Food and Industrial Microbiology

Module 3. Microbiology of food commodities

Lesson 9
MICROBIAL SPOILAGE OF CEREALS AND BAKERY FOODS

9.1 Introduction

Cereals are important foods which provide bulk of our dietary requirements. They are also source of carbohydrates which are metabolized by body for energy generation. Besides cereals also provide minerals, proteins and vitamins. India produces a large variety of cereals such paddy, wheat, maize, barley millets like, jowar, bajra, ragi. Various types of products are prepared from cereals. Cereal products can be broadly classified into the following groups:

• Whole cereals where only the husk of the grain is removed, e.g. rice, wheat, gram, lentils, etc.

• Milled grain products are made by removing the bran and usually the germ of the seed and then crushing the kernel into various sized pieces. These include wheat flour, maida, semolina (rawa), etc.

• Processed cereals like weaning food, breakfast cereals, etc.

• Ready mixes like cake mix, idli mix, vada mix etc.

The country is self sufficient in grain production and is the second largest rice producer in the world with a 20% share. But due to constantly increasing population there is still a shortfall in cereals. A large amount of these cereals are spoilt every year due to various factors.

9.2 Spoilage Factors

The grains are low moisture commodities due to which they are less susceptible to spoilage and have greater shelf-life. The spoilage mainly occurs due to moisture absorption during storage leading to fungal growth at high temperature and humidity. Before bulk packaging and storage, the whole grains are fumigated to reduce microbial load and increase storage period. The factors influencing the quality of cereals are:

9.2.1 Physical

Physical losses are caused by spillages, which occur due to use of faulty packaging materials.

9.2.2 Physiological

Physiological losses include respiration and heating in grains, temperature, humidity and oxygen.

9.2.3 Biological

Biological losses occurs due to micro-organisms, insects, rodents, etc.

The sources of contamination in cereals are:

• Soil
• Air
• Insects
• Natural microflora of harvested grains

9.3 Cereal Grains and Flours

At initial stages, the grains are contaminated by Pseudomonas, Micrococci, Lactobacillus and Bacillus. The initial bacterial population may vary from 10³ to 10⁶ per gram while mold population may be more than 10⁴ spores per gram.

Due to low moisture content grains and flours usually have long shelf life if these are properly harvested or stored under proper conditions as microbial growth is not supported. If due to any reason they attain moisture, the microbial growth may occur with molds growing at initial stages of moisture while yeasts and bacteria may grow with increasing moisture.

Spoilage of stored grains by molds is attributed to the following factors:

• Type and number of microorganisms
• Moisture content of more than 12-13%
• Storage temperature
• Physical damage

Most common species of molds are Aspergillus, Rhizopus, Mucor, Fusarium. A significant aspect of spoilage of molds is production of mycotoxins, which may pose danger to health.

The process of flour making such as washing, milling reduce the microbial content. Moisture content of less than 15% does not allow growth of molds. Most molds and bacteria in flours can grow only above 17% moisture, thus moistening of flours is essential for spoilage by microbes (Fig 9.1).

Fig. 9.1 Stem rot and head blight of wheat and barley- Fusarium culmorum and Fusarium graminearum

Storage fungi- Penicillium, Aspergillus and Fusarium when grain stored under moist conditions.

9.4 Spoilage of Bread

Bread is a major product prepared using flours. Dough is prepared from flours which undergo fermentation for which desirable microorganisms must grow. If this fermentation exceeds the required limits, it causes souring. Excessive growth of proteolytic bacteria reduces the gas holding capacity which is otherwise required for dough rising. Spoilage of bread is usually of two types viz. moldiness and ropiness.

During bread making, it is baked at very high temperature, thereby there are less chances of survival of microorganisms. Thus the contamination usually occurs when cooling is done as well as during packing, handling and from the environment. The molds which are prevalent are Rhizopus stolonifer (referred as bread mold), Penicillium expansum, Aspergillus niger. Mucor and Geotrichum also develop.

Ropiness in bread is usually due to bacterial growth and is considered more prevalent in home made breads. The chief causative organism is Bacillus subtilis or B. licheniformis. These are spore forming bacteria with their spores surviving baking temperatures. These spores can germinate into vegetative cells, once they get suitable conditions as heat treatment activates them. In ropiness, the hydrolysis of bread flour protein (gluten) takes place by proteinases. Starch is also hydrolysed by amylases, which encourage ropiness. The manifestation of ropiness is development of yellow to brown color and soft and sticky surface. It is also accompanied by odor.

Another type of spoilage of bread is chalky bread which is caused by growth of yeast like fungi Endomycosis fibuligera and Trichosporon variable. This spoilage is characterized by development of white chalk like spots.

An unusual spoilage of bread is Red or Bloody bread, which is due to the growth of bacteria Serratia marcescens. This organism produces brilliant red color on starchy foods giving blood like appearance. Neurospora and Geotrichum may also be involved in imparting pigmentation during spoilage of bread.

Some spoilage of bread are shown in Figure 9.2 to 9.4.

Fig. 9.2 Green spored mold- Penicillium expansum

• Bread mold- Rhizopus stolonifer.
• White cottony mycelium and black spots
  

Fig . 9.3 Red bread mold- Neurospora sitophila

• Ropiness of home-made breads- Bacillus subtilis (Bacillus mesentericus).
• Ropyness due to hydrolysis of flour protein by proteinase of the bacillus and capsulation of bacillus

Fig. 9.4 Chalky bread

• Chalky bread—chalk like white spots due to yeast like fungi ----Endomycopsis fibuligera and Trichonospora variable