Maturity of Green Peas

The maturity of peas is dependent on the temperature development and correlates with the number of heat units reached. They influence the growth and maturity, (i.e. the tenderometer) at the time of the planned harvest.

3 important growth factors influence the development of green peas



Temperature generates growth of peas when

  • light (global radiation) for assimilation and
  • water (osmose) for the transportation of nutrients

are available in sufficient quantities!

The requirement for temperatures of peas are measured in Heat Units (HU – WE)
until the green harvesting stage has been reached.

Establishment of Heat Units

The temperature of a day is measured 1 m above ground.


1.

minimum temperature + maximum temperature
2

= mean day temperature



2.

mean day temperature - 4,5 = Heat Units




Why are 4,5 °C substracted from the mean day temperature?

40° F = 4,44 °C


This basic temperature measure in °C originates from the conversion of Fahrenheit to Celsius.
It varies for different species.
For the growth of peas this measure was determined at 40°F. Below this temperature no development and growth of peas is possible.

Determination of maturity

Each variety requires a certain amount of heat units to reach maturity which accumulates from the daily temperature readings after deduction of a basic temperature under which no growth is possible.
With green peas the planting therefore does not adjust itself - as with other groups - only to the most favourable conditions in spring, but preferably to the planned and calculated day on which the crop must be processed in the factory within a few hours – as a rule in only 150 minutes.
Based on the ideal harvesting day it must be counted back to be able to find the right date for sowing.
15 heat units have to be accumulated between two consecutive sowing dates in order to achieve one day difference in harvesting.
Because in Central Europe we reckon with 15 H.U. per day during the harvesting period from mid June to the beginning of August.
This average can be adjusted in other growing regions in case of deviating conditions.

The relative time of maturity in days of:

PREMIUM = 0 days 680 HU

AMBASSADOR = 12 days 855 HU


855 – 680
12

= 15



We therefore calculate with 15 HU for each additional day to mature during the time of maturing for peas in Central Europe in our descriptive lists. To achieve a difference in time for maturity of one day by using two dates of sowing - following each other – of the same variety at least 15 HU between these two dates must be reached.
The temperatures are therefore measured during the total planting period to calculate the HU!
The difference in days during the harvesting period can therefore not be transferred in the ratio 1:1 on to the sowing period.

  • 1 day difference during harvesting time with 15 HU
  • can be equal 3 days difference during sowing time with 5 HU respectively!
  • the difference between 2 calendar days is 24 hours
  • the difference between 2 sowing days is 15 HU


The anticipated date for harvesting the respective variety can only be calculated in relation to the first sown i.e. variety planted at the earliest date.


Example: Determination of HU during the time of sowing


1. Sowing on 25th March, variety: WINNER, harvest planned 23rd June (3rd day of harvesting in sequent)
Temperatures on 26th March: min. 1,5°C, max. 15°C = HU 5,3* on 26th March
Temperatures on 27th March: min. 4,2°C, max. 15°C = HU 6,6* on 27th March
Temperatures on 28th March: min. 0,4°C, max. 15°C = HU 4,7* on 28th March

Sum = WE: 16,6* are reached



2. Sowing on 29th March, variety: WINNER, harvest planned 24thJune! (4thday of harvesting)

= 3 days difference during sowing time comply with only 1 day difference during harvesting time!

* calculated with modified formular for the determination of Heat Units from our computer programme ROPEAS.

Example for determining harvest date relative to other varieties


-
relative maturity variety 1 in days in field 1
0 day
-
relative maturity variety 2 in days in field 2 4 days
-
difference in the earliness of both fields
(soil quality, microclimate etc.)

1 day
-
accumulated heat units from sowing date
of variety 1 to the sowing date of variety 2,
calculated 45 HU, comply with


3 days
expected difference in maturity of
variety 1 and variety 2

8 days


We calculate with:

  • the maturity of the variety
  • the earliness of the field
  • and the difference of heat-units between two sowing dates.


Determination of harvesting date


“Speedies” – 150 Minutes Peas
The optimum time from vining to freezing should not be more than 150 minutes. Within this time frame the loss in quality will still be small. Frozen peas which are processed within 150 minutes are called speedies in the U.K.


The Tenderometer
To determine the quality of peas and also the optimum harvest date, mechanical or electronic tenderometers are used by which a crushed sample of fresh peas is measured to give an objective result of thenderness and consistence.
We recommend harvesting at average tenderometer readings of between 95 and 115.
Preferably after blanching a test can be used, that especially in the USA has given good results.
The peas are graded in so called salt graders (quality grader ) according to their specific weight. The overmature peas will sink to the bottom in the salt solution (sinker ), the tender peas will drift at the top of the solution.
This technique is also commercially used in processing lines to improve quality.


Grading of Peas in Salt Graders
The Sink test (or salt water classification) is a technique used in the USA to separate peas of a high AIS value. Peas classified as A grade because of their low tenderometer reading can nevertheless sink in salt water because of their poor texture (high AIS value). For 85000 lbs of peas 1US ton (2000 lbs) of salt is required. Most processors use hydrometers to measure the salt saturation level. They prefer a 30-34% saturation level in the salt tank.
Many processors use a separate tank filled with concentrated salt water solution, which automatically feeds this solution into the saltwater tank and keeps a30-34% level.
The water temperature in the salt tank is very important. In the ideal case processors try to maintain a temperature between 21-22°C. This is difficult and generally the temperature fluctuates between 24-29°C. The reason is the fact that after blanching the peas are still warm. The peas which sink are sorted out.
In the laboratory this procedure is carried out simply in glass containers. The "sinkers" are counted and should not exceed a percentage stipulated in advance.



AIS-Alcohol-insoluble dry matter


Principle:
AIS is the portion of starch, plant fibre and several proteins in vegetable. As sugar, during the maturity process, will be transferred into starch, the AIS is a measure for the maturity stages of (starchy) vegetable.
% AIS is also the amount of dry matter in % which can not be dissolved in 80% proof alcohol, if the analysis is carried out as follows:

Equipment:
Scales, PC 440, Mettler
Drying cabinet LP 12, Mettler, drying time 20 minutes, power switch pos.3.
Drying cabinet LP 15, Mettler, drying time 20 minutes, power switch pos.9.
Filter clip with 9 cm funnel
Water jet pump
Filter, s&s 589² white tape, diam. 9cm²
250ml cup, spray bottle, strainer, glass spatula with rubber tip.

Liquid:
80% proof alcohol: 5 l 96% proof fine sprit + 1 l dest. water.

Pretreatment of filter:
Dry filters for 5 minutes in drying cabinet ,weigh on PC 440 ( 3 decimals), note weight.

Pretreatment of vegetable:
Defrost vegetable in microwave for 2 min. or in warm water for approx.1 hour. Fresh vegetable does not require pre treatment. Drain canned vegetable in strainer for 2 minutes.

Analysis:
Rinse pretreated vegetable in 1 l of cold water, drain for 2 minutes. Weight 200 g of drained vegetable in a 250 ml cup. Blend 200 g of vegetable and 100 ml destilled water in a blender for 3 minutes. Weigh on PC 440 scale 10 g of mixture in a 250 ml cup (3 decim.).Blend the above mixture with 150 ml of 80% proof alcohol. When solution has settled start to filter through dried and weighed filter paper (filter clip and 9 cm funnel), rinse cup with spray bottle and 80% proof alcohol, sticky particles to be removed with glass spatula and rubber tip. Rinse with 80% proof alcohol. Dry filter in drying cabinet for 20 minutes. Note weight (3 decim).

Vegetable mixture (g) x 200
300

= Vegetable weighed

Vegetable mixture dehydrated (g) x 100
Vegetable weighed

= % AIS


If drying cabinet LP 12 is used add 0,2 to the result. (%AIS+0,2)

5.0thumb
PDF "AIS - diagram"

 

General AIS-comparison of round peas and wrinkled peas


AIS of variety WINNER


AIS of variety LAREX


Green peas: Foliage ratio
The relative ratio of the weight of vined green peas to the remain of the plant correlates with the ideal date for harvesting a variety. When vining a sample to assess the maturity we can establish besides the tenderometer also the so called kernel : foliage ratio. It is data of experience which during the season may be helpful additional information. The results vary with the individual climatic and growing conditions of each year.