Processing of Horticultural Crops (1+2)

Steps involved in canning of fruits and Vegetables (Contd..)

3. Washing: Fruit and vegetables are generally washed with water to remove dust, dirt and adhering surface micro-flora. Fruits like peach, apricot etc are lye peeled so not washed before peeling. On the other hand, washing after peeling removes vitamins and minerals and should be discouraged. Different methods of washing include soaking or agitating in water, washing with cold or hot water sprays etc.

• Mechanical washers involve agitating or tumbling the commodity on moving belts or revolving screens while they are immersed in water or subjected to water sprays.
• Washing by using high pressure sprays is most satisfactory.
• Detergents are frequently used in the wash or rinse water.
• Vegetables may be soaked in dilute solution of potassium permanganate or chlorine (25-50 ppm) for disinfection.
• The water temperature should be kept low to keep the fruit firm and to reduce leaching losses.
• High pressure sprays should not injure the fruits.

Bacteria and other contaminants can accumulate in the wash water and hence appropriate cleaning and chlorination practices be followed.

4. Sorting and grading: Sorting and grading ensures the removal of inferior or damaged commodity. For sorting, inspection belt can be used, in addition to trained personnel who detect poor quality produce unsuitable for canning.

• Automatic colour sorters can be used for sorting to reduce labour cost.
• The fruit and vegetables are graded to obtain uniform quality with respect to size, colour etc. after preliminary sorting.
• Grading can be done either manually or with the help of grading machines.
• For mechanical grading, the fruit and vegetables are passed over screens with holes of different diameter.
• Different types of mechanical graders include screen grader, roller grader, rope or cable grader etc. Screen graders fitted with vibrating screens of copper with circular openings are most commonly used. A set of six screens is generally provided to accommodate different sizes.
• Soft and berry fruits are generally graded manually.
• Plums, cherries and olives are graded whole while peaches, apricot, pears, mangoes etc are graded after cutting them into halves or slices for canning.
• White button mushrooms are graded on cap size basis. Only healthy and light buttons with cap diameter up to 2.5cm and compact head are graded as A grade while, cap diameter beyond 2.5cm as B grade.

5. Peeling, coring and pitting: These are the primary unit operations for preparing fruit and vegetables for canning. Depending upon the type of commodity, peeling and coring methods are selected such as (1) by hand or knife (2) by machine (3) by heat treatment (4) by using lye solution. Cores and pits in fruits like apple, peach, apricot etc are removed by hand or by machine (de-corer).

Typical examples of different methods of peeling fruit and vegetables are given in the Table-7.4.

a) Peeling by hand: Many fruit and vegetables are peeled and cut by hand with the help of peeling knives. The peeling knife with a curved blade and a special guard to regulate the depth of peeling can be used for uniform peeling in case of irregular fruit shapes.

Table-7.4: Common methods for the peeling of fruit and vegetables

S.No.	Peeling method	Commodity
1.	Knife peeling (manual)	All fruit and vegetables
2.	Mechanical knife peeling	Apple, pear, pineapple
3	Hot water peeling	Tomato
4	High pressure steam/water peeling	Potato, tomato
5	Abrasive peeling	Potato, ginger, carrots
6	Flame peeling	Brinjal, onion, garlic
7	Lye peeling	Orange segments, peach, apricot, nectarines, pears.

b) Mechanical peeling: Mechanical peeling, coring and cubing machines are used for peeling pears, apples, carrots, turnip, potatoes etc. Similarly, automatic peelers are used for peeling of peaches and cherries.

c) Mechanical /Knife peeling: Mechanical knife peelers are used for peeling of fruits like apples and pears. In mechanical knife peeler either stationary blades are pressed against surface of rotating food commodity or the rotating blades are pressed against the stationary food to remove the skin.

d) Abrasive peeling: It is used for peeling potatoes, ginger, carrots etc. The food commodity is fed on to the carborundum rollers or placed into a rotating bowl which is lined with carborundum crystal acting as abrasive surface. With the continuous supply of water, the rotating abrasive surface removes the skin from the surface of the food.

e) Flame peeling: Flame peeling is used in onions, garlic and brinjal. The peeler consists of a conveyor belt which carries and rotates food through a furnace heated to more than 1000oC. The outer layer and root hairs of onion are burnt off and charred skin is removed manually.

f) Peeling by heat or hot water: In this method peaches and potatoes are scalded in steam or boiling water to soften and loosen skin, which is subsequently removed manually. Infra-red heat peeling can also be used for peeling of apples and tomatoes.

g) Flash steam peeling: In flash-steam peeling, the fruit and vegetables are fed into a slow rotating (4-6 rpm) pressure vessel. High pressure steam (1500 kPa) is then introduced into the rotating vessel to expose all food surfaces to the steam for specified period depending upon the type of fruit. When the pressure is instantly released, the steam formed under the skin causes the surface of the food to flashes off. Most of the peeled material is discharged with the steam and finishing is done with additional water sprays to remove any skin traces.

h) Lye peeling: Lye is an boiling aqueous solution of caustic soda (Sodium hydroxide) or Potassium hydroxide (1-2%) used in conjunction with ample water supply and heat source for peeling. Fruit and vegetables like peaches, nectarines, apricot, sweet orange segments, carrots and sweet potatoes are peeled by dipping them in boiling caustic soda (1-2%) for 1-2 minutes (depending upon the strength of lye, temperature/maturity and nature of fruit or vegetable) followed by dipping in cold water. The hot lye loosens the skin from the flesh underneath which is removed by gentle rubbing of fruit by hand. The fruit can also be dipped in a dilute solution of hydrochloric acid or citric acid for few seconds to neutralize the alkali. The method is very quick and efficient to reduce wastage and peeling cost. The effectiveness of lye peeling depends upon lye concentration and temperature, product holding time and agitation.

• Lye peeling equipment varies from simple stainless steel (SS) pan for lye solution with SS baskets as cages for holding the food commodity to fully automatic system.
• In cottage and small scale canning units, the peeling system consists of three SS tanks attached in series, the one of which is having provision for steam, the second tank contain dilute solution of citric acid or hydrochloric acid while the third is filled with tap water.
• The fruit or vegetables placed in perforated SS crates/ basket or cage are dipped in the first tank which contains boiling hot lye solution. After 1-2 minute of dipping treatment, the crates are immediately dipped in second tank to neutralize the lye and final washing is carried out in third tank.

6. Cutting/halving/ slicing: After peeling, the fruits are halved or cored either manually or mechanically. However, peeled fruit should always be kept submerged in either water, containing 1-2 % salt solution or acid to avoid enzymatic browning. Peaches, apricot, pears, tomatoes etc are peeled before canning. However, the fruits which are canned retain better nutrients as compared to peeled fruits.

7. Blanching: Treatment of fruit and vegetables with boiling water or steam for short periods followed by immediate cooling prior to canning is called blanching. The basic objectives of blanching are as under:

• To inactivate enzymes
• To clean the product initially to decrease the microbial load and to preheat the product before processing
• To soften the tissue to facilitate compact packing in the can
• To expel intracellular gases in the raw fruit to prevent excessive pressure built up in the container.
• To allow improved heat transfer during heat processing
• To ensure development of vacuum in the can and to reduce internal can corrosion.

Blanching is carried out either by hot water or using live steam. Water blanching is generally of the immersion type or spray type as the product moves on a conveyer. Only soft water should be used for blanching as hard water toughens the tissue and destroys the natural texture.

8. Prevention of browning: Some fruits which cannot be blanched due to their delicate tissue structure are treated with some chemicals to prevent oxidative browning, occurring due to exposure to oxygen during peeling and slicing. Oxidative browning is caused by action of oxidase enzyme with catechol and tannins and is common in peach, apple, potato, mushroom, cherry, apricot, grapes and persimmon. Pineapple, tomato and melons are however not prone to browning. Common methods used to prevent browning are as under:

• Sulphite treatment: Fruits are dipped in a solution containing 2000-4000 ppm SO2 for 2-5 minutes. SO2 fumigation can also be used commonly for grapes dehydration.
• Acids: Common acids used to increase acidity include citric, fumaric, tartaric, acetic, phosphoric etc. Low pH of solution is known to act as inhibitor for enzyme polyphenol oxidase thus inhibits the browning of fruits. The peeled fruits, slices or cut surfaces are dipped in a 1-2 % citric acid solution to prevent browning.
• Antioxidants: Ascorbic acid is commonly used as an antioxidant in most canned fruits. It acts as an inhibitor of peroxidase in some fruits like kiwi fruit. It also reduces quinones, which are generated by polyphenol oxidase upon oxidation of polyphenols to phenolic compounds thus preventing their conversion to brown pigments. Ascorbic acid can be used as such or mixed in dry sugar, citric acid or in syrups.
• Sugars: Sugar syrup is used to prevent browning in peeled and sliced fruits by inhibiting oxidation by partially excluding air in the tissues. Sugar is mixed with ascorbic acid and citric acid as an effective agent against loss of texture, colour and flavour. Addition of chitosan in filtered apple and pear juices also prevents enzymatic browning.
• Salt: Dipping of peeled and sliced fruit and vegetables in 1-2% salt solution also prevent enzymatic browning, as salt acts as inhibitor for polyphenol oxidase.

9. Filling in cans: Tin cans are washed in hot water or in steam jet to remove any adhering dust or foreign matter. The cans are then sterilized by dipping in hot water tank or the cans are passed through a steam sterilizing tunnel before use. Generally plain cans are used however, for coloured fruits like plums, black grapes; strawberries etc lacquered cans are employed. The fruit and vegetable either slices, halves or whole are filled into the cans keeping in view the declared drain weight.

10. Syruping or brining: The cans are filled with hot sugar syrup (35-55%) for fruits and with hot brine (2-10%) concentration for vegetables. The purpose of syruping or brining is to help in transfer of heat within the food pieces during processing. It also improves the taste of the canned product, fill up the inter-space between the fruit or vegetables in the can. The syrup or brine is added to the can at a temperature of 79-820C, leaving 0.32-0.47cm head space either manually or in automatic machines. In automatic machines, the prepared syrup or brine is drawn into the cans through a horizontal pipe having a row of small holes. The cans travel on a continuous belt in an inclined position below the syrup or brine pipe and get filled, the overflowing excess syrup is pumped back into the syrup tank by a centrifugal pump.

11. Exhausting: Exhausting is a unit operation in which practically all air from the contents in the can is removed before sealing. The purpose of exhausting and creation of vacuum is to create an anaerobic environment in the can that would inhibit microbial spoilage. The removal of air from the contents also reduces the risk of corrosion and pin holing of the tin plate and discoloration of can contents. Exhausting helps in better retention of vitamin C. Expansion and shriveling of contents during heating help to avoid over filling or under filling of the can. (Corn and peas expand on boiling in brine while strawberries shrivel upon heating in sugar syrup). The vacuum in can prevents bulging of the can during storage at higher attitudes or in hot climate. It also prevents excessive pressure and strain during sterilization.

Methods of exhausting: There are generally three methods of exhausting the cans to remove headspace gas and creation of vacuum.

a) Heat/thermal exhausting: Heat exhausting is used in cans. The can covered with the lid or loosely sealed or clinched is passed through a tank of hot water at about 82-87oC or on a moving belt through a covered steam box. In water exhaust box, the cans are placed in such a manner that the level of water is 1.3-2.5 cm below their tops. The time of exhausting varies from 5 to 25 minutes depending upon the nature of the product. At the end of the exhausting, the temperature at the centre of the can should be about 79oC. During exhausting, the steam replaces the air inside the can and it is sealed while still hot.

b) Steam flow or steam-vacuum closing: In this system, high steam pressure is injected into the can headspace (at 100oC for 5-8 minutes) just prior to closing. Thus, all the air inside the can is quickly replaced with steam, which will condense and form vacuum following seaming. Steam vacuum closure coupled with hot fill, assures very high vacuum in the can.

c) Mechanical vacuum sealing: In high speed mechanical vacuum sealing, the cans filled with the product and covering syrup or brine, are passed through a clincher that clinches the cans (first operation roll seam) but does not form an airtight seal. The cans are subjected to a vacuum for a short period of time to remove the free headspace air but not all dissolved gases within the product. However, during this process some syrup may be drawn along with the dissolved air. To avoid syrup spillage, a pre-vacuumizing step before vacuum closing is recommended. High vacuum closing is also used in case of glass jars where the jar is placed in a closed chamber in which high vacuum is maintained.

12. Seaming/closing: Immediately after exhausting, the cans are sealed by using a double seamer. Double seaming is a two step operation. In the first operation, the can lid is inserted on the can body hook by holding and rotating the lid-in-position can between two rollers. This operation is called as clinching; during which first operation roller gently guides the lid in the body hook. The next step is to press the seam using the second operation roller, which results in an appropriate overlap of the body hook and cover hook which results in an appropriate countersink. Between the cover hook and body hook lies a layer of sealing compound which ensures the sealing process. The critical parameters for an ideal hermetic seam are body hook, cover hook, seam thickness, seam width and overlap which need to be carefully controlled to prevent leakage in the can.
Immediate closing of the cans is required to prevent excessive cooking of the surface of the product. Double seamers are of different designs and capacities like hand operated, semi-automatic and fully automatic. Modern double seamers operate at high speeds (300 cans per minute) while liquid products are sealed in cans at speed of up to 1600 per minute.

13. Coding/Embossing: Coding of lid of the can is essential to identify the can, once it is closed. The code provides the necessary information about the product like name of canning unit, product packed in the can, date of packing; lot number etc. Coding is done on the second lid (end cover) of the can just before sealing.

14. Heat processing: The cans after sealing are immediately transferred to the heating retorts to achieve sterilization of contents. Heat processing consists of heating cans to a predetermined time and temperature of heating to eliminate all possibilities of microbial spoilage. Over cooking should be avoided as it spoils the texture, flavour and appearance of the product. Generally all fruits and acid vegetables can be processed satisfactorily in boiling water (100oC) as the presence of acid retards the growth of bacteria and their spores. While non acidic vegetables (except tomato and rhubarb) are processed at higher temperatures of about 115-121oC under pressure. It needs to be ensured that required temperature reach the centre of the can. The temperature at the centre of the can should be maintained for sufficiently long period to destroy spores of more heat resistant bacteria.

Processing methods: Processing methods differ with the kind of fruit and vegetables to be processed. The cans containing most fruit and acid vegetables (pH < 4.5) are heated in open cookers, continuous non-agitating cookers and continuous agitating cookers.

• Open cooker consists of stainless steel (SS) or galvanized iron tanks to which perforated water pipes are placed underneath the false bottom to supply the steam for heating of water. The sealed cans are placed in SS or GI crates and immersed in the tank containing boiling water.
• In continuous non-agitating cookers, the cans travel in boiling water in crates carried by over-head conveyors on a continuous moving belt.
• While in continuous agitating cookers, the sealed cans while moving on the belt are rotated by a special mechanical device to agitate the contents of the cans. This helps in reducing the processing time.
• For low acid foods like vegetables (pH > 4.5), with hard texture, the processing is carried out in a pressurized vessel (retorts) at elevated temperatures (= 110oC) under higher steam pressure (2-3 atmospheres). The retorts vary in shape and size (horizontal or vertical), type of operation (batch to continuous, non-agitating to agitating) and with different types of heating media such as water, steam, steam/air or flame.
• In small scale canning units, vertical stationary retorts are generally used. They are made of cast iron cylinders and are fitted with a lid which can be bottled steam tight. They are provided with steam and water feeds, drain cock, safety valve, pressure gauge and thermometers.