Institute of Agrophysics Polish Academy of Sciences, P.O.Box 201, 20-290 Lublin, Poland
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The investigations on rapeseed postharvest handling has been carried out in the Institute of Agrophysics for many years. They cover the whole technological process of rape handling from harvesting to processing. The aim of the present study was to evaluate the decrease of seed quality during harvesting, cleaning, drying and storing as well as indicate the reasons of the mechanical damage occured during handling. Therefore the rapeseed samples were colected at all technological operations and the amount of damaged seeds was checked. At the same time laboratory investigations were performed. The compression test between two parallel heating plates was performed and mechanical strength of whole seed was calculated according to wide range of seed moisture content and temperature. The results showed considerable decrease of rapeseed quality (understanding as number of mechanical damage) during postharvest handling. The most negative effects were observed at low moisture content (below 6% w.b.) and high temperature of seeds (over 50°C). The most negative operation was harvesting, when 50 - 90% of total number of seed damage occured.Total percentage of damaged seed was from 1.6 till 7.5% of its whole amount. This results were confirmed by laboratory tests. Moisture content influenced seed strength stronger then temperature. Maximal strength was noticed at 6% w.b. and decreased both with increase and decrease of moisture content. With the increase of temperature the strength of seed decreased.
KEYWORD damage, quality, harvesting, postharvest, strength, moisture content, temperature
Harvesting and postharvest operations negatively influenced quality of rapeseed. Harvester subsumbles and other machines used during postharvest handling caused signifficant mechanical damage to seeds i.e.: skin rupture, seed fracture etc. The damage resulted from mechanical interaction between biological material (seeds) and steel, rubber etc. (working elements of machines).
Some researchers described quality of rapeseed according to its maturity (Daun and Burch 1984; Cenkowski et al. 1992) or the influence of drying temperature on quality features of seeds (Cenkowski et al. 1989; Fornal et al. 1995). Some changes in physical state of seeds during postharvest handling was investigated by Stepniewski and Szot 1995. Stepniewski at al. 1994 described the influence of various drying conditions on some technological features of rape. Wider the same topic was studied by Nellist 1978 and Nellist and Bruce 1987, who included other oil seeds and cereals. The influence of seed damage on protein and fat quality was studied by Fornal et al. 1992. There were also attemps to evaluate mechanical properties of rape seeds (Bilanski 1966, Cenkowski et al. 1991, Davison et al. 1975, Szot and Kutzbach 1992 and Szot and Stepniewski 1995).
The aim of the present study was to evaluate rapeseed mechanical resistance to external loads at differentiated moisture content and temperature of seeds. All parameters were choosen according to data collected during field experiments (range of seed moisture content and temperature during and after drying). The strength of a single seed - mechanical parameters was chosen to describe changes of rapeseed mechanical properties under various external conditions. The sources of damage at postharvest processing was also described and the amount of broken seed was given at all stages of handling.
First field experiment was carried out in order to check the changes of rape quality features during postharvest handling. The following operations were examined (Fig. 1):
unloading, cleaning, wet seed storage, drying, dry seed storage. Between each successive operation the seed was transported. The transport was carried out using various types of conveyors: worm, belt, Redler, chutes and bucket elevators. The unloading was done on a tippler, then the seed was poured into a charge hopper, from which a worm conveyor directed it to a bucket elevator. The elevator took the seed to a chute which filled a fanner with flat coarse-cleaning screens. The fanner separated only the larger contaminants, such as parts of stems and pods, cereal seeds, etc. Next the seed was placed in wet seed silos, from where it was moved to the dryer. Drying medium was hot air, of a temperature of 110 - 130°C at the inlet to the dryer. Dry rape seed was transported to a containers for storage.
Fig. 1. The scheme of typical postharvest technological process of rapeseed.
A - F - sampling points 1. tippler 2. silo 3. cleaner 4. dryer 5. bucket elevator 6. screw conveyer 7. belt conveyer 8. chute
Seed samples were taken before and after every operation. At each sampling point a sample was taken every 15 min for 5 hours a day. All samples were taken from the moving flow of seed. Samples were delivered to a laboratory, where first their moisture content was determined, and then after drying the seed at room temperature to the air-dry level, the qualitative features of the seed were determined (i.e. the level of macrodamage, microdamage and amount of unusable contaminants). Macrodamage were considered as broken parts of cotyledons and seed cover as well as seed with visible cracks of seed cover. The content of this fraction was expressed in terms of weight percentages of 10g sample. Likewise, unusable contaminants separated from 10g sample were expressed as weight percentages. After the separation of seeds with macrodamage and unusable contaminants, the level of microdamage was determined. Ten groups of 100 seeds each were taken and placed on wet tissue, where they swelled and, an hour later, it was possible to count the number of seeds with cracked seed cover. The level of microdamage was expressed in percentages as the number of seeds per one hundred. The material investigated during this part of an experiment was commonly grow Ceres winter rapeseed variety.
The stend for rapeseed strength measurements was set up around an INSTRON model 6022 strength tester. The stend consisted of the following:
• an air tight seed container, washed over with heating fluid
• a thermometer
• a thermostat with heating fluid pump
• a contact thermometer for temperature control
• a measurement head
• a thermal chamber, one of the heater plates of which was attached to the measurement head and the other one was fixed to the strength tester.
An analog signal from the measurement head was converted into a digital signal in the measurements interface and then registered by the computer. The specially elaborated programme allowed also to calculate seed strength.
A mechanical parameter of rapeseed was taken under consideration, i.e. strength of single seed (maximal tensile stress, which acted on seed cover). From the practical point of view knowledge of strength of rapeseed is very significant and could help to eliminate seed damage. Therefore a model of elastic shell with compressible liquide inside was used to calculate maximal stress of compressed seed (Davison et al. 1975).
Polo winter rapeseed variety was under investigation. The moisture content varied as following: 3, 6, 12 and 18% w.b., and temperature of seed was in the range from 10 till 60°C at every 10°C. This levels were chosen according to data colected at field experiment.
Monitoring of rapeseed post-harvest handling established that the fundamental cause of the occurrence of mechanical damage to rapeseed was combine harvesting. The content of mechanically damaged seeds after harvesting had a considerable effect on the final content of that fraction. This final amount of broken seeds considerably depended also on the weather conditions during harvest and on the weather during the whole growing season. Similary the amount of unusable admixture depended on the state of a plantation.
Fig.2. Changes of the amount of seed micro- and macrodamage during rapeseed postharvest handling
At the begining of handling the amount of broken seed (macro and microdamage) in samples was at the average 7.48% and increased to the average 9.1% in samples after drying (Fig. 2). The amount of broken seed slighty decreased till 8.3% at the end of the whole technological process in the purchasing centre. This decrease resulted from the decrease in the level of microdamage - from 3.67% before drying to 2.52% at the end of the full cycle. The level of macrodamage increased slowly from 5.31% to 6.09% until the final operation, i.e. transport of dry rapeseed, when it decreased to 5.77%. The decrease in the number of seeds with microdamage during drying could have been caused by the expansion of it into macrodamage, the level of which increased in the operation. The temperature and water stress during drying increased seed coat microcracks.
The content of unusable admixture showed a tendency to decrease in the successive operations. However the differences observed were statistically insignificant, even during cleaning. Rape seed delivered to the purchasing centre contained on average 3.2% of unusable admixtures, while at the end of the cycle their content was 2.94%.
The moisture content of seed before drying strongly depended on weather conditions close before drying. Initially the moisture content varied from 13 till 19%. However in dry sommer the moisture content was at the level of 9 - 10%. In the most cases drying was performed correctly and the final seed moisture was from 5 till 6%.
From the above it is seen, that harvest operation had a basic importance for the amount of broken rapeseed in the whole handling. Level of mechanically broken seed was different within succiding years, but decided about the total level of this fration at the end of processing. Postharvest operations had only slight influence on the amount of this fraction, however it was possible to notice characteristic changes within all handling procedure. These changes could suggest some improvements in the postharvest technological process in order to avoid mechanically broken seeds. Especially in accordance to the earlier results (Stępniewski et al. 1991) it is possible to make some conclusions i.e.: elimination of pneumatic transport of seed, cooling of the seed after drying, or replacement of some sort of conveyors for another - better from the breakage point of view.
Rapeseed strength differed according to their moisture content and temperature. Bigger influence had moisture. The lowest strength values were noticed at the lowest moisture content (3%) and the highest between air-dry moisture (6%) and 12%. With the increase of seed moisture above air-dry one the strength decreased. The differences were from 31.4MPa (6%) till 16.5MPa (3%) at the same temperature level 20°C.
Fig. 3. The average values and the approximation of the relationship of maximal stress to moisture content and temperature of Polo rapeseed
Similary the seed temperature significantly influenced maximal stress of skin. With the increase of temperature seed strength decreased. Maximal strength was noticed at 10°C and the lowest at 60°C. The average strength value of air-dry seed varied from 21.9 till 28.7 MPa at the whole temperature range. The influence of temperature was stronger at higher levels (up from 40°C) and wasn't so big bellow 30°C.
Temperature influenced seed strength stronger at higher moisture levels (12 and 18%) and the lowest influence was noticed for the most dry seed (3%).
The multiple regression analisys allowed to print surface, which described the changes of maximal tensile stress in the seed cover according to moisture and temperature. It also allowed to find the coefficients of determination R2 = 0.44 and coefficient of correlation R = 0.63.
The above described results confirmed, that over-dry seed undergo damage easier, what was observed in the practice as a big number of damaged seed occured during transport after drying. The higher temperature expand this phenomena, so cooling after drying seems to be neccesary. The same undergo for very moist seed (18%). Their strength was also low as in the case of over-dried seed. Therefore all operations on very moist or very dry rapeseed should be suspended or conducted in very delicate way, because of their high brittleness. Especially transport of such a seed should be limit to the minimum.
1. Rape seeds are subject to mechanical damage during combine harvesting, and the level of damage sustained in the course of harvest has a decisive effect on the content of damaged seeds at the end of the cycle of postharvest processing at the purchasing points.
2. The level of macrodamage decreased or did not change significantly during the initial operations after delivery to the purchasing point until drying, when it increased. The final level of macrodamage remained constant after drying, or decreased slightly, remaining insignificantly higher than the initial level of macrodamage after the combine (5.3% after the combine and 5.77% after drying).
3. The level of microdamage increased until the operation of drying (from 2.17% to 3.66%) where it decreased (to 2.39%), after which it increased insignificantly (to 2.52%). The decrease in the number of seeds with microdamage during drying could have been caused by the expansion of some of the instances into macrodamage, the level of which increased in the operation. A probable cause of the increase was the interoperation transport.
4. The content of unusable admixture generally decreased in the course of rape seed processing. The operation of seed cleaning was found to be ineffective (only larger parts of stems and siliques were separated), which resulted in the high content of the contaminants at the end of post-harvest processing, exceeding the value permitted by the applicable standard.
5. Moisture content influenced strength of seed stronger then temperature. The highest strength was noticed at air-dry moisture 6% and decreased both with increase and decrease of seed moisture. The range of rapeseed strength varied from 16.5 MPa till 31.4 MPa.
6. The increase of temperature significantly decreased seed strength at all levels of moisture.
7. The influence of temperature and moisture content on seed strength was in the relation, i.e. strength decreased more with the increase of temperature at the highest moisture level 18% and wasn't so distinct at lower moisture contents.
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