Rational for Food Preservation

Question 1

What causes about 10-20% of all agricultural commodities lost each year (pre-harvest deterioration)?

Answer 1

Weeds, insects, microorganisms, rodents and birds.

Question 2

All foods (agricultural & aquatic products) undergo varying degrees of deterioration after harvest (post-harvest) and during storage.

True or false?

Answer 2

True

Question 3

What kinds of deterioration happens to food?

Answer 3

Losses occur in the nutritional value, safety, and aesthetic appeal (colour, texture, flavour).

Food is subject to physical, chemical and biological deterioration.

Food deterioration involves: heat, cold, light, oxygen, moisture, dryness, food enzymes, microorganisms and macroorganism.

Question 4

What is the objective of food preservation technologies?

Answer 4

To delay the onset of spoilage and to enable the creation of new food products from the resource commodities.

Question 5

What are the good, the bad, and the ugly microorganisms?

Answer 5

Question 6

Which microorganisms contribute to producction of fermented foods?

Answer 6

Microorganisms can be put to good use for the production of fermented foods, but that they can also be the causes of spoilage and food borne diseases.

Bacteria, yeasts and moulds may contribute to production of fermented foods, as well as food spoilage and food borne disease.

Viruses although not considered true microorganism they can be agents of food borne disease but do not cause food spoilage, nor are they used to produce fermented foods.

Question 7

What microorganism grow fastest in food?

Answer 7

Bacteria

Question 8

What two forms do bacteria exist in?

Answer 8

• vegetative cells, which are actively metabolizing cells, consume nutrients and produce waste products.
• spores, which are the dormant form of the bacterial cell. The spore is analogous to the seed of a green plant. All of the genetic material is contained within the spore. When favourable conditions are encountered, the spore germinates and produces an actively metabolizing bacterial cell capable of cell division).

Question 9

How do yeasts reproduce?

Answer 9

Budding

Question 10

Compare yeast to bacteria.

Answer 10

They generally grow more slowly than bacteria but can tolerate more severe environmental conditions than bacteria.

For example yeasts are not inhibited by pH to the same extent as bacteria

Yeasts can grow in many foods with low water activity that would normally inhibit growth of bacteria

Some yeasts are used to produce fermented foods and beverages.

Question 11

Compare mould to yeast.

Answer 11

Moulds are filamentous and are also found on most foods of agricultural and aquatic origin. Most moulds produce spores.

Like yeasts, moulds can grow on foods that have a low pH and also in foods with low water activities that would inhibit growth of bacteria and yeasts.

Some moulds are used in the production of mould-fermented foods (e.g. mould ripened cheeses), but most moulds are agents of food spoilage and many also produce toxins (mycotoxins) under favourable conditions.

Question 12

What is clostridium botulinum?

Answer 12

In food preservation and processing, the spores of Clostridium botulinum, an anaerobic bacterium, are of great concern because the spores are very heat resistant.

Clostridium botulinum is also the group of bacteria that produces the toxin that causes the very serious illness, botulism.

Question 13

How are microorganisms characterized?

Answer 13

On the basis of temperature ranges and oxygen requirements over which growth occurs.

Question 14

What is a thermophile?

Answer 14

Thermophiles grow best at high temperatures.

Most thermophilic organisms are spoilage-causing.

Question 15

What is a mesophile?

Answer 15

Mesophiles grow well in moderate temperatures.

Many mesophiles have an optimum growth temperature of 37°C.

Most spoilage and disease-causing organisms are mesophiles.

Question 16

What are psychrophiles and psychrotrophs?

Answer 16

Psychrophiles grow well in cold temperatures, while Psychrotrophs have adapted to living and multiplying in cold environments.

Psychrotrophs are the major cause of spoilage in refrigerated foods

Question 17

What type of microorganism grows only in the presence of dissolved oxygen?

Answer 17

AEROBIC
grow only in the presence of dissolved oxygen
(Bacteria, yeast, mould)

Question 18

What kind of microorganism grow only in the absence of dissolved oxygen?

Answer 18

ANAEROBIC
grow only in the absence of dissolved oxygen
(Bacteria)

Question 19

Which microorganism can grown in the presence or absence of oxygen?

Answer 19

FACULTATIVE ANAEROBE
can grow in the presence or absence of oxygen
(Bacteria, yeast)

Question 20

What are pests?

Answer 20

Pests are major contributors to post-harvest losses

Rodents alone ruin >30,000,000 tonnes of food each year worldwide

Insects are believed to destroy 5-10% of the U.S. grain crop annually, and in some parts of the world this figure can be as high as 50%

Pests will damage the food and open it to microbial contamination

Rodent, insect and bird control are important factors in the agriculture and food industries.

Question 21

What are parasites?

Answer 21

Parasites can cause damage to food quality.

An example is the visible appearance of parasitic cysts in fish flesh which lowers the quality and market value of infested products.

Parasites such as Trichinella spiralis in pork and Anisakis species in some types of fish can also cause health problems in humans if they are ingested through inadequately cooked or improperly processed foods, infested with those parasites.

Question 22

Give an example of deterioration of food quality by enzymes.

Answer 22

The softening of fruit tissue and the changes in flavour and colour during storage after picking are examples of deterioration of food quality by enzymes.

It is enzymatic action that causes the softening of apple tissue during storage, with texture changing from that of a crisp apple to a somewhat soft and mealy apple with a concomitant loss of sweetness.

Enzymes in the apple tissue degrade pectins that cement the cells together, leading to the textural change.

Question 23

What causes the browning of apples?

Answer 23

This browning phenomenon is known as enzymatic browning and is catalyzed by an enzyme known as polyphenol oxidase which catalyzes the oxidation of colorless phenols in the tissues to brown colored compounds, as shown in the following equation:

Question 24

How can enzymes be inactivated?

Answer 24

Enzymes can be inactivated by means of heat, chemicals (e.g. antioxidants), and by controlling the gaseous environment.

Question 25

How are reaction rates affected by temperature?

Answer 25

Rates of reactions generally increase as the temperature increases until an optimum is achieved, after which further increases in temperature cause the rates of reaction to decrease because of inactivation of microorganisms or enzymes or because of inhibitory effects on chemical reactions.

Question 26

How do extreme temperatures affect food?

Answer 26

Excessive heat also denatures proteins, breaks emulsions, removes moisture from foods (drying out), and destroys vitamins.

Cold temperatures can also deteriorate food. A well known example is “chill injury”, the change in texture and discolouration of fruits and vegetables when they are exposed to freezing temperatures.

Freezing temperatures can deteriorate liquid foods such as milk, causing emulsions to break and fat to separate, and denaturing protein causing it to curdle or coagulate.

“Freezer burn” due to loss of moisture can occur in solid and liquid foods.

Question 27

Explain how the gain or loss of moisture may affect food.

Answer 27

Water loss during storage (e.g. wilting of lettuce in the refrigerator), or water uptake (e.g. by dehydrated foods) can lead to deterioration.

Retrogradation of starch, resulting in staling of bread, is caused by packing of linear starch molecules leading to the exclusion of water that was previously absorbed during gelatinization. The bread becomes tough and develops a dry texture.

Changes in water activity (free versus bound water) can influence chemical and enzymatic reactions and microbial growth.

Question 28

Explain why oxygen is an important factor in food quality.

Answer 28

Many oxidative reactions lead to deterioration in the quality of food and, in some cases, to losses in nutritive value.

Oxidative deterioration is often accelerated by light.

Oxidation of vitamins and colour pigments can lead to the deterioration of nutritive quality and aesthetic appeal of foods.

Question 29

What is rancidity?

Answer 29

For example, deterioration frequently occurs because of oxidation of the fats in food products. The development of rancidity in breakfast cereals, vegetable oils and oil-based products, and in deep-fried foods is due to reaction of oxygen with fats, particularly those with high unsaturated fatty acid content. This type of rancidity is known as oxidative rancidity.

This is in contrast to rancidity induced in foods upon the release of free fatty acids by very high temperatures or by the action of lipase enzymes, either endogenous or produced by spoilage-causing microorganisms.

The latter rancidity is known as hydrolytic or lipolytic rancidity.

Question 30

Describe how physical abuse may affect food quality.

Answer 30

Physical abuse causes tissue disruption and release of enzymes into tissues which can lead to changes such as enzymatic browning mentioned earlier.

Furthermore, improper packaging, for example, can cause crushing and tissue damage, making foods such as fruits and vegetables particularly susceptible to microbial invasion as well as enzymatic and chemical reactions.

Question 31

Explain how time affects food quality.

Answer 31

For the majority of foods, quality will decrease with time.

Food preservation, packaging and storage practices are aimed to maintain this quality for as long as possible (shelf life); however, eventually the quality of any food will decrease with time.

Question 32

What are perishable foods?

Answer 32

Perishable foods are those foods that are not processed or are only minimally processed and have a shelf life of less than 60 days.

Spoilage of perishable foods is usually caused by microbial growth or senescence1

Examples of perishable foods are: meat, leafy vegetables, soft fruits, and milk.

Question 33

What are semi-perishable foods?

Answer 33

Semi-perishable foods last between 2 to 6 months as a result of some form of preservation method.

Examples of semi-perishable foods are: ice cream, cheeses, and dry snack foods.

Question 34

What are shelf-stable foods?

Answer 34

Shelf-stable foods have a shelf life greater than 6 months.

Examples of shelf-stable foods are: cereal grains, dehydrated pasta, some frozen foods, canned foods, and dehydrated vegetables.

Question 35

What is senescence?

Answer 35

Fruits and vegetables continue to respire after harvest.

Respiration is fueled by carbohydrate metabolism that generates adenosine triphosphate (ATP) needed to promote various reactions in the tissues. When nutrients become exhausted, the tissues begin to deteriorate (soften, change colour, rot, produce off-odours).