Vegetable
processing
General introduction
In
developing countries agriculture is the mainstay of the economy. As such, it
should be no surprise that agricultural industries and related activities can
account for a considerable proportion of their output. Of the various types of
activities that can be termed as agriculturally based, fruit and vegetable
processing are among the most important.
The
main objective of fruit and vegetable processing is to supply wholesome, safe,
nutritious and acceptable food to consumers throughout the year.
Fruit
and vegetable processing projects also aim to replace imported products like
squash, yams, tomato sauces, pickles, etc., besides earning foreign exchange by
exporting finished or semi-processed products.
Practically
any fruit and vegetable can be processed, but some important factors which
determine whether it is worthwhile are:
·
The demand for a particular fruit or
vegetable in the processed form;
·
The quality of the raw material, i.e. whether
it can withstand processing;
·
Regular supplies of the raw material.
A. Small-Scale Processing. This is done by
small-scale farmers for personal subsistence or for sale in nearby markets. In
this system, processing requires little investment: however, it is time
consuming and tedious. Until recently, small-scale processing satisfied the
needs of rural and urban populations. However, with the rising rates of
population and urbanisation growth and their more diversified food demands,
there is need for more processed and diversified types of food.
- Intermediate-Scale Processing. In this scale of processing, a group of small-scale processors pool their resources. This can also be done by individuals. Processing is based on the technology used by small-scale processors with differences in the type and capacity of equipment used. The raw materials are usually grown by the processors themselves or are purchased on contract from other farmers. These operations are usually located on the production site of in order to assure raw materials availability and reduce cost of transport. This system of processing can provide quantities of processed products to urban areas.
- Large-Scale Processing. Processing in this system is highly mechanised and requires a substantial supply of raw materials for economical operation. This system requires a large capital investment and high technical and managerial skills. Because of the high demand for foods in recent years many large-scale factories were established in developing countries. Some succeeded, but the majority failed, especially in West Africa. Most of the failures were related to high labour inputs and relatively high cost, lack of managerial skills, high cost and supply instability of raw materials and changing governmental policies. Perhaps the most important reason for failure was lack of adequate quantity and regularity of raw material supply to factories. Despite the failure of these commercial operations, they should be able to succeed with better planning and management, along with the undertaking of more in-depth feasibility studies.
FAO
maintains (in FAO, 1992c), that the basis for choosing a processing technology
for developing countries ought to be to combine labour, material resources and
capital so that not only the type and quantity of goods and services produced
are taken into account, but also the distribution of their benefits and the
prospects of overall growth. These should include:
- increasing farmer/artisan income by the full utilisation of available indigenous raw material and local manufacturing of part or all processing equipment;
- cutting production costs by better utilisation of local natural resources (solar energy) and reducing transport costs;
- generating and distributing income by decentralising processing activities and involving different beneficiaries in processing activities (investors, newly employed, farmers and small-scale industry);
- maximising national output by reducing capital expenditure and royalty payments, more effectively developing balance-of-payments deficits through minimising imports (equipment, packing material, additives), and maximising export-oriented production;
- Maximising availability of consumer goods by maximisation of high-quality, standard processed produce for internal and export markets, reducing post-harvest losses, giving added value to indigenous crops and increasing the volume and quality of agricultural output.
In
order to assure preservation in long term storage, it is necessary to reduce
respiration and transpiration intensity to a minimum possible; this can be
achieved by:
o
Maintenance
of as low a temperature as possible (down to 0° C),
o
Air
relative humidity increased up to 85-95 % and
o
CO2
percentage in air related to the vegetable species.
When
vegetables are maturing in the field they are changing from day to day. There
is a time when the vegetable will be at peak quality from the stand-point of
colour, texture and flavour.
This
peak quality is quick in passing and may last only a day. Harvesting and
processing of several vegetables, including tomatoes, corn and peas are rigidly
scheduled to capture this peak quality.
After
the vegetable is harvested it may quickly pass beyond the peak quality
condition. This is independent of microbiological spoilage; these main
deteriorations are related to:
a)
loss of sugars due to their consumption during respiration or their conversion
to starch; losses are slower under refrigeration but there is still a great
change in vegetable sweetness and freshness of flavour within 2 or 3 days;
b)
production of heat when large stockpiles of vegetables are transported or held
prior to processing.
c)
the continual loss of water by harvested vegetables due to transpiration,
respiration and physical drying of cut surfaces results in wilting of leafy
vegetables, loss of plumpness of fleshy vegetables and loss of weight of both.
Reception
This
covers qualitative and quantitative control of delivered vegetables. The
organoleptic control and the evaluation of the sanitary state, even if they are
very important steps in vegetables' characteristics assessment, cannot
establish their technological value.
On
the other hand, laboratory controls do not precisely establish their technological
properties because of the difficulty in putting into showing some deterioration
when using rapid control methods.
One
correct method of vegetable quality appraisal is their overall evaluation based
on the whole complex of data that can be obtained by combining an extensive
organoleptic evaluation with simple analysis that can be performed rapidly in
plant laboratory. These analysis can be:
A.
Refractometric
extract (tomatoes, fruit, etc.);
B.
Specific
weight (potatoes, peas, etc.)
C.
Consistency
(measured with Tenderometers, penetrometers, etc.)
D.
Boiling
tests, etc.
Temporary
storage
This
step should be as short as possible and better completely eliminated.
Vegetables can be stored in:
I.
simple
stores, without artificial cooling;
II.
in
refrigerated stores; or, in some cases,
III.
in
silos (potatoes, etc.).
Simple
stores should be covered, fairly cool, dry and well ventilated but without
forced air circulation which can induce significant losses in weight through
intensive water evaporation; air relative humidity should be at about 70-80%.
Refrigerated
storage is always preferable and in all cases a processing centre needs a cold
room for this purpose, adapted in volume I capacity to the types and quantities
of vegetables (and fruits) that are further processed.
Washing
Washing
is used not only to remove field soil and surface micro-organisms but also to
remove fungicides, insecticides and other pesticides,
Sorting
This step covers two separate operations:
a) removal of non-standard vegetables (and fruit) and
possible foreign bodies remaining after washing;
b) quality grading based on variety, dimensional,
organoleptical and maturity stage criterion.
Skin
Removal/peeling
Some vegetables require skin
removal. This can be done in various ways.
a) Mechanical
b) Chemical
c) Thermal
Size reduction
This step is applied according to
specific vegetable and processing technology requirements.
Blanching
The special heat treatment to
inactivate enzymes is known as blanching. Blanching is not indiscriminate
heating. Too little is ineffective, and too much damages the vegetables by
excessive cooking, especially where the fresh character of the vegetable is
subsequently to be preserved by processing.
This heat treatment is applied
according to and depends upon the specificity of vegetables, the objectives
that are followed and the subsequent processing / preservation methods.
Two of the more heat resistant
enzymes important in vegetables are catalase and peroxidase. If these are
destroyed then the other significant enzymes in vegetables also will have been
inactivated. The heat treatment to destroy catalase and peroxidase in different
vegetables are known, and sensitive chemical tests have been developed to
detect the amounts of these enzymes that might survive a blanching treatment.
Catalase and peroxidase inactivation tests are presented in section 9.2.9.
Because various types of vegetables
differ in size, shape, heat conductivity, and the natural levels of their
enzymes, blanching treatments have to be established on an experimental basis.
As with sterilisation of foods in cans, the larger the food item the longer it
takes for heat to reach the centre. Small vegetables may be adequately blanched
in boiling water in a minute or two, large vegetables may require several
minutes.
Blanching as a unit operation is a
short time heating in water at temperatures of 100° C or below. Water blanching
may be performed in double bottom kettles, in special baths with conveyor belts
or in modern continuous blanching equipment.
In order to reduce losses of hydrosoluble
substances (mineral salts, vitamins, sugars, etc.) occurring during water
blanching, several methods have been developed:
o Temperature
setting at 85-95° C instead of 100° C;
o Blanching
time has to be just sufficient to inactivate enzymes catalase and Peroxidase.
o Assure
elimination of air from tissues.
Blanching parameters for some vegetables
|
Vegetables
|
Temperature, °C
|
Time, min.
|
|
Peas
|
85-90
|
2-7
|
|
Green beans
|
90-95
|
2-5
|
|
Cauliflower
|
Boiling
|
2
|
|
Carrots
|
90
|
3-5
|
|
Peppers
|
90
|
3
|
Canning
Canning is a method of preserving
food in which the food contents are processed and sealed in an airtight
container. Canning provides a typical shelf life ranging from one to five
years, although under specific circumstances a freeze-dried canned product,
such as canned, dried lentils, can last as long as 30 years in an edible state.
Large quantities of vegetable
products are canned. A typical flow sheet for a vegetable canning operation
(which also applies to fruit for the most part) covers some food process unit
operations performed in sequence: harvesting; receiving; washing; grading; heat
blanching; peeling and coring; can filling; removal of air under vacuum;
sealing/closing, retorting/heat treatment; cooling; labelling and packing. The
vegetable may be canned whole, diced, puréed, as juice and so on.
Other than sterilization,
no method is perfectly dependable as a preservative. For example, the
microorganism Clostridium botulinum
(which causes botulism), can only be eliminated at temperatures
above the boiling point.
Technology
for vegetable powder processing
This technology has been developed in
recent years with applications mainly for potatoes (flour, flakes, granulated),
carrots (powder) and red tomatoes (powder). In order to obtain these finished
products there are two processes:
a) Drying of vegetables down to a final water content below
4% followed by grinding, sieving and packing of products;
b) Vegetables are transformed by boiling and sieving into
purées which are then dried on heated surfaces (under vacuum preferably) or by
spraying in hot air.
Potato
crisp/chip processing
The most important steps involved in
potato crisps processing are:
- Selecting, procuring and receiving potatoes
- Storage of potato stock under optimum conditions
- Peeling and trimming the tubers
- Slicing
- Frying in oil
- Salting or applying flavoured powders
- Packaging
Drying/dehydration
The
technique of drying is probably the oldest method of food preservation
practiced by mankind. The removal of moisture prevents the growth and
reproduction of micro-organisms causing decay and minimises many of the
moisture mediated deterioration reactions.
Heat
and mass transfer
The
two important aspects of mass transfer are:
1.
The
transfer of water to the surface of material being dried and
2.
The
removal of water vapour from the surface.
In
order to assure products of high quality at a reasonable cost, dehydration must
occur fairly rapidly. Four main factors affect the rate and total drying time:
·
The
properties of the products, especially particle size and geometry;
·
The
geometrical arrangement of the products in relation to heat transfer medium
(drying air);
·
The
physical properties of drying medium/ environment;
·
The
characteristics of the drying equipment.
Surface
area generally the fruit and vegetables to be dehydrated are cut into small
pieces or thin layers to speed heat and mass transfer. Subdivision speeds
drying for two reasons:
·
Large
surface areas provide more surface in contact with the heating medium (air) and
more surface from which moisture can escape;
·
Smaller
particles or thinner layers reduce the distance heat must travel to the centre
of the food and reduce the distance through which moisture in the centre of the
food must travel to reach the surface and escape.
Vegetable
juices
Vegetable juices are natural
products constituted from cellular juice and a part of crushed pulp, from the
tissues of some vegetables. These juices contain all valuable substances from
the vegetables: vitamins, sugars, acids, mineral salts and pectic substances.
The most important of these products is tomato juice; in a lower proportion
there are also other juices (carrots, beet, sauerkraut, etc.).
Tomato juice
This product is characterised not
only by its organoleptical properties (taste, colour, flavour) but also by its
vitamin content close to those of fresh tomatoes. Modem technology is oriented
to a maximum maintenance of organoleptic properties and of vitamin content.
At same time, it is important to
assure juice uniformity by avoiding cellulosic particle sedimentation. Juice
stability is assured by a flash pasteurization which assures the destruction of
natural micro-flora, while keeping the initial properties.
The modern technological flow-sheet
covers the following main operations:
PRE-WASHING is carried out by
immersion in water, cold or heated up to 50° C (possibly with detergents to
eliminate traces of pesticides). This operation is facilitated by bubbling
compressed air in the immersion vessel/equipment.
WASHING is performed with water
sprays, which in modern installations have a pressure of 15 at or more.
SORTING/CONTROL on rolling sorting
tables enables the removal of non-standard tomatoes - with green parts, yellow
coloured, etc.
CRUSHING: in special equipment
PREHEATING at 55-60° C facilitates the
extraction, dissolves pectic substances and contributes to the maintaining of
vitamins and natural pigments. In some modern installations, this step is
carried out under vacuum at 630-680 mm Hg and in very short time.
EXTRACTION of juice and part of pulp
(maximum 80%) is performed in special equipment / tomato extractors with the
care to avoid excessive air incorporation. In some installations, as an
additional special care, a part of pulp is removed with continuous centrifugal
separators.
DE-AERATION under high vacuum of the
juice brings about its boiling at 35-40° C.
HOMOGENISATION is done for mincing
of pulp particles and is mandatory in order to avoid future potential product
"separation" in two layers.
FLASH Pasteurization at 130-150° C,
time = 8-12 see, is followed by cooling at 90° C, which is also the filling
temperature in receptacles (cans or bottles).
ASEPTIC FILLING
CLOSING OF RECEPTACLES is followed
by their inversion for about 5 to 7 minutes.
COOLING has to be carried out
intensely.
Full cans do not need further
pasteurization because the bacteria that have potentially contaminated the
tomato juice during filling are easily destroyed at 90° C due to natural juice
acidity.
For bottles, it may be possible to
avoid further sterilisation if the following conditions can be respected:
washing and sterilising of receptacles, cap sterilisation (with formic acid),
filling and capping under aseptic conditions, in a space with UV lamps. In so
far as this is quite difficult to achieve it may be necessary to submit bottles
to a pasteurization in water baths.
The main characteristics of high
quality tomato juice are:
- natural red colour;
- taste and flavour of fresh tomatoes;
- uniformity (without pulp sedimentation);
- total soluble solids: 6% minimum;
- total soluble substances (by refractometer): 5% minimum;
- vitamin C: 15 mg/100ml minimum.
Concentrated tomato products
Tomato
paste
The product with highest production
volumes among concentrated products is tomato paste which is manufactured in a
various range of concentrations, up to 44% refractometric extract. Tomato paste
is the product obtained by removal of peel and seeds from tomatoes, followed by
concentration of juice by evaporation under vacuum.
In some cases, in order to prolong
production period, it may be advisable or possible to preserve crushed tomatoes
with sulphur dioxide as described under semi-processed fruit "pulps".
Technological flow-sheets run
according to equipment/ installation lay-outs, which are especially designed
for this finished product. Manufacturing steps fall into three successive
categories:
- obtaining juice from raw materials;
- juice concentration and
- tomato paste pasteurization.
Benefits
and drawbacks
Benefits
Benefits of food processing include
toxin removal, preservation, easing marketing and distribution tasks, and
increasing food consistency. In addition, it increases yearrly availability of
many foods, enables transportation of delicate perishable foods across long
distances and makes many kinds of foods safe to eat by de-activating spoilage
and pathogenic micro-organisms. Modern supermarkets would not exist without
modern food processing techniques, long voyages would not be possible and
military campaigns would be significantly more difficult and costly to execute.
Processed foods are usually less
susceptible to early spoilage than fresh foods and are better suited for long
distance transportation from the source to the consumer. When they were first introduced,
some processed foods helped to alleviate food shortages and improved the
overall nutrition of populations as it made many new foods available to the
masses.
Processing can also reduce the
incidence of food borne disease. Fresh materials, such as fresh produce and raw
meats, are more likely to harbour pathogenic micro-organisms (e.g. Salmonella)
capable of causing serious illnesses.
The extremely varied modern diet is
only truly possible on a wide scale because of food processing. transportation of
more exotic foods, as well as the elimination of much hard labour gives the
modern eater easy access to a wide variety of food unimaginable to their
ancestors.
The act of processing can often
improve the taste of food significantly.
Mass production of food is much
cheaper overall than individual production of meals from raw ingredients.
Therefore, a large profit potential exists for the manufacturers and suppliers
of processed food products. Individuals may see a benefit in convenience, but
rarely see any direct financial cost benefit in using processed food as
compared to home preparation.
Processed food freed people from the
large amount of time involved in preparing and cooking "natural"
unprocessed foods.[5] The increase in free time allows people much more choice
in life style than previously allowed. In many families the adults are working
away from home and therefore there is little time for the preparation of food
based on fresh ingredients. The food industry offers products that fulfill many
different needs: From peeled potatoes that only have to be boiled at home to
fully prepared ready meals that can be heated up in the microwave oven within a
few minutes.
Modern food processing also improves
the quality of life for people with allergies, diabetics, and other people who
cannot consume some common food elements. Food processing can also add extra
nutrients such as vitamins.
Drawbacks
Any processing of food can have slight effects on its
nutritional density. Vitamin C, for example, is destroyed by heat and therefore
canned fruits have a lower content of vitamin C than fresh ones. The USDA
conducted a study in 2004, creating a nutrient retention table for several
foods.A cursory glance of the table indicates that, in the majority of foods,
processing reduces nutrients by a minimal amount. On average any given nutrient
may be reduced by as little as 5%-20%.
Another safety concern in food processing is the use of food
additives. The health risks of any additives will vary greatly from person to
person; for example sugar as an additive would be detrimental to those with
diabetes. In the European Union, only food additives (e.g., sweeteners,
preservatives, stabilizers) that have been approved as safe for human
consumption by the European Food Safety Authority (EFSA) are allowed, at
specified levels, for use in food products. Approved additives receive an E
number (E for Europe), which at the same time simplifies communication about
food additives in the list of ingredients across the different languages of the
EU.
Food processing is typically a mechanical process that
utilizes large mixing, grinding, chopping and emulsifying equipment in the
production process. These processes inherently introduce a number of
contamination risks. As a mixing bowl or grinder is used over time the food
contact parts will tend to fail and fracture. This type of failure will
introduce in to the product stream small to large metal contaminates. Further
processing of these metal fragments will result in downstream equipment failure
and the risk of ingestion by the consumer.
Food manufacturers utilize industrial metal detectors to
detect and reject automatically any metal fragment. Large food processors will
utilize many metal detectors within the processing stream to both ensure reduced
damage to processing machinery as well risk to the consumer. The first
industrial level metal detector pioneered by Goring Kerr was introduced back in
1947 for Mars Incorporated.
Bibliography
ADM. 1992. Food applications of citric acid
and its salts. ADM Technical Bulletin. Decatur, III., USA
ANON.1960. Food Composition Tables. Rome:
FAO.
Manuals of Food Quality Control.
FAO Food and Nutrition Paper. No. 14. Rome: FAO.
Traditional food plants. FAO Food
and Nutrition Paper No. 42. Rome: FAO.
Traditional food plants. FAO Food
and Nutrition Paper No. 42. Rome: FAO.
Webliography
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