Principles of fresh food
Department of Food and Dairy Technology
• Storage life of foods can be prolonged or shortened depending on the
conditions of storage and physical abuse the food products receive
prior to, during and following storage.
• In overall analysis the cool storage of nearly all foods, fresh or
preserved is beneficial to the retention of quality.
Nature of harvested crops
• The quality of harvested fruits and vegetables is dependent on the
conditions of growth and on post harvest treatments.
• The climatic factors and cultural practices are complex.
• The length of storage is a function of composition, resistance to attack
microorganisms, the external conditions of temperature, and the gases
in the environment.
• A mature harvested fruit contains a variety of oxidizable substrates and
the molecular machinery required to perform oxidative reactions.
• Respiration is the major process of concern and its mechanism is essentially the
same in fruits as in other plant and animal life.
• In fruits, as in other materials, biological oxidations involve a number of metabolic
pathways in which synthetic and degradative reactions are interdependent.
• Ripening in fruit is a critical period of transition from the stages of cell
disorganization and death. Ripening means those changes in sensory factors of
color, texture and taste which render the fruit acceptable to eat.
• These changes may be detected by analyzing transformations in pigments, pectin,
carbohydrates, acids and tannins, etc.
• Climacteric- Stages of the fruit associated with ethylene production and rise in
• Respiration – oxidative breakdown of complex substrate molecules –
starches, sugar and organic acids to simpler molecules- co2 and H2o.
• Events of senescence and ripening – signaled by abrupt changes in
• Aerobic respiration-3 complex reaction – catalyzed by a number of
specific enzymes – add an energy containing phosphate group to the
substrate molecule , rearrange the molecule, breakdown the molecule
to simpler one.
• metabolic pathways
• Tricarboxylic acid
• Electron transport system
• Pentose Phosphate pathway
Metabolic interrelations among the various
The climacteric pattern of respiration and
associated changes in fruit ripening
• After harvest the decline in the rate of oxygen uptake and the CO2 evolution
to a low value is followed by a sharp rise to a peak, termination in a post
• The peak to minimum ration tend to increase with temperature.
• This ratio varies among fruits.
• For example, it is much higher for a banana than for an apple.
• Fruit reaches the eating stage at the climacteric peak or sometime after the
peak, depending on the species and conditions of storage.
• Unique changes take place during the sharp rise in respiration from the
pre-climacteric minimum to the climacteric peak.
• The slope of the rise varies with species, maturity, temperature and
oxygen and CO2 content of the storage chamber.
• Various anabolic and catabolic reactions are associated with the
ripening of fruit.
• Example: depolymerisation include the hydrolysis of starch to glucose
in the banana and the breaking of polygalacturonic chains of pectin.
• Tissues with vegetative or Floral meristems- asparagus, broccoli- high
• Nuts and tubers – Low respiration rate
Climacteric Fruits Non-Climacteric Fruits
Fig Grape fruit
• A multitude of physiological processes is present in edible plant at the time of
• Example: on removal of vegetative parts (fruits, roots, stem) from the parent plant,
the tissues are deprived of their normal supply of water, minerals and organic
molecules( sugars, hormones) which previously translocated to them from other
parts of the plant.
• Although photosynthic activity of the plant is then negligible, most tissues retain
capabilities of a diversity of metabolic reactions which are specific to a given
commodity and variety.
• These are seen in events such as rotting, ripening, sprouting, toughening and
• The kind and intensity of activity in detached plant parts determines to
a large extent their storage life.
• Seeds, fleshy roots, tubers and bulbs are morphologically and
physiologically designed to maintain the tissues in a dormant
condition until favorable environmental situations are available for
germination and growth.
• Metabolic activity is low but completely halted in such tissues.
• Fruits, leaves and stems – physiologically conditioned for senescence
rather than dormancy.
Plant product storage
• Fresh fruits and vegetables maintain life processes during storage so
long as they are alive, they are able to resist the growth of spoilage
microorganisms to some extent.
• Antimicrobial agents and senescence inhibitors can be used to delay
the onset of spoilage during storage.
• Two methods of applying these agents are
1. a spray or dip in a solution or suspension in water/ wax
Treatments to control post harvest decay of
Heat evolved by living tissues
• Freshly harvested fruits, vegetables and grains are alive.
• These living tissues respire and released energy in the form of heat.
• The amount of heat released varies with the commodity and increases
as the temperature of the storage chamber increases.
• Reducing the rate of respiration could prolong the storage life.
Temperature of cold storage rooms
• Temperature control in cold storage rooms is most important.
• Variations from desired conditions may be most damaging.
• These variations can be prevented if the storage rooms are well insulated,
have adequate refrigeration equipment and the spread between the
temperature of the refrigerating coils and the temperature of the storage room
• The difference between the temperature of the refrigerant and the temperature
of the storage room is important in maintaining desired humidity.
Recommended storage temperature, relative humidity,
approximate storage life and average freezing points of
fruits and nuts.
Metabolism a function of temperature
• The metabolism of living tissue is a function of the temperature of the environment.
• Living organisms have a temperature which is optimum for growth.
• Higher temperatures are injurious.
• Lower temperature greatly retard metabolism.
• Low temperatures near the freezing point of water, are effective in reducing the rate at which respiration
• Such temperatures have been found to be valuable in short term preservation of foods.
• For every 10°C the temperature is lowered, it may be estimated that the rate of a reaction will be halved.
• Storage of food at temperatures near 0°C to 5°C may be anticipated to prolong the period in which foods may
• Lower temperatures not only decrease the respiration rate but also halt the growth of spoilage
• Three types of microorganisms with an optimal temperature for
Creating energy deficits
• Ice has been employed since early times to prolong the storage life of foods.
• It is the energy deficit of ice which has great utility.
• When all the energy deficit of ice is supplied, water remains; the temperature of the water food
substrate begins to come into equilibrium with the environment.
• Protective mechanisms may prolong this process by insulation.
• The temperature of the water food substrate reaches that where microorganisms can multiply, the
food will deteriorate very rapidly.
• Ice is therefore used presently where its characteristics are of value.
• One feature of ice in cooling foods is that ice does not desiccate the food.
Creating energy deficits mechanically
• One of the important invention of man is mechanical refrigeration.
• Ammonia gas takes up energy as it is expanded.
• This heat is taken from the atmosphere or chamber or environment.
• The expanded ammonia gas is then compressed.
• This requires energy to be exerted on the system.
• The compressed gas is now hot
• Heat is removed from the compressed gas by means of running water or circulating air over the tubes containing
the hot gas.
• The gas is liquefied. The cycle is then repeated. With such a system accurate temperature control is possible. The
refrigeration developed may be made to work directly on the food.
Schematic representation of a simple
ammonia refrigeration system
Refrigeration load needed
• Refrigeration requirement for a chamber of fruits and vegetables
• Initial temperature of the food
• Final storage temperature
• The rate of respiration and heat evolved
• Specific heat of food
• Amount of food to be placed in the room
• Heat load would be obtained by multiplying the specific heat for the food by the
number of degrees the temperature will be lowered by the weight of food.
• Btu- amount of heat required to raise one pound of water one degree Fahrenheit at
or near the water’s point of maximum density.
• Cooling process- fruits or vegetables live on and evolve heat
Specific heat of foods
• The specific heat of food is required in calculating the refrigeration load.
• The specific heat for a food can be estimated from the equation:
• Example: apples have 85% moisture.
• The specific heat of apples can be estimated to be 0.008× 85+0.20 or 0.88
Specific heat = 0.008 (% H2O in food) + 0.20
Cold injury to fruits and vegetables
• Fruits and vegetables are susceptible to cold injury at above freezing temperatures.
• There is wide variation in fruits and vegetables in their injury due to freezing.
• Some foods are not injured with a slight exposure to freezing temperatures; other
foods can be frozen and thawed several times without permanent injury.
• Living tissues must be kept living if the food values and eating quality are to be
maintained by cold storage practices.
Animal product storage
• Microorganisms causing spoilage of fresh meat, poultry, eggs and
most dairy products have optimum temperature for growth at 20°C to
• Pseudomonads –major cause of spoilage for most fresh meats, poultry,
eggs and bulk tank raw milk.
• Most critical factors influencing spoilage rate are temperature and initial level of contamination.
• Small changes in the temperature in the range of 0- 7°C have an effect on refrigerated shelf life of beef,
poultry and pork.
• Coli-aerogens and micrococci are found at vey low levels in spoiled beef held at 4°C.
• These groups increase to moderate levels at 9°C and become a significant portion of the spoilage
population at 15°C.
• Pseudomonads dominate the spoilage flora at all temperatures; with chicken meat pseudomonads
dominate at 1°C and become of less significance as holding temperature increased.
• Acinetobacter and members of the family Enterobacteriaceae are of little consequence at 1°C but
increases significantly with the higher temperatures of 10°C and 15°C.
• The importance of temperature as well as time in holding milk cannot be overstated.
• Manufacturing-grade milk received into a plant with bacterial counts in the millions requires little time
to spoil further.
• This is especially true for milk received at 7°C or more, where the generation time may be only 8hr or
less compared to 12hr at 5°C and 16hr at 2°C. this is due to the psychrotrophic nature of the majority
of bacteria in the milk.
• Coliforms do not rapidly multiply in adequately refrigerated milk.
• Psychrotrophic coli-aerogens bacteria may constitute 5 to 20% of the psychrotrophic microflora of
farm bulk tank milk at time of retailing.
• These may increase only 100 or 1000 fold in 3 days at 3 °C to 5 °C, yet rarely attain dominance in milk
at temperatures of less than 8 °C to 10°C.
• The spoilage of eggs is also depends upon the degree of refrigeration.
• Tests on raw albumen for 3 weeks at 5°, 10°, 15°C and 20 °C reveal that pseudomonas sp.
Dominate at 5°C and decrease in significance as the holding temperatures approaches 20°C.
• conversely, Enterobacter liquifaciens was not detected at 5°C and increased significantly as
the holding temperatures was raised to 20°C.
• It is clear that temperature markedly influences the rate of spoilage of animal products and
the types of bacteria.
• Environmental temperature influences the types and numbers of bacteria present in a
• Aseptic packaging has extended refrigerated shelf life of 6 weeks to 6 months.
Storage of eggs
• Eggs should be stored at the lowest temperature possible yet not permit
the interior of the egg to solidify.
• Shell characteristics also affect the storage quality of eggs, as thick shells
withstand the solidification better than thin shelled eggs.
• It is considered that -1°C is the ideal storage temperature for eggs.
• A relative humidity between 82-85% is generally considered optimum for
• Eggs may pick up odor quickly during storage therefore should not be
stored in the same room with other commodities.
Effect of cold storage on quality
• An unrefrigerated fruit or vegetable usually spoils rapidly and soon has
little food value for man. If similar fruits and vegetables are held
temporarily in cool storage, life processes are retarded, but the net result is
a longer period in which the food is acceptable for man to eat.
• It is not to be expected that a fruit after seven months storage will be
identical to a freshly harvested fruit. Occasionally it is necessary to
refrigerate fruits and vegetables in common storage chambers.
Storage of grains
• Grain stores better and more cheaply than other major foods.
• Storage is not an end in itself.
• Storage usually repeated in transporting grain from producer to processor to
• as a minimum, grain must be stored from one harvest to the next.
• Storage of grains occurs on farms.
• Grain stores better when it is sound, clean and especially when it is dry.
• High moisture increases storage hazards.
• Moisture can be determines accurately and grain can be dried without damage to
• The milling, baking quality, malting quality of wheat and corn can be damaged
when the kernels become too hot during drying.
• Technological changes are occurring steadily in the grain industry.
• Examples from the milling and baking industries are found in α amylase
activity and protein content.
• The importance of controlling α amylase is seen when mechanical work
during mixing dough is used to reduce the fermentation period in this
• Bread of acceptable quality can be made from flour of lower protein content.
• When the protein content is reduced the moisture content of the bread is
more difficult to control.
• This difficulty can be offset by mechanically damaging the starch granules
during milling so that the starch will additional moisture.
• Damaged starch is more susceptible to attack by α amylase with consequent
deterioration of the crumb texture of the loaf.
• The α amylase in grain increases rapidly if the grain becomes wet after harvest and
starts to sprout.
• It can increase before visual evidence of sprouting is apparent.
• The first step in germination is the development and activation of enzymes.
• Their function is to transform the contents of the kernel, making them available to
nourish the new plant until the emerging roots and shoots take over the task.
• Control of α amylase can be exercised by visual examination for signs of incipient
sprouting or even weathering.
• Protein content of flour can be controlled by blending wheat's of appropriate protein
• The primary aim during storage is simply to prevent deterioration in quality. This is
done by indirectly through control of moisture, air movements and by preventing attack
my microorganisms, insects and pests.
• Sinha (1973) describes a grain bulk or storage unit as a man made ecological
system in which living organisms and their non living environment interact.
• Deterioration results from interactions among physical, chemical and biological
• The environment of the grain physical variables such as temperature, Carbon
dioxide, oxygen , moisture and an array of organic compounds which are the by-
products of biological activity in the bulk.
• Atmospheric temperature and intergranular air temperature – safe
and prolonged storage
• Mites do not develop below 5°C, nor insects below 15 °C
• Fungi do not develop below 0°C
• Effect of temperature on an organism is correlated with the amount
of moisture present
• Moisture content of below 13% arrest the growth of microorganism
• Maintaining Moisture content below 10% limit –storing grains
• Moisture content- changeable from season to season and from
climatic zone to another.
• Both grain and microbial associates- respire by same physiological
• Metabolism to produce energy- occurs either in presence or absence
• Effects – loss of weight, gain in moisture content, rise in the level of
co2 and rise in temperature of air.
Control of insects and mites
• Protection of grains- chemical treatments
• Use of fumigants, pesticides- prolonged protection against invasion by
• Methyl bromide, ethylene oxide, hydrogen cyanide, ethylene
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