FREEZING OF GREEN PEASMoksha Chib13FET1003

Why freezing ?▪ Canning has extensively been used as a preservation technique for green peas ▪ Canned peas even though retain their nutritive value, but no one claims that they taste like

freshly cooked green peas. It still leaves much room for enhancement in the flavor and color standpoint

▪ Freezing preservation, if properly carried out, comes much closer to retaining the natural fresh flavor

▪ If the peas are properly blanched, quickly frozen & then stored at a sufficiently low temperature, the colors of the vegetable are retained perfectlyQUALITY

ATTRIBUTESFROZEN

FOODS (% score)

CANNED FOODS (% score)

DRIED FOODS (% score)

OTHERS ( %

score)Freshness 71 14 5 10

Taste 56 30 7 7Appearance 62 28 3 7

Nutritive value 51 20 11 18Value of money 47 29 10 13

Ease of preparation 40 52 3 3

INDIVIDUAL QUICK FREEZING OF PEAS

Individual Quick Freezing (IQF)

▪ Freezing of solid foods/pieces/grains like peas, cut beans to a very low temperature (-18°C) rapidly

▪ Rapid lowering of temp makes it possible to pass quickly below the critical temp of 0-5°C at which intracellular liquids i.e. water freeze and forms ice crystals

Advantages over Slow Freezing:1. Ice crystals formed are much smaller which causes less damage to the cell structure &

texture 2. Shorter freezing period, which means less time is allowed for the diffusion of salts & the

separation of water in the form of ice3. Product is quickly cooled below the temp, at which no microbial growth occurs, preventing

decomposition of food during freezing4. Inherent speed & greater output and hence, greater capacity for the commercial freezing

plants with the resultant cost reduction

Process chart for quick freezing of green peas

Peas Size Grading

Washing/ Dry

CleaningPeeling/

Preparation

BlanchingPre-cooling

or chilling to 5-7°C

Freezing (IQF) at -

18°CWeighing & Packaging

Frozen Peas

Selection of varieties suitable for freezing

▪ Proper varieties at optimum maturity & freshness are selected▪ Variety must yield well (high insect resistance, high edible part recovery, resistance to

unfavorable climatic conditions & uniformity of quality) & have a growth habit which will permit harvest by a mowing machine

▪ Must be tender & sweet at optimum maturity▪ Must turn to a brilliant green color on blanching▪ Must not have a tendency to split during blanching, cooling & packaging and easily podded▪ Must not tend to develop off-flavors during preparation for freezing

Varieties of peas commonly used for canning, the Alaska, Surprise, Perfection etc. do not yield particularly desirable frozen products as they are relatively light in color & small-seeded as compared to varieties commonly grown in market gardens & for shipment during off-season.Most retail markets demand large seeded, tender peas, high in sugar and low in starch.

Suitable varietiesThe following varieties are recommended by IRAI for commercial scale cultivation and processing. Garden pea varieties are grouped into i) Smooth or soft seeded and, ii) Wrinkle seeded. The latter are sweeter. Each group has a) Early b) Mid Season and c) Late Varieties, divided according to their period of maturity. GROUP VARIETY DESCRIPTIONEarly Asauji Smooth seeded variety suitable for early sowing, takes 60-65 days

to be ready for green pod harvest MeteorEarly Badger

Wrinkle seeded, high yielding, dwarf variety which takes 65 days for harvest

Arkel Wrinkle seeded, high yielding, dwarf variety, takes 60 days to be ready for harvesting

Mid- season Bonneville Wrinkle seeded, double podded, medium tall variety, takes 85 days to be ready for harvesting

Late season N.P 29 Plants tall, wrinkle seeded, takes about 100 days to harvest maturity

For hilly areas

Early Giant Earle, tall plants, dark green pods with wrinkled seedsAlderman Early, tall plants, pods more or less straight, 9cm in length

SIZE GRADINGIs done by grading drums. These screen drums, revolving, allow the immature peas to drop into a waste bin & sort the others into 3 sizes: Large , small and medium

WASHING OF PEAS to remove any field contamination

PEELING OF PEAS to separate the peas from the pod

Largest starchy peas and smallest peas lacking ‘body’ are eliminated

BlanchingPurpose of blanching:1. The inactivation of enzymes (catalase, peroxidase, oxidase) to prevent any

enzymatic activity which may lead to off-flavors & odors during cold storage & thawing

2. Secondary benefits:▪ Destroys vegetative cells of microorganisms on the surface, complementing the

action of washing▪ Fixes or enhances the green color▪ Eliminates any remaining insecticide residues▪ Eliminates the off-flavors produced by gases & other volatile substances that

may have formed during the time between harvesting & processing The duration of blanching varies according to the method employed, product type & variety, product size, degree of ripening & the blanching temperature.

Heat Treatment in water at 98°C (Time)

Residual Enzyme Activity (%) Quality Evaluation after one year’s

storageColor Flavor

Lipoxygenase

Catalase

Peroxidase

None (Control)

100 100 100 Discolored

Strong off-flavor

2.5 sec 80 36 65 Discolored

Off-flavor

5 sec 62 28 52 Discolored

Good

10 sec 6 2 34 Good Good15 sec 1 0.3 23 Good Good3 min - - 0.3 Good Good

Relationship between the Heating Time, Residual Enzyme Activity & Quality Retention of Frozen Peas

It is found that the quality of frozen peas after undergoing blanching was superior if a certain level of peroxidase activity remained at the end of the blanch. Total absence of peroxidase indicates over-blanching. Peas must be blanched until 2-6.3% of peroxidase activity is retained.

Most optimum temperature range for blanching of peas was found to be 76.6-87.77°C. • Below this temp

did not entirely inhibit the formation of off-flavor

• Above this temp resulted in loss of fresh flavor & formation of ‘cooked’ flavors

Blanching time in water at 100°C is 1-1.5 min

Individual Quick Blanching (IQB)▪ Drying the peas with warm air followed by spreading them in a

single layer on belt & heating with steam ▪ These are then held in a multi-layer chamber for thermal

equilibrium ▪ Pros: Faster & more uniform blanching with improved color retention ,

reduced nutrient losses & less leaching of soluble giving better yield & giving increased energy savings

Pre-Cooling followed by IQFPRE-COOLING ▪ Peas are dropped into a shaker washer where they are cooled by cold water spray

(5-7°C)▪ Peas can also be cooled with a fine fog of water, most of which evaporates or is

absorbed INDIVIDUAL QUICK FREEZING▪ Fluidized belt freezers are used▪ The product is kept in suspension using a sufficiently powerful stream of cold air (-

79°C) which is forced up through the layer of the food▪ Since each piece is kept loose & free flowing by air pressure, freezing time is far less

than usual air blast freezing▪ Peas generally fluidize easily and freeze in about 4 minutes!

Fluidization causes products to behave like fluid because they are suspended by refrigerated air that flows from underneath the perforated belt or tray. The air pressure is equal to the weight of the product.

The airflow is evenly distributed rather than being concentrated in one section. Through fluidization, products are separated from one another and frozen more quickly. This preserves the product’s flavor and texture, while minimizing dehydration which tends to occur with slower freezing rates.

The product enters the infeed and proceeds to 2 freezing zones:

1. Initial Freezing Zone: here the airflow is higher. The product is rapidly crust-frozen, locking in moisture so the product’s weight, appearance and quality are preserved before it proceeds to the next and final stage of the freezing process.

2. Final Freezing Zone: The temperature is lower, so the product becomes fully frozen.

A weir, or retaining plate, separates the two zones, and ensures the proper holding time for products in each zone.

Evaporators: They continously remove moisture from the cold air

3D Airflow system: This system blasts air at differnet speeds & in different directions, separating snow & residual moisture form the air stream

Agitators/ Pulsators: Gently shake the product back and forth horizontally & vertically in order to prevent any formation of holes & helps in maintaining a uniformly fluidized bed

Trays: Their size and pattern are customized to accommodate peas so that they do not fall through the perforations during the freezing process

Air Modulator Sheets: Adjust the airflow and the temperature. These are present underneath the tray or the belt

Packaging & Cold Storage▪ Airtight containers which are moisture-vapor-proof

& cold resistant are preferred. Metal containers need not be lacquered

▪ Parchment bags or wraps enclosed in cartons may also be used for dry frozen peas. Liquid pack with 2% brine can be added to cover the product in order to produce highest quality peas

▪ Cold storage at -18°C is preferred with -9°C as the highest permissible limit

▪ Special refrigerating equipment can be used to produce single-frozen or glazed peas rather than a solid mass

Storage Temp -12°C -18°C -24°CStorage life of Frozen Peas

6 months

24 months >24 months

References

▪ Freezing Preservation of Fruits, Fruits juices and Vegetables by  Tressler, Donald Kiteley, 1894-; Evers, Clifford F

▪ Quick Freezing Preservation of Foods: Foods of plant origin by J. S. Pruthi

▪ “Fluidization and FLoFREEZE IQF freezers: the revolution continues” by FMC FoodTech