Food and Industrial Microbiology

Module 3. Microbiology of food commodities

Lesson 8
MICROBIAL SPOILAGE OF VEGETABLES

8.1 Introduction

Vegetables form an integral part of diet due to their role in providing various types of vital nutrients such as carbohydrates, minerals, vitamins, roughage etc. Vegetables being a part of fresh produce, contain high moisture which makes them highly perishable foods and hence more prone to spoilage. Microorganisms gain entry into vegetables from various sources. These sources include:

• Soil
• Water
• Diseased plant
• Harvesting and processing equipments
• Handlers
• Packaging and packing material
• Contact with spoiled vegetables

The conditions in which vegetables are stored and transported after harvesting also contribute to rate of spoilage. Other than microbial, sources, the spoilage of vegetables can also occur due to the activity of native enzymes.

8.2 Types of Spoilage in Vegetables

The microbial spoilage of vegetables is predominately of following types

8.2.1 Spoilage due to pathogens

The plant pathogens which infect stem, leaves, roots, flowers and other parts or the fruit itself.

8.2.2 Spoilage due to saprophytes

Vegetables have general microflora inhabiting them. These organisms under certain conditions can grow on these vegetables and spoil them. The list of these organisms is given in Table 8.1. There are certain secondary invaders which may enter the healthy food or grow after growth of pathogens.

It is well known that plant diseases are mostly caused by fungi. Thus most of the spoilage causing pathogens in vegetables are fungi. Fungi have specific characteristics when spoiling food as it leads to mushy areas which may be water soaked. The fungi produce characteristic spores which may be pigmented. The pigmentation helps in identification of the type of spoilage by fungi. The bacterial diseases too cause spoilage of vegetables but to a lesser extent. Figure 8.1 represent bacterial and fungal diseases of tomato.

Table 8.1 Normal microflora of vegetables

Table 8.2 The major types of spoilages by pathogens in vegetables

Spoilage in vegetables is largely affected by composition of vegetable. The non acidic foods are thus spoiled by bacterial rot while acidic foods with dry surfaces are more prone to mold spoilage. The product on which organism grows and types of organisms growing largely determine the character of spoilage.

Bacterial Soft Rot

Caused by Erwinia carotovora and Pseudomonas such as P. marginalis. Bacillus and Clostridium spp. are also implicated.

Breaks down pectin, giving rise to a soft, mushy consistency, sometimes a bad odour and water-soaked apperance.

Vegetables affected- onions, garlic, beans, carrot, beets, lettuce, spinach, potatoes, cabbage, cauliflower, radishes, tomatoes, cucumbers, watermelons.

Some types of spoilage in vegetables by bacteria are shown in the Figure 8.1 to 8.7

Fig. 8.1 Bacterial and fungal disease of tomato

Soft rot in tomato caused by Erwinia carotovora (left)

Blue mould rot in tomato caused by Penicillium spp (right)

Fig 8.2 Bacterial soft rot in tomato, capsicum, and potato

Fig 8.3 Black leg of potatoes Erwinia carotovora var. atroseptica

Fig 8.4 Black rot of cabbage and cauliflower- xanthomonas campestr



Fig. 8.5 Bacterial wilt of beans Corynebacterium flaccumfaciens

Fig 8.6 Slime of lettuce- Pseudomonas marginalis

Fig. 8.7 Sliminess/souring- bacteria

8.2.3. Fungal spoilage of vegetables

Penicillium, Cladosporium, Rhizopus, Aspergillus spp. are responsible for various defects in vegetables.

Some types of spoilage in vegetables by fungi are shown in the Figure 8.8 to 8.13.

Gray mold rot – caused by Botrytis cinera in vegetables. Favoured by high humidity and warm temperature

Fig. 8.8 Gray mold rot in peas and tomato

8.2.3. Fungal spoilage of vegetables

Penicillium, Cladosporium, Rhizopus, Aspergillus spp. are responsible for various defects in vegetables.

Some types of spoilage in vegetables by fungi are shown in the Figure 8.8 to 8.13.

Gray mold rot – caused by Botrytis cinera in vegetables. Favoured by high humidity and warm temperature

Fig. 8.9 Black mold rot- Aspergillus niger, dark brown to black mass

Fig. 8.10 Pink mold rot- Trichothecium roseum

Fig. 8.11 Fusarium rot- Fusarium spp.

Fig. 8.12 Green mold rot- Cladosporium and Trichoderma

Fig. 8.13 Brown rot- Sclerotinia spp.

Table 8.3 Examples of fungal spoilage of vegetables