Crop Process Engineering

Lesson 8. Fruits and Vegetables Processing

 8.1 Freezing

 Freezing is a low temperature preservation process where the product is frozen at -38 ⁰C and stored -18 ⁰C. Freezing is cheaper than canning and frozen products are close to fresh products and of better quality the metabolic activity and spoilage due to post harvest chemical are retarded by freezing. Through the product preserved by freezing retains their quality appreciably, the major disadvantage of the process is that the low temperature has been maintained during handling, transportation and storage before the product is finally consumed. Suitable fruits and vegetables for freezing are mango slices, pulp, pineapple slices, guava slices, orange segments, peas, carrot, cauliflower, beans, etc.

 When compared to the most other food preservation methods, freezing requires the least amount of food preparation before storage and under optimum conditions it has the best nutrient, flavour, and texture retention. Since food remains microbiologically safe during freezing, its shelf life is determined by chemical and physical changes that occur during storage.

 IQF refers to Individual Quick Freezing of every particle/aggregate using fluidization in a stream of very cold air. Fluidization leads to high heat transfer co-efficient and therefore very rapid freezing and hence better quality. For example, freezing of green peas in an air blast freezer may take 3-4 h whereas it is only 10-12 min by IQF. This results in for better texture and there is no humb or block formation. Some of other important IQF products are frozen fruit dices and cut vegetables.

 Equipments of freezing

 1) Freezer/cold room

2) Quick freezing Equipments- fluidized bed freezer, automatic package freezer, continuous plate freezer, continuous can freezer

3) Direct Immersion equipments- Brine Freezer, Frog freezer, and Bartlett freezer

4) Rotating cold drum

5) Foot operated polythene bag sealer- sealing polyethylene bags of different gauges after filling of fruits and vegetables

 Methods of freezing

 i) Quick (0 to -40 ⁰C for 30 min.)

ii) Sharp (-15 to -29 ⁰C for 3-72 h)

iii) Cryogenic (-196 ⁰C)

iv) Dehydro-freeing (50 % moisture removed)

8.2 Dehydration

 Dehydration is the removal of moisture from fruits and vegetables by artificially produced heat under controlled condition of temperature, relative humidity and air flow. In dehydration, sufficient moisture is removed so that the product is free from spoilage; but this must be done in such a way to preserve food value as far as possible. Rate of dehydration is so done in hygienic condition to have products of uniform colour than sun dried. Dehydration reduces the bulk, requires less storage space and usually cheaper that the other methods of preservation. The suitable fruits and vegetables for dehydration are grape, date, fig, raw mango, anola, ber, litchi, apricot, banana, apple, carrot, leafy vegetables, etc. FPO specification for dehydrated fruits and vegetables are;

 1) Moisture content shall not exceed 20 per cent and 24 per cent (w/w) respectively.

2) Fruits and vegetables used for drying be clean, wholesome and shall be practically free from insect or fungal attack.

3) Dehydrated products may contain permitted preservations.  

 Equipments used for Dehydration

 Blanching unit:  It can be used for pre-treatment (steam blanching) of fruits and vegetables before processing.

 Types of dryers used: Kiln drier- pieces; Cabinet/tray drier- pieces; purees, liquids; tunnel- pieces; continuous conveyer belt-purees, liquids; Belt trough-pieces; air lift- granules; spray- liquid; vacuum-purees, liquid, pieces; Fluidized bed-small pieces; Drum or roller-Liquid, purees.

 Vacuum drier- It is used for dehydration of fruits and vegetables without any change in sensory attributes.

8.3 Canning

Canning is a method of food preservation. Canning may be defined as heating and sealing of food material in a hermetically sealed container.

a) Fruit Canning:  The most important fruits for canning are mango, pineapple, guava, litchi, cherry. Strawberry, jackfruit, etc.

 Specification

- The head space in the can shall not be more than 1.6 cm.

- The drained weight of the fruit shall not be less than 50 per cent and fruit should be firm

- No preservative shall be added

- No artificial colour shall be present

- The can shall not show any positive pressure at sea level and shall not show any sign of bacterial growth when included at 37 ⁰ C for a week.

Equipments required for canning

- Fruit cutter/ slicer

- Blancher (water/steam)

- Canning equipments (double seamer, flanger, can reformer)

- Retorting equipment

- Can opener

- Exhaust Unit

- Canning material (Tin plate can, Aluminium can, TFS can)

b) Vegetable Canning: In vegetable canning, the specific requirements for brine strength, exhaust, processing temperature, time and types of cans, etc., are needed. Suitable vegetables for canning are cauliflower, carrot, peas, okra, beans, cabbage, etc. Mushrooms can also be canned.

 Specifications

- The head space in the can shall not be more than the 1.6 cm.

- The drained weight of preservative of the vegetables shall not be less than 55 percent except in tomato (50%)

- No addition of preservative and no artificial colour shall be present except in the case of peas.

- The can shall not show any sign of bacterial growth when incubated at 37⁰ C for a week.

 Process of Canning

 1) Selection of fruits and vegetables

i)   Fruits and vegetables should be absolutely fresh.

ii) Fruits should be ripe, but firm, and uniformly mature. Over-ripe fruits should be rejected  because they are infected with microorganisms and give a poor quality product.

      Unripe fruits should be rejected because they generally shrivel and toughen on canning.

iii) All vegetables except tomatoes should be tender.

iv)Tomatoes should be firm, fully ripe and of deep red colour.

v)  Fruits and vegetables should be free from dirt.

vi) They should be free from blemishes, insect damage or mechanical injury.

 2) Grading:

The selected fruits and vegetables are graded according to size and colour to obtain uniform quality. This is done by hand or by machines such as screw grader and roller grader. Fruits like berries, plums and cherries are graded whole, while peaches, pears, apricots, mangoes, pineapples, etc., are generally graded after cutting into pieces or slices.

 3) Washing:

It is important to remove pesticide spray residue and dust from fruits and vegetables. One gram of soil contains 1012 spores of microorganisms. Therefore, removal of microorganisms by washing with water is essential. Fruits and vegetables can be washed in different ways. Root crops that loosen in soil are washed by soaking in water containing 25 to 50 ppm chlorine (as detergent). Other methods of washing are spray washing, steam washing, etc.

 4) Peeling: The objective of peeling is to remove the outer layer. Peeling may be done in various ways. (hand peeling, steam peeling, mechanical peeling, lye peeling, flame peeling).

 5) Cutting: Pieces of the size required for canning are cut. Seed, stone and core are removed. Some fruits like plum from which the seeds cannot be taken out easily are canned whole.

 6) Blanching:

It is also known as scalding, parboiling or precooking. Fruits are generally not blanched leaving the oxidizing enzyme system active. Sometimes fruit is plunged for a given time-from half to, say, five minutes, according to variety-into water at from 180 °F to 200 ^oF, and then immediately cooled by immersion in cold water. The object is to soften the texture and so enable a greater weight to be pressed into the container without damage to the individual fruit. Blanching is usually done in case of vegetables by exposing them to boiling water or steam for 2 to 5 minutes, followed by cooling. The extent of blanching varies with the toed. This brief heat treatment accomplishes the following:

 i) Inactivates most of the plant enzymes which cause toughness, discolouration (polyphenol oxidase). mustiness, off-flavour (peroxidase), softening and loss of nutritive value.

ii) Reduces the area of leafy vegetables such as spinach by shrinkage or wilting, making their packing easier.

iii) Removes tissue gases which reduce sulphides.

iv) Reduces the number of microorganisms by as much as 99%.

v) Enhances the green colour of vegetables such as peas, broccoli and spinach.

vi) Removes saponin in peas.

vii) Removes undesirable acids and astringent taste of the peel, and thus improves flavour.

viii) Removes the skin of vegetables such as beetroot and tomatoes which helps in their peeling.

 7) Cooling: After blanching, the vegetables are dipped in cold water for better handling and keeping them in good condition.

 8) Filling:

Before filling, cans are washed with hot water and sterilized but in developing countries these are subjected to a jet of steam to remove dust and foreign material. Automatic, large can-filling machines are used in advanced countries but choice grades of fruits are normally filled by hand to prevent bruising in India. Hand filling is the common practice. After filling, covering with syrup or brine is done and this process is called syruping or brining.

 9) Exhausting: The process of removal of air from cans is known as exhausting. After filling and lidding or clinching, exhausting is essential. The major advantages of exhausting are as under:

 i)  Corrosion of the tinplate and pin holing during storage is avoided.

ii) Minimizes discolouration by preventing oxidation.

iii) Helps in better retention of vitamins particularly vitamin C.

iv) Prevents building of cans when stored in hot climate or at high altitude.

v) Reduces chemical reaction between the container and the contents.

vi) Prevents development of excessive pressure and strain during sterilization.

 Containers are exhausted either by heating or mechanically. The heat treatment method is generally used. The cans are passed through a tank of hot water at 82 to 87 °C or move on a belt through a covered steam box. In the water exhaust box, the cans are placed in such a manner that the level of water is 4-5 cm below their tops. The exhaust box is heated till the temperature of water reaches 82 to 100 ⁰C and the centre of the can shows a temperature of about 79 °C. The time of exhausting varies from 6 to 1 a minutes, depending on the nature of the product. In the case of glass jars or bottles, vacuum closing machines are generally used. The bottles or jars are placed in a closed chamber in which a high vacuum is maintained.

 It is preferable to exhaust the cans at a lower temperature for a longer period to ensure uniform heating of the contents without softening them into pulp. Exhausting at high temperature should be avoided because. The higher the temperature, the more is the volume of water vapour formed, and consequently the greater the vacuum produced in the can.

 10) Sealing:

Immediately after exhausting the cans are sealed airtight by means of a can sealer. In case of glass jars a rubber ring should be placed between the mouth of the jar and the lid, so that it can be sealed airtight. During sealing the temperature should not fall below 74 °C.

11) Processing:

Heating of foods for preserving is known as processing, however, in canning technology processing means heating or cooling of canned foods to inactivate bacteria. Many bacterial spores can be killed by either high or very low temperature.   Such drastic treatment, however, affects the quality of food. Processing time and temperature should be adequate to eliminate all bacterial growth. Moreover, over-cooking should be avoided as it spoils the flavour as well as the appearance of  the product. Almost all fruits and add vegetables can be processed satisfactorily at a temperature of 100 °C, i.e., in boiling water.The presence of acid retards the growth of bacteria and their spores. Further, they do not thrive in heavy sugar syrup which is normally used for canning of fruits. Vegetables (except the more acid ones like tomato and rhubarb) which are non-acid in nature, have a hard texture, and proximity to soil which may infect them with spore-bearing organisms are processed at higher temperatures of 115 to 121 ⁰C.

The sourness of fruits and vegetables is due to their acid content (measured in pH) which has a great influence upon the destruction of microorganisms. The lower the pH the greater is the ease with which a product can be processed or sterilized. Fruits and vegetables can be classified into the following four groups according to their pH value

Bacterial spores can be more easily destroyed at pH 3.0 (fruits) than at pH 5.0 to 6.0 (vegetables, except tomato and rhubarb). Bacterial spores do not grow or germinate below. pH 4.5. Thus, a canned product having pH less than 4.5 can be processed in boiling water but a product with pH above 4.5 requires processing at 115 at 121 ^oC under a pressure of 0.70 to 1.05 kg/cm² (10 to 15 lb/sq inch). It is essential that the centre of the can should attain these high temperatures.

 The temperature and time of processing vary with the size of the can and the nature of the food: the larger the can, the greater is the processing time. The processing time 'for different canned fruits and vegetables is given in the tables Under 'Canning of Fruits' and 'Canning of Vegetables'. Fruits and acid vegetables are generally processed in open type cookers, continuous non-agitating cookers and continuous agitating cookers, while vegetables (non-acid) are processed under steam pressure in closed retorts known as automatic pressure cookers. In India, small vertical stationary retorts (frontispiece) are generally used for canned vegetable processing. The sealed cans are placed in the cookers, keeping the level of water 2.5 to 5.0 cm above the top of the cans. The cover of the cooker is then screwed down tightly and the cooker heated to the desired temperature. The period of sterilization (processing) should be counted from the time the water starts boiling. After heating for the required period the cooker is removed from the fire and the petcock is opened. When the pressure comes down to zero the cover is removed and the cans are taken out.

 12) Cooling: After processing. The cans are cooled rapidly to about 39 °C to stop the cooking process and to prevent stack-burning. Cooling is done by the following methods:

 i)  dipping or immersing the hot cans in tanks containing cold water;

ii) letting cold water into the pressure cooker specially in case of vegetables;

iii) Spraying cans with jets of cold water; and

iv) exposing the cans to air.

 Generally the first method, i.e., dipping the cans in cold water, is used. If canned products are not cooled immediately after processing, peaches and pears become dark in colour, tomatoes turn brownish and bitter in taste, peas become pulpy with cooked taste and many vegetables develop flat sour (become sour).

 13) Storage:

After labeling the cans, they should be packed in strong wooden cases or corrugated cardboard cartons and stored in a cool and dry place. The outer surface of the cans should be dry as even small traces of moisture sometimes induce rusting. Storage of cans at high temperature should be avoided, as it shortens the shelf-life of the product and often leads to the formation of hydrogen swell. The marketable life of canned products varies according -to the type of raw materials used. Canned peach, grapefruit, pineapple, beans, spinach, pea etc., can be stored for about two years, while pear, apricot, carrot, beetroot, tomato, etc., can be stored for a comparatively short period only.