3 Preparatory Operations in Fruits and Vegetables
Ms. Akanksha Wadhera
Preparatory operations are those processes, which transform raw materials into a form that can be packed efficiently into appropriate containers. The selection of raw materials as well as selection of preparatory operations should be done in such a way so as to minimize the waste and to maximize the product yield. The raw materials represent a major cost factor. Hence, it is very important to procure the raw materials that comply with the specifications laid down by the manufacturer of the plant so as to minimize the defect levels. Since preparatory operations generate labor and plant costs, it is essential that these steps must be carefully designed in order to decrease the defect levels and hence decrease the overall cost. Product attributes that affect the preparatory operations are:
1. Size and shape
2.Color
3.Texture and functional properties
4.Maturity
5. Mechanical damage
6.Insect, animal, fungal and microbial damage
7.Extraneous matter
Flow chart showing the different stages in the preparation of fruits and vegetables
Reception and storage of raw materials
All lots of raw material should be inspected for evidence of spoilage or insect infestation at the time of delivery to the plant. Problems of spoilage should be dealt with before the product is brought into the plant in order to avoid contamination of other raw material. As soon as the raw material arrives at the plant, sampling and testing should be carried out to determine compliance with the mutually agreed upon specifications.
Raw materials may have to be stored in the plant due to a number of factors, such as due to temporary oversupply of raw material, or due to temporary power cuts, or due to spread of the production over a longer period (in case where the processes are semi- automatic), or in the case where mixed products with different harvest dates are to be canned, such as in case of peas, potatoes, carrots and beans.
Special attention should be paid to the design and construction of storage areas and rooms to facilitate their cleaning and to prevent infestation and contamination. Fruits like strawberry, peach, plums and apricots should be processed as soon as they are received because they if they are stored, there will be an appreciable loss of quality, which in turn will affect the yield and final product quality.
Vegetables like tomato, mushrooms, lettuce can be stored for short periods under refrigerated condition. Refrigeration or cooling units can also act as dehydrators if relative humidity is not controlled. Hence, the products that are stored under refrigerated conditions should be packaged properly otherwise there will be excessive quality deterioration. Root vegetables such as potato and fruits like apples, pear etc. can be stored in bulk. Potatoes are generally stored in the dark to discourage budding. Fruits are generally stored under controlled atmosphere and low temperatures to delay maturation, onset of senescence as well as to prevent accumulation of heat due to respiration.
Cleaning
The main purpose of cleaning is to remove undesirable foreign material and it should be designed to obtain:
1. Complete removal of separated contaminants and avoidance of recontamination
2. Maximum separation efficiency consistent with minimum wastage of desirable material
3. Minimum quantity and concentrations of residues
The foreign material found on fruits and vegetables can be grouped under the following headings:
| Types of contaminant | Examples |
| Metals | Ferrous and non- ferrous metals, bolts, fillings |
| Minerals | Soil, sand, stones |
| Animal | Hair, bone, insects, larvae |
| Plant | Leaves, twigs, weed seeds, pods, skins |
| Chemicals | Herbicides, pesticides, fertilizers |
| Microbial cells | Soft rots, fungal growth, yeasts |
| Microbial products | Colors, flavors, toxins |
Cleaning should take place as soon as possible opportunity in a food process to prevent damage to subsequent processing equipment by stones or metals, to prevent time and money from being spent on processing contaminants which are then discarded and also to prevent subsequent loss of remaining bulk by microbial growth during storage or delays before processing. Cleaning is thus an effective method of reducing food wastage, improving the economics of processing and protecting the consumers.
Cleaning Methods
The selection of a cleaning method is determined by the nature of the product to be cleaned, the types of contaminant to be removed and the desired condition of the cleaned material. There are basically two types of cleaning methods, which are generally used either singly or in combination- dry cleaning and wet cleaning.
Dry cleaning
Dry cleaning methods are used for products that are smaller, have greater mechanical strength and possess lower moisture content. The main advantages of dry cleaning methods are that, these methods are generally inexpensive and involve cheaper equipment than wet cleaning methods and produce a concentrated dry effluent which may be disposed of more cheaply but it suffers from various disadvantages such as, it is prone to production of dust, which can be a source of product recontamination and in some cases, a fire and explosion hazard.
The main groups of equipment used for dry cleaning are:
- aspirators
- magnetic separators
- Separators based on screening of foods
Forced air separators or aspirators use large quantities of low-pressure air and consume considerable energy. Control of dust is necessary for safety and health reasons and to prevent the spread of contaminants. This method should not be used for products, which are sensitive to oxidation.
Magnetic cleaning is generally used to remove magnetic contaminants that can damage preparatory equipment from the raw materials. Rotating or stationary magnetic drums, magnetic belts, magnets located over belts carrying the food, through which the food is passed, are used involving permanent and electromagnets. The contaminants that adhere to the magnets should be frequently removed to prevent a buildup that can be swept into the product stream, which results in a gross localized recontamination.
Screens remove contaminants that have sizes different from the product. A screen in a frame is the simplest form used which has now been largely replaced by continuous types, such as centrifugal screens and the flat bed screens.
Cleaning by abrasion between food particles or between the food and moving parts of cleaning machinery is used to loosen and to remove adhering contaminants. Trommels, tumbler vibrators, abrasive discs and rotating brushes are used for this purpose. Attention should be paid to dust removal to limit recontamination, protect operators and prevent dust explosions. Other newer methods of cleaning include- electrostatic cleaning, radio-isotope separation as well as X-ray separation.
Wet Cleaning
Wet cleaning is more effective than dry cleaning for removing soil from root crops or dust and pesticide residues from soft fruits or vegetables. It is also dustless and causes less damage to foods than dry methods. Different combinations of detergents and sterilizers at different temperatures allow flexibility in operation. Use of warm cleaning water may accelerate chemical and microbiological spoilage unless careful monitoring is done over washing times and subsequent delays before processing. The main disadvantage of these methods is that these methods produce large volumes of effluent which have high concentrations of biological oxidation demand (BOD) and chemical oxidation demand (COD). Hence, this requires an additional effluent disposal cost. To reduce costs, recirculated, filtered and chlorinated water is used whenever possible. Examples of wet cleaning equipment include- spray washers, brush washers, ultrasonic cleaners and flotation tanks.
Soaking is the simplest wet cleaning method. It is often used as preliminary stage in the cleaning of root vegetables heavily contaminated with adhering soil. The soil is softened and partly removed along with stones, sand and other abrasive materials that could damage the machinery used in subsequent cleaning or preparation. The efficiency of soaking is improved by agitating the raw material to be cleaned by means of propeller- stirrers built into the tank. Alternatively, raw material is fed into a rotating horizontal perforated drum partially submerged in a water tank. The use of warm water and detergents increase the efficiency but the use of chemicals may affect the texture of the food, e.g. sodium hexametaphosphate softens peas while some metal ions toughen peas and peaches destined for canning. Chlorination is used to decrease bacterial load of water in the soak tank. High levels of chlorine affect the food, e.g. potatoes develop blackened flesh.
Spray washing is one of the most widely used methods for wet cleaning of fruits and vegetables. The surface of the food is subjected to water sprays. The efficiency of spray washing depends on several parameters such as water pressure, volume of water, temperature, the distance of the foods from jets, the time of spraying and the number of spray jets used. A small volume of water at high pressure is the most effective combination. This method may have a disadvantage in case of ripe fruits such as strawberries and tomatoes, which may get damaged during this process.
Sound waves of frequencies above 16 kHz are used for ultrasonic cleaning. When a fluid is exposed to ultrasonic waves at frequencies in the range of 20-100 kHz, a rapidly alternating pressure in the path of the waves is created which leads to the rapid formation and collapse of bubbles in the fluid. These effects, called cavitation and decavitation cause the release of energy causing a violent agitation of particles immersed in the fluid. This phenomenon is used to loosen the dirt adhering on the surface of vegetables and grease and wax on fruits.
Flotation washing method is based on the differences in buoyancy of the desired and undesired parts of the raw material to be cleaned. For example, rotten apples sink in water and are removed by fluming the fruits in a tank and collecting the good fruits as overflow. Heavy debris can be removed by fluming dirty product over a series of adjustable weirs arranged in series. The less buoyant contaminants are trapped by the weirs and remain behind, leaving the product contaminated only by material of same or greater buoyancy, which can be separated by passage over a vibrating screen under water sprays. Flotation washer effectively removes stones, dirt and plant debris from peas, beans, dried fruits and similar materials.
Dewatering
Wet cleaning results in a cleaned product that may have some excess water adhering to it. To carry out dewatering, fruits maybe passed over vibratory screens or specially designed rotary screens. In the case of cleaned peas for freezing, centrifuges may be used.
Sorting
Sorting may be regarded as a separation operation based in the differences in physical properties of the raw materials or products. It is an important operation in controlling the effectiveness of many processes in food industry. Separation based on single properties such as color, size, shape or weights of the raw material are generally considered as sorting operations. For example, sorted fruits and vegetables are better suited for mechanized operations of peeling, pitting and blanching. Sorting methods include- weight sorting, size sorting, shape sorting and color sorting.
(A)Weight sorting- The weight of a food product is proportional to the cube of its characteristic dimension and hence, weights sorting is more precise compared to dimensional sorting. Fruits such as apples, pears and citrus fruits, vegetables such as potatoes, carrots and onions are sorted by weight.
(B) Size sorting- Two types of screens are used for size separation such as:
(i) Variable aperture screens using cable, belt, roller or screw sorter
(ii) Fixed aperture screens using stationary, vibratory, rotary, gyratory or reciprocating screens
(C) Shape sorting- Usually, this method is adopted when raw materials contain undesirable material even after size or weight sorting and cleaning. The raw material to be sorted is fed into an upward-moving belt held at an inclination. The smaller shaped fruits easily roll down while desired material moves upward along the belt and is discharged at the upward stream.
(D) Color sorting- It uses optical properties of foods to separate desired materials from contaminants. Optical properties such as reflectance and transmittance of foods are useful in a number of ways. Reflectance properties are used to indicate-
I.Raw material maturity
II. Presence of surface defects III. Extent of heat processing
Transmittance properties of foods are useful in indicating the internal properties of foods such as ripeness and presence of any core defects in fruits and vegetables. The color sorter functions on the basis of photometric scanning of each food unit as it moves past or rotated before a photocell. The signal generated by the photocell is compared to an adjustable pre-set standard signal automatically, and any deviation between the two signals is amplified and is used to activate the sorting gear.
Grading of fruits
Grading is the quality separation on the basis of an overall assessment of those properties, which affect the acceptance of the raw material for processing, and finished food product for consumer acceptance and safety. It strictly means “ the assessment of overall quality of a food using a number of attributes”. For example, apples are graded with the assistance of coloured cards that show the required characteristics of different grades in terms of colour distribution across the fruit, surface blemishes and size and shape of the fruit.
Peeling of fruits
Peeling is used in the processing of many fruits and vegetables to remove unwanted or inedible material, and to improve the appearance of the final product. The main consideration is to minimize costs by removing as little of the underlying food as possible and reducing energy, labour and material costs to minimum. The peeled surface should be clean and undamaged. There are five main methods of peeling:
1. Flash steam peeling
2. Knife peeling
3. Abrasion peeling
4. Caustic peeling
5. Flame peeling
Flash steam peeling is generally used for root crops, where in the raw material is fed in batches into a pressure vessel which is rotated at 4-6rpm. High-pressure steam is introduced and all food surfaces are exposed to the steam by the rotation of the vessel for a predetermined time, which differs according to the type of food. High temperature causes rapid heating of the surface layer but the low thermal conductivity of the product prevents further heat penetration, and the product is not cooked. The pressure is then suddenly released which causes steam to form under the skin, and the surface of the food „flashes off‟. Most of the peelings are discharged with the steam, and the water sprays are needed only to remove any remaining traces. Main advantages of this method include-lower water consumption, minimum product loss, good appearance of the peeled surface, high throughput and the production of a more easily disposable concentrated waste.
Knife peeling is particularly suitable for citrus fruits where the skin is easily removed by pressing the stationary blades against the surface of rotating fruits or vegetables.
In abrasion peeling, food is fed onto carborundum rollers or placed into a rotating bowl, which is lined with carborundum. The abrasive surface removes the skin, which is continuously washed with running water. The main advantages of this method include low energy costs since this operates at room temperature, no heat damage, low capital costs and a good surface appearance of the product. The major limitations of this method are- a higher product loss than flash peeling; production of large volumes of dilute waste which are difficult and expensive to dispose off as well as relatively low throughputs as all pieces of food need to contact the abrasive surfaces.
Caustic peeling uses a dilute solution of sodium hydroxide (lye) heated to 100-120°C. Food is dipped in 10% sodium hydroxide and the softened skin is removed with rubber disks or rollers. These processes reduce water consumption and product losses and give a concentrated skin waste which can be disposed of easily.
Flame peeling is generally used for products like onion and garlic, which have an outer „paper shell‟. In this method, peeler consists of a conveyor belt which carries and rotates the food through a furnace heated to 1000°C. As the product moves in, root hairs and the outer paper shell is burned off and the charred skin is removed by high-pressure water sprays. Average product losses using this method are about 9%.
Size reduction
Size reduction or comminution is the unit operation in which the average size of solid pieces of food is reduced by the application of grinding, compression or impact forces. The main reasons for size reduction are:
1.To obtain particles that can easily and effectively be packed into containers for canning
2.To produce particles that have almost a regular shape and size to facilitate the efficiency and uniformity of the thermal process.
3.To present the final product in an attractive form to the consumer
Different methods of size reduction include- chopping, cutting, slicing and dicing. Fruits like plums, prunes and cherries are pitted and used for canning. Others, such as peaches, apricots and pears, which have stones, are cut into halves along their longitudinal axis. Some fruits are also divided into approximately four equal parts. Others may be sliced or cut into pieces with uniform size and shape such as cubes.
Blanching
It is one of the main methods used to destroy enzymic activity in vegetables and some fruits, prior to further processing. It is not intended as a sole method of preservation but as a pre-treatment, which is normally carried out between the preparation of raw material and the later operations. The factors that influence blanching time are:
- Type of fruit or vegetable
- Size of the pieces of food
- Blanching temperature
- Method of heating
The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam or a bath of hot water. The yield of food from the blanching operation is the most important factor in determining the commercial success of a particular method. In some methods, the cooling stage may result in greater losses of product or nutrients than the blanching stage, and it is therefore important to consider both blanching and cooling when comparing different methods.
Blanching inevitably causes some changes to sensory and nutritional qualities. It causes a modification of the tissue structure. Flexibility of many products is increased by the application of moist heat, which facilitates the packing operation with a minimum of physical damage and permits a greater weight-to-volume ratio. Hence, it is important in control of the fill of the container. Similarly, intercellular air as well as gases are removed during blanching which prevent difficulty during retorting and affect the attainment of F-value during thermal processing.
Also, blanching helps in reduction of superficial microbial and chemical contamination (pesticide residues). It also helps in enzyme inactivation, which prevents any adverse effects in color, flavour as well as textural changes.
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REFERENCES:
- Fellows, P.J. (2000). “ Food Processing Technology”. CRC Press. Woodhead Publishing Limited. Cambridge, England.
- Hui, Y.H. (2006). “Handbook of fruits and fruit processing”. Blackwell Publishing
Suggested Readings
- Fellows, P.J. (2000). “ Food Processing Technology”. CRC Press. Woodhead Publishing Limited. Cambridge, England.
- Hui, Y.H. (2006). “Handbook of fruits and fruit processing”. Blackwell Publishing.
Web References