not the microorganism responsible. Thus, Alternaria citri is responsible for black rot in oranges, Asp. niger in figs, and Alt. alternata in tomatoes and melons.

Although there are several important bacterial causes of market diseases, particularly bacterial soft rots that are caused by Erwinia carotovora, fruits’ lower pH, due to naturally present acids, often inhibits the growth of bacteria. Consequently, fungi (both yeast and molds) are the dominant microorganisms in many fruits and include both spoilage and innocuous types. Common genera include members of Aspergillus, Penicillium, Mucor, Alternaria, Cladosporium and Botrytis spp. (Brackett, 1994). Although fungi are largely responsible for fruit spoilage, not all fungi that have been insolated from fruit are spoilage fungi.

Yeasts occurring on fruits are evenly divided between ascosporogeneous and imperfect species. Saccharomyces, Hanseniaspora, Pichia, Kloeckera, Candida, and Rhodotorula are among the most common genera (Splittstoesser, 1987). Populations of yeasts on fruits, can be high, e.g., averages of 38,000–680,000 cfu/g were isolated from grapes, and damaged or defective fruits can contain as many as

10 million cfu/g of fruits. In contrast, sound apple contains only about 1000 yeast cells/g (Brackett, 1994). Defense mechanisms in fruits appear to be highly effective against nearly all fungi. Only a relatively few genera and species are able to invade a particular fruit type and cause serious losses. Some fungi are highly specialized pathogens, attacking only one or two kinds of fruits; others have a more general ability to invade fruit tissue. Common spoilage fungi found in fresh fruits are listed in Table 6.2. The most important fungal diseases of fruits are briefly described below by fruit type.

While spoilage of fruits is distinct and separate from safety considerations, it has been noted that that there is an apparent association of Salmonella contamination of fresh fruits and vegetables with bacterial soft rot (Wells and Butterfield, 1997). The survival and growth of Escherichia coli O157:H7 in apples, appears to be enhanced by wounds from plant pathogens, physical damage, or insects (Janisiewicz et al., 1999a; Dingman, 2000; Riordan et al., 2000), while preventing post-harvest decay of apples helps prevent the growth of the E. coli 0157:H7 (Janisiewicz et al., 1999a).

Citrus fruits. Rotting of citrus fruit throughout the world is commonly caused by Penicillium italicum and Pen. digitatum, termed blue rot and green rot, respectively. Infection can occur at any stage after

MICROORGANISMS IN FOODS 6

Table 6.2 Common fungi spoiling fresh fruits a Fruit

Spoilage Citrus

Fungus

Penicillium digitatum

Green rot

Oranges, lemons

Penicillium italicum

Blue rot

Alternaria citri

Stem end, black rot

Geotrichum candidum

Sour rot

Whisker mould Pome fruits Apples, pears

Penicillium ulaiense

Penicillium expansum

Blue rot

Penicillium solitum

Blue rot

Phlyctema vagabanda

Bulls eye rot

Rhizopus stolonifer

Transit rot

Stone fruits Peaches, apricots

Black to brown spots Cherries, plums

Alternaria sp.

Monilinia fructicola

Brown rot

Nectarines

Rhizopus stolonifer

Transit rot

Trichothecium sp.

Pink rot

Bananas

Lasiodiplodia theobromae

Cushion rot

Colletotrichum sp.

Crown rot

Nigrospora oryzae

Squirter rot

Fusarium semitectum

Soft rot

Figs

Aspergillus niger

Black rot

Fusarium spp.

Soft rot

Hanseniaspora uvarum

Souring

Tropical fruits

Stem end rot Avocadoes

Lasiodiplodia theobromae

Anthracnose Mangoes

Colletotrichum spp.

Stem end rot Papayas

Phomopsis sp.

Stem end rot Soft fruits

Diplodia sp.

Botrytis cinerea

Grey rot

Strawberries

Rhizopus stolonifer

Leaking rot

Raspberries

Mucor piriformis

Leaking rot

Phytophthora cacotorum

Leather rot

Grapes

Botrytis cinerea

Grey rot

Pineapples

Fusarium sp.

Brown rot

Penicillium sp.

Brown rot

Tomatoes

Alternaria alternata

Black rot

Rhizopus stolonifer

Watery rot

Geotrichum candidum

Sour rot

Melons

Colletotrichum lagenarium

Anthracnose

Alternaria alternata

Black rot

a From Hall and Scott (1977), Beuchat, 1978, Ryall and Pentzer (1982), Splittstoesser (1987), and Snowdon (1991).

harvest. Initiation requires damage to skin tissue, which readily occurs in modern bulk handling systems. Decay spreads from fruit-to-fruit by contact (Snowdon, 1990).

In lemons and limes, Geotrichum candidum causes sour rot, a pale, and soft area of decay that later develops into a creamy and slimy surface growth (Butler et al., 1965; Morris, 1982; Snowdon, 1990). Infection usually occurs in over-mature fruit after long and high temperature storage. Black centre rot of oranges, caused by Alt. citri, appears as an internal blackening of the fruit.

Penicillium ulaiense is a recently identified citrus pathogen. Closely related to Pen. italicum, this species has caused losses in areas such as California, where fungicidal control of Pen. italicum has been effective (Holmes et al., 1993, 1994). Less common and usually less serious spoilage of citrus can be produced by a variety of fungi (Table 6.3).

Pome fruits. The most destructive fungal spoilage agent of apples and pears is Pen. expansum, which causes a blue rot. Penicillium expansum grows at low temperatures, so cold storage only retards, rather

331 Table 6.3 Other fungi spoiling fresh fruits a

FRUITS AND FRUIT PRODUCTS

Fruit

Spoilage Citrus

Fungus

Aspergillus niger

Black rot

Oranges, lemons

Botrytis cinerea

Grey mould rot

Diaporthe sp.

Stem end rot

Sclerotinia spp.

Cottony rot

Septoria sp.

Septoria rot

Trichoderma sp.

Cocoa-brown rot

Fusarium sp.

Brown rot

Phytophthora sp.

Brown rot

Diplodia sp.

Stem end rot

Stem end rot Pome fruits Apples, pears

Phomopsis sp.

Botrytis cinerea

Grey mould rot

Phytophthora sp.

Brown rot

Venturia sp.

Black spot

Physalospora obtusa

Black rot

Black to brown spots Stone fruits Peaches, apricots

Alternaria sp.

Grey black rot Cherries, plums

Cladosporium herbarum

Watery, tan rot Nectarines

Diplodia sp.

Geotrichum candidum

Sour rot

Aspergillus niger

Black rots

Botrytis cinerea

Grey rot

Penicillium expansum

Blue rot

Monilinia fructicola

Brown rot

Figs

Alternaria spp.

Brown to black spot

Botrytis cinerea

Grey mould rot

Penicillium spp.

Blue mould

Kloeckera apiculata

Souring

Tropical fruits

Stem end rot Avocadoes

Lasiodiplodia theobromae

Anthracnose Mangoes

Colletotrichum spp.

Stem end rot Papayas

Phomopsis sp.

Stem end rot Soft fruits

Diplodia sp.

Grey black rot Strawberries

Cladosporium spp.

Watery white rot Raspberries Grapes

Sclerotinia spp.

Cladosporium spp.

Black rot

Penicillium spp.

Blue mould

Watery soft rot Blueberries

Rhizopus stolonifer

Alternaria spp.

Woolly mould

Botrytis cinerea.

Grey mould rot

Mummification Tomatoes

Monilinia spp.

Cladosporium herbarum

Grey black rot

Botrytis cinerea

Grey mould rot

Rhizoctonia solani

Soft rot

Melons

Cladosporium spp.

Black rot

Fusarium spp.

Pink rot

Penicillium spp.

Blue mould rot

a From Hall and Scott (1977), Ryall and Pentzer (1982), Splittstoesser (1987), Snow- don (1991).

With the advent of effective fungicidal control of Pen. expansum, Pen. solitum has emerged as a serious problem in apples (Pitt et al., 1991).

Core rot in apples, is the growth of any variety of molds initially in the seed cavity and then spreading to the mesocarp tissue. Molds that have been associated commonly with this type of spoilage are Alt. alternata, Botrytis cinerea, Penicillium spp., Coniothyrium spp., Pleospora herbarum, and Pestalotia laurocerasi (Combrink et al., 1985).

Botrytis cinerea causes grey rot in cold-stored pears and, less commonly in apples (Hall and Scott,

MICROORGANISMS IN FOODS 6

invades through wounds or abrasions and can spread rapidly in packed fruit. Other fungi that commonly cause rots of pome fruits are listed in Table 6.3.

Fire blight is a bacterial disease of pear and apple trees that is caused by Erwinia amylovora. It is treated by the use of antibiotics such as streptomycin, tetracycline or oxytetracycline. However, it appears that transfer of resistance genes is transposon-mediated leading to an increased incidence of antibiotic resistance among commensal bacteria associated with the fruit (Schnabel and Jones, 1999).

Stone fruits. Stone fruits (peaches, plums, apricots, nectarines, and cherries) are all susceptible to brown rot caused by Monilia fructicola or the closely-related species Mon. fructigena and Mon. laxa. Brown rot colonises water-soaked spots on the fruit, which within 24 h, becomes brown enlarging and deepening, rapidly, then producing a dusting of pale brown conidia. The whole fruit may rot in 3–4 days (Hall and Scott, 1977; Snowdon, 1990).

Transit rot, caused by Rhizopus stolonifer, is so named because it usually develops in the high humidity conditions established in boxed fruit during transport. It produces a soft rot in the fruit, which then becomes surrounded by a coarse and loose “nest” of mycelium. Growth spreads rapidly, engulfing fruit adjacent to the originally infected one, and sometimes all the fruits in a box, in only 2–3 days.

Penicillium expansum causes blue mould rot in cherries and plums, but is uncommon in other types of stone fruits (Ryall and Pentzer, 1982). Other spoilage fungi of stone fruits are listed in Table 6.3. An important bacterial pathogen of peaches and apricots is Xanthomonas campestris pv. pruni. It causes a market disease referred to as bacterial spot. Like fire blight, it is treated with streptomycin, tetracycline, or oxytetracycline, which has lead to concerns about the emergence of antibiotic resistance among commensal microorganisms associated with these fruits (Schnabel and Jones, 1999).

Grapes. Botrytis cinerea is regarded as the highly desirable “noble rot” in certain wine grapes (Coley- Smith et al., 1980), but it is by far the most serious cause of spoilage in table grapes (Ryall and Pentzer, 1982). In the early stages of invasion, the fungus develops on stems and inside the berry; later growth then expands rapidly through tight bunches where humidity is high and large “nests” of rot may quickly develop.

Infection by black Aspergillus species, Asp. niger, Asp. carbonarius, and Asp. aculeatus, occurs on damaged grapes in warmer climates. This damage may result from insect or mechanical penetration, splitting due to rain before harvest, or infection by pathogenic fungi such as Botrytis and Rhizopus (Snowdon, 1990; Leong et al., 2004).

Penicillium species do not usually attack grapes, before harvest, but are common in stored grapes (Barkai-Golan, 1974; Hall and Scott, 1977; Ryall and Pentzer, 1982).

Berries/soft fruit. The two principal fungal rots observed with most berry and related soft fruit crops are caused by Botr. cinerea and Rhiz. stolonifer (Dennis, 1983a). Botrytis causes soft rots in cane berries such as raspberries and loganberries, but a firm, dry rot in strawberries. In both cases, the fruit becomes covered with a growth of grey mould. Losses in strawberries can be high as the fungus spreads by

contact and forms “nests” of rotting fruit. Initial contamination generally occurs in the field and can lead to substantial crop losses if not treated with fungicides, particularly during flowering. Botrytis cinerea infections at the stem end of the fruit appear to be related to an initial contamination of the flower, with the mold lying dormant until maturation of the fruit. This accounts for approx., 15% of strawberry spoilage by Bot. cinerea. The majority of spoilage is due to the mold infecting multiple points on the surface of the fruit post-harvest. The mold is able to grow on strawberries at temperatures as low as 0 ◦

C, so refrigeration retards, but does not prevent spoilage. Shelf-life can be extended through the use of low dose ionizing radiation and/or modified atmosphere packaging. Botrytis cinerea is also

333 Rhizopus stolonifer causes a large proportion of marketing losses of all berry fruits, and in some

FRUITS AND FRUIT PRODUCTS

regions, is a major cause of market losses in strawberries harvested late in the season. The mold is associated with a market disease of strawberries referred to as “leak” disease, wherein the rotting fruit has a wet appearance and ultimately collapses completely, exuding juice. This infection is favored by holding the fruit at temperatures above 20 ◦

C, allowing the fungus to spread rapidly. Mucor piriformis causes a similar condition in strawberries. Like Bot. cinerea, initial infection with this mold can occur during flowering or on the surface of intact fruit. Additional mold species that are linked to spoilage of soft fruits are Colletotrichum gloeosporioides, Mucor hiemalis, Rhizopus sexualis, and various species of the genera Penicillium, Cladosporium, Alternaria, and Stemphylium.

Yeasts are normal colonizers of strawberries, being present at up to 10 5 CFU/g in macerates of mature berries (Buhagiar and Barnett, 1971). Despite the presence of a wide variety of yeast species on

strawberries, spoilage of this soft fruit by yeasts is rare (Dennis, 1983b). Figs. The invasion of Smyrna figs by yeasts was documented by Miller and Phaff (1962). This type of

fig is pollinated by the fig wasp, which at the same time introduces yeasts (e.g., Candida guilliermondii) and bacteria (Serratia species). These microorganisms do not cause spoilage themselves, but at maturity attract Drosophila flies, which carry the spoilage yeasts Hanseniaspora uvarum, Kloeckera apiculata and Torulopsis stellata. These spoilage yeasts produce “souring” of the figs due to acid production.

Growth of Asp. flavus and the associated production of aflatoxins in figs, has been recognized as

a serious problem (Buchanan et al., 1975). Black Aspergillus species can also infect figs, with the possibility of ochratoxin A formation ( ¨ Ozay and Alperden, 1991, Doster et al., 1996).

Tomatoes. With an internal pH of 4.0–4.5, tomatoes can be affected by fungal and bacterial market diseases. The primary spoilage bacterium is Erwinia carotovora subsp. carotovora, which causes bac- terial soft rot. A number of market diseases are also associated with bacterial plant pathogens that lead to crop losses or defects that reduce the economic value of a crop. For example, “bacterial speck” on mature tomatoes is associated with infection of the plant with Xan. campestris pv. vesicatoria, Xan. campestris pv. tomato, and Cornyebacterium michiganense pv. michiganense (Getz et al., 1983).

Several of those produced by fungi are important. Alternaria rots of tomatoes appear as dark brown to black, smooth, and slightly sunken lesions, which are of firm texture and can become several centimetres in diameter. The cause is Alt. alternata, which attacks fruit damaged by mechanical injury, cracking due to excessive moisture during growth, or chilling (Snowdon, 1991).

Chilling injury also allows the entry of other fungi also. Cladosporium rot caused by Cladosporium herbarum and grey mould rot due to Bot. cinerea can both be potentiated by chilling injury. Botrytis cinerea can also affect mechanically-damaged green fruit, on which it forms “ghost spots”, small whitish rings, often with darker centers. Rot can spread rapidly at higher temperatures, during packing and transport (Ryall and Lipton, 1979; Snowdon, 1991).

Rhizopus species appear to be able to attack almost any kind of fruit or vegetable, and the tomato is no exception. “In severe cases of Rhizopus rot, as there seems to be no mild ones, the fruit resembles a red, water filled balloon” (Ryall and Lipton, 1979). When the fruit collapses, grey mycelium, a fermented

odor and white to black spore masses become visible. The disease starts in cracked or injured fruit, but may spread by contact thereafter.

Sour rot in tomatoes is caused by Geo. candidum. Lesions are a light greenish grey and may extend as a sector from end to end of the fruit. Tissues remain firm at first, but later weaken and emit a sour odor. This disease invades only damaged or cracked fruit, and is disseminated by Drosophila flies (Ryall and Lipton, 1979).

Tomatoes grown without stakes or trellises can develop soil rot caused by Rhizoctonia solani. Small brown spots of this disease develop concentric rings, when they grow to 5 mm or more in diameter.

MICROORGANISMS IN FOODS 6

Melons. The relatively neutral pH values of most melons make them susceptible to spoilage by both bacteria and fungi. Soft rot is the primary bacterial spoilage condition and is most often associated with Erw. carotovora.

Watermelons sometimes develop anthracnose from Col. lagenarium. This disease forms circular or elongated welts that are initially dark green and later become brown, disfiguring the melon surface. Pink Colletotrichum conidia may become visible, if humidity remains high (Snowdon, 1991).

Cantaloupes and rock melons may be affected by several different diseases, the most important being Alternaria rot due to Alt. alternata. Mould invasion usually takes place at the stem scar, pro- ducing dark brown to black lesions and eventually invading the flesh, forming firm, and adherent areas.

Cladosporium species can also invade melons through the stem scar, forming a rot similar to that caused by Alternaria. In both cases, prompt shipping and correct cool storage will limit the losses from

these diseases. Several Fusarium species can invade melons, especially when storage temperatures are high or storage periods become excessive. Penicillium species may also occasionally cause problems under these conditions (Ryall and Lipton, 1979; Snowdon, 1991).

Tropical fruit. Fruits from tropical areas are susceptible to a different array of diseases than those grown in subtropical or temperate climates. Study of such diseases is still a developing science with many pressing problems, not the least being that most tropical and subtropical fruits are injured by low temperatures and so disease control cannot be assisted by refrigeration. Tropical fruits generally do not tolerate temperatures below 8–10 ◦

C and suffer from chill injury when stored at temperatures of 1–2 ◦ C to kill off fruit flies (Chan, 1997). Bananas are the most important tropical fruit in international trade. Most post-harvest diseases of bananas are due to fungal rots in the stalks and crowns, rather than on the sides of the fruit (Eckert et al., 1975). A comprehensive study of bananas shipped from the Windward Islands to England showed that nearly 20 fungal species can cause crown rots. The most important were Col. musae (synonym Gloeosporium musarum) and Fusarium semitectum, with several other Fusarium species also significant (Wallbridge, 1981) (Table 6.3).

The major rots of other tropical fruits are usually anthracnoses, brown, or black spots on the skin that reduce crop value and may eventually destroy the fruit. Anthracnoses are usually caused by Col- letotrichum species (often referred to as Gloeosporium species in the literature). Stem end rots due to Las. theobromae (= Botryodiplodia theobromae) occur in most tropical tree fruits (Snowdon, 1990).

Among the viruses, the papaya ringspot virus is the most widespread and damaging virus infecting papaya and cucurbits worldwide. This economic burden provided the impetus for the recent development of viral resistant transgenic papaya in Hawaii (Swain and Powell, 2001).

C Pathogens Bacterial pathogens. Pathogenic bacteria are not normally associated with fruit; however, it is possible

for pathogens to be present due to faecal contamination. Historically, fruits were considered as low risk food, and had been implicated with illness only on isolated occasions. This in part reflects the fact that fruits are traditionally considered acidic foods that would not support the growth of most foodborne pathogens. However, as discussed above, on fruits with lower acid contents, such as melons, apples, and tomatoes, survival of enteric pathogens may be prolonged or growth may even occur (Escartin et al., 1989; Asplund and Nurmi, 1991; Madden, 1992; Golden et al., 1993). Growth in particular, is likely to occur on cut surfaces of fruit. The potential for fruits to serve as a potential route of transmission was reinforced during the 1990s, when a series of foodborne illness outbreaks was attributed to the

335 consumption of unpasteurised apple juice/cider and orange juice. These outbreaks were caused by three

FRUITS AND FRUIT PRODUCTS

different pathogenic microorganisms: E. coli O157:H7, Salmonella spp., and Cryptosporidium parvum (see Chapter 14). These outbreaks provided the impetus for a more detailed examination of the potential for fruit to serve as a vehicle for foodborne disease (Conner and Kotrola 1995; Semanchek and Golden, 1996; NACMCF, 1999; Sewell and Farber, 2001).

A number of outbreaks of salmonellosis have been associated with fresh tomatoes (Wood et al., 1991; CDC, 1993; Beuchat, 1996), watermelon (Gayler et al., 1955; Lawson et al., 1979; Blostein, 1993), and cantaloupes (Anonymous, 1993, Del Rosario and Beuchat, 1995; Beuchat, 1996; Sewell and Farber, 2001; Anderson et al., 2002). In the latter case, there have been a series of outbreaks associated with S. Poona (Sewell and Farber, 2001; Anderson et al., 2002) and to a lesser extent S. Chester (Ries et al., 1990) and S. Oranienburg (Sewell and Farber, 2001). Investigations of these outbreaks largely traced the cantaloupes to a farm in Mexico and identified contaminated water used for irrigation and post-harvest washing and cooling as a source of the Salmonella. Recently, an outbreak of S. Newport infections in the United States was traced to mangoes (Sivapalasingam et al., 2003). In this instance, the source of the microorganisms appeared to be cooling water used after the fruit had been heated to eliminate larva of the Medfly before entry into the country.

Growth of Clostridium botulinum and toxin production was reported to be possible in stored tomatoes, if growth of mold (Alternaria or Fusarium, but not Rhizoctonia) also occurs (Draughon et al., 1988). However, in a later experiment with tomatoes inoculated with spores of C. botulinum (type A and type B)

and Alternaria, and stored under passively modified (sealed, 1.0–2.9% O 2 ) or controlled atmosphere (1% O 2 , 20% CO 2 , and balance N 2 ) storage, botulinum toxin was not detected until after tomatoes became inedible due to mould growth (Hotchkiss et al., 1992). It was concluded that the risk of botulism from consumption of stored tomatoes was insignificant. The growth of C. botulinum on pre-cut pieces of can- taloupe and honeydew melons under different atmospheres, indicated that properly refrigerated (<7 ◦ C) melon pieces did not support neurotoxin formation (Larson and Johnson, 1999). Toxin production did not occur during abuse temperature (15 ◦ C), before gross spoilage. However, if the competing microflora was reduced by UV treatments, toxin production at 27 ◦

C did occur with only marginal spoilage. Viruses, protozoan and parasites. Enteric viruses may be present on fruits, as a result of human faecal

contamination either before or after harvest. Noroviruses (formerly Norwalk and Norwalk-like viruses) and Small Round Structured Viruses (SRSVs) and hepatitis A virus are the major concerns. Workers preparing fruits for consumption are an important source of contamination, but workers at other points in production and processing or polluted water may also be sources. There have been few studies of the survival of hepatitis A or Noroviruses, however, the major influences on survival of pathogenic bacteria mentioned above also determine the survival time of enteric viruses (Cliver, 1983). Some substances present in fruits cause reversible inactivation of viruses (Cliver and Kostenbader, 1979). However, a study of survival of poliovirus in fresh raspberries indicated no loss of viability over a 2-week refrigerated storage period (Kurdziel et al., 2001). A variety of fruits and fruit juices have transmitted hepatitis A (Cliver, 1983) or viral gastroenteritis (Caul, 1993).

Potentially, fruit could serve as the vehicle for a variety of protozoan diseases transmitted by an oral– faecal route, such as apple cider, which has been associated with Cryp. parvum outbreaks. However, the most prominent outbreaks during the 1990s, involved the yearly association of Cyclospora cayetanensis infections with raspberries imported from Guatemalan to Canada and the United States (CDC, 1997b; Herwaldt and Ackers, 1997; Shellabear and Shah, 1997; Soave et al., 1998; Herwaldt et al., 1999; Sterling and Ortega, 1999; Sewell and Farber, 2001). The protozoan is most closely related to the genus Eimeria, a common cause of diarrheal disease in a wide variety of birds and animals (Soave et al., 1998). However, the only known reservoir for Cyc. cayetanensis to date is humans. Thus, the most likely source of contamination of the raspberries associated with the outbreaks is either the farm or

MICROORGANISMS IN FOODS 6

packinghouse workers or the water used for irrigation or treatment of the plants. However, to date, the source of the protozoa in the production and packing house environment, has not been identified.

Mycotoxins. The mycotoxin problems likely to occur in fresh fruits, are the formation of patulin in apples by Pen. expansum, aflatoxins in figs by Asp. flavus, and ochratoxin A in grapes from Asp. carbonarius. Apple rots and mouldy grapes are conspicuous, and consumers can be expected to remove them before fruit is eaten fresh. Hence, patulin and ochratoxin A are more of a problem in juice manufacture, and are discussed in Chapter 22.

Aflatoxins carry over from contaminated figs in the production of fig wine (M¨oller and Nilsson, 1991). Degradation of aflatoxins occur at a constant rate, with a half-life calculated to be 115 days. However, most figs are consumed after drying, so aflatoxins in figs are discussed below.