DECREASING THE TEMPERATURE OF POPPING CORN COBS AS A PROCESS REDUCING THE LOSSES OF MECHANICAL SEEDING

Marek Domin¹, Franciszek Kluza^2
1 Department of Refrigerating Engineering and Power Engineering of Food Industry, Agricultural University in Lublin, Poland
² Department of Refrigeration and Food Industry Energetics, University of Life Sciences in Lublin, Poland

ABSTRACT

The purpose of the study was an attempt to characterize the process of seeding the popping corn cobs at lowered temperature, with particular regard to the relations between the losses of grain weight and the temperature values. Corn cobs were picked up at the moment when the corn reached the moisture of about 15%. The seeding of corn cobs was performed in a device of the authors’ own design, after the temperature was lowered. The studies were conducted at six temperature levels, namely 17, 0, -5, -10, -15 and -38°C. After establishing the weight of the obtained grain, the weight of the grains left on the cobs and the weight of damaged corn, the possible dependence was determined between the grain losses accompanying the seeding of corn cobs and their temperature. It was found out that lowering corn temperature of cobs from 17°C to -15°C led to a statistically significant decrease of the losses of grain weight. The border values of grain weight losses in this range of temperature of corn cobs were, respectively, 16.4% and 1.0%.

Further lowering of the temperature of the cobs to the lowest value of -38°C led to further reduction of the losses but it proved to be statistically insignificant.

Key words: popping corn, mechanical seeding, freeze corn cobs.

INTRODUCTION

Maize is a valuable cultivated plant, with high fertility and a broad use. Poland is the country with favourable conditions for the cultivation of this plant. Differentiated climatic conditions in Poland, however, enforce strict regionalization of maize cultivation and the proper choice of cultivars. Cultivation for grain should be mostly concentrated in the south of Poland, where the climatic conditions are the best for this direction of cultivation [1].

Maize corn contain a considerable amount of sugars, protein, vitamins (A, B₁, B₂, C, PP) as well as numerous microelements (chromium, zinc, copper and selenium). Maize also contains slight quantities of calcium and iron [8] (tab. 1).

Usable cultivars of maize are divided into flint corn, sweet corn and popping corn. The latter is characterized by high parameters of expanding. It contains protein and carbohydrates, which give it high nutritious values [10]. Popping corn is harvested in October [6]. After the harvest the maize has the moisture of about 16-35% (it requires drying to the moisture of 12-14%) [7]. Under-dried grain is susceptible to frost in the relation that is directly proportional to its moisture. A germ in the grain with the moisture of 16-25% loses its germinating ability already around 0°C, while at the moisture of less than 15% it keeps this ability even at the temperature of less than -20°C.

Too late post-harvest drying of maize can cause the development of fungi from Fusarium family, which eliminates the infected part of the material from further processing [12]. Low temperature is a factor effectively inhibiting the development of the fungus and mould on maize corn. During a dry and sunny autumn maize is harvested after the first frosts, when the temperature of the cob settles at the level of much less than 0°C.

The harvest of popping corn takes place at two stages. The first consists of removing the whole cobs from the plant and these are next dried up. In the second stage they are seeded (threshed). A similar method is applied during the harvest for seeds and grain [9, 11]. As a result of mechanical injuries of corn or leaving them in the core on the cob during the harvest and the threshing, losses are generated, which, according to the studies conducted in Germany [5] and France [2, 4], reach from 10% to 27% of the grain weight. Such a big proportion of maize grain losses causes that the ways of reducing them are sought. The present studies were intended to determine the effect of lowered temperature on the course of the process of seeding the cobs of new popping corn cultivars with the grain meant for consumption or the sowing material. An attempt was made to establish the optimum temperature of seeding the material and pointing to the cultivar with the best predispositions for this process.

MATERIAL AND METHODS

The material consisted of new cultivars of popping corn:

• Płomyk – a cultivar of medium fertility, cobs with the length of about 12 cm, 1-2 cobs on a plant, well-filled with orange grain, with very good abilities to break at the moisture of about 12%,

• Perłowa – a cultivar of medium fertility, cobs with the length of about 12 cm, 1-2 cobs on a plant, well-filled with pearl-white grain,

• Dobosz – medium late cross-breed of popping corn with yellow-orange grain.

The studies used the maize cobs that did not show any disease infection or pest infestation. The cobs of similar shape were picked up at the stage of technological maturity, when the moisture of corn, marked by means of a drier method (according to PN), was about 15%.

Directly after the harvest, neglecting the process of drying up, the cobs of particular cultivars were sorted out into 6 samples and next cooled to the seeding temperature. The process of separating the grain from the cobs took place at the cob temperatures of 17, 0, -5, -10, -15 and -38°C. The losses were the corn left on the cobs after the process of removing them and the corn mechanically injured (crushed, broken), which – in the case of popping corn – are not suitable for further use. [3].

The grain separated from the cobs was weighed and then the injured corn were separated. Those that remained on the cob were removed manually and their weight was determined.

The percentage losses of the weight of the raw material in relation to the total weight of the samples were determined by means of statistical programs and calculation sheets. Using the proper software (CurvExpert), regression curves, correlation coefficients, standard errors were established and the significance of differences was determined (Tukey’s method, significance level at α = 0.05) between the obtained characteristics at different temperatures of the process. Additionally, MTZ and the seeding density of the grain were marked.

RESULTS

The marked mean moisture of the grain (14.8-15.1%) (tab. 2) qualified the maize for harvest. Slight differences in the moisture level between particular samples testified to balanced ripening.

Corn of pop-corn are characterized by relatively low dimensions, which makes the MTZ value relatively low (tab. 3).

The determined values of losses of maize grain losses converted into percents are listed in tables 4, 5, 6.

The level of losses of seeded popping corn are statistically analyzed, obtaining regression curves (figs. 1, 2, 3), correlation coefficients, standard errors (tab. 7) and significance of differences of the levels of losses between the conditions of seeding characterized by particular temperatures of the seeded raw material.

Regression relations were determined using the model of exponential fitting. The best fitting to the experimental results was obtained every time by means of the equation y = ae^bx (tab. 7), where “y” was the start level, and “x” described the temperature of the seeds during the seeding process.

In order to find out which of the studied maize cultivars is characterized by the lowest losses in the suggested method of seeding, the following were specified: weight losses as a results of grain remaining on the cobs (fig. 4), the level of losses as a result of injured corn (fig. 5) and total losses (fig. 6).

The highest losses, both in the case of the grain remaining on the cobs and the proportion of mechanically injured grain, were observed in the case of seeding the cobs with the temperature of 17°C of Perłowa cv. (7% grain remained on the cob, 9% injured grain). Lowering the temperature of the cobs directly before the seeding reduced the disproportions of the generated losses.

The lowest losses were observed when the cobs of Dobosz cv. at the temperature of -15% were seeded, when only 0.4% of the grain weight remained on the cob and 0.4% were mechanically injured.

The analysis of the significance of differences of the losses of the grain weight with differentiated temperature during the process of seeding showed that cooling the maize cobs to the temperature of -15°C directly before the seeding caused significant reduction of grain weight loss. Differences in the losses below the temperature of -15°C are not significant despite further reduction of the proportion of grains that were injured or were not seeded.

CONCLUSIONS

1. Lowering the temperature of maize cobs to -15°C before the process of threshing (seeding) reduced the total losses of grain from 16.4% to 1.6%. Accepting, after the data of GUS (Central Statistical Office) for the year 2004, the average national yield at the level of 10.5 t·ha^-1, it gives the mean saving of the raw material of 1.6 t·ha^-1.

2. Lowering the temperature significantly affects the reduction of the susceptibility of corn to injuries, as a consequence decreasing the proportion of crushed and broken corn, from 9% at the temperature of 17°C to 0.8% at -15%.

3. The method used in the present studies can be recommended to the producers of maize for seeds or “popcorn”, which are the cases when any mechanical injury completely eliminates the corn.

4. Dobosz cv. showed the best predispositions for seeding with the use of the process of cooling the cobs of popping corn. During the seeding of the cobs of this cultivar the losses were the lowest, both in the case when corn remained on the cob and when they were injured mechanically. Cooling the cobs before the threshing process lowered the total losses from over 12% to 0.7%.

REFERENCES

1. Brzezicka A., 1988. Kukurydza na ziarno [Maize for grain]. WOPR Rejowiec z. 22/88 [in Polish].

2. Carre M-A., 1994. Le mais francais a conquis nos industriels. Cultiwar 369, 30-3.

3. Domin M, Kluza F., 2003. Sposób przygotowania kolb kukurydzy do omłotu [Manner of preparing maize cobs for threshing]. Zgłoszenie patentowe nr. P 363599 z dnia 20.11.2003 [in Polish].

4. Gautier X., 1994. Le mais et ses industries. AGPM Montradon.

5. Jesch P., 1994. Der Stärkemarkt als Konjunktubarometer. Mais 22 (1), 34-35.

6. Katalog odmian 2002. Kukurydza [A catalogue of cultivars 2002. Maize]. KWS Polska Sp. z o. o. w Poznaniu [in Polish].

7. Kumider J., 1995. Poszukiwanie wyróżników fizykochemicznych dojrzałoœci przetwórczej kukurydzy cukrowej [Looking for physico-chemical discriminants of processing maturity of sweet corn]. Przemysł Spożywczy 49, 7, 247-249 [in Polish].

8. Meuser F., Pahne N., 1993. Möglichkeiten konzeptioneller Veränderungen bei der Stärke Gewinnung. Materiały z sympozjum: Nachwachsende Rohstoffe – Perspektiven für die Chemie – 5-6. 05. 1993, Frankfurt, Landwirtschaftsverlag Münster, 115-144.

9. Puszyk T., 2002. Na kolekcji odmian kukurydzy [On a collection of maize cultivars]. Rolnicze Wieœci 10, 5 [in Polish].

10. Waligóra H., 1992. Kukurydza jadalna [Edible maize]. Wielkopolski Zwišzek Producentów Kukurydzy Poznań [in Polish].

11. Waligóra H., 1998. Uprawa i wykorzystanie kukurydzy [Cultivation and utilization of maize]. Nowoczesne Rolnictwo 5, 4, 10-11 [in Polish].

12. Zalewski K., Login A., 1999. Zawartoœć mykotoksyn w kolbach kukurydzy zbieranej po przemrożeniu roœlin [Content of mycotoxins in the cobs of maize harvested after freezing the plants]. Zesz. Probl. Post. Nauk Roln. 469, 2. 613-619 [in Polish].

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