Effects on Pellet Properties and Energy Use When Starch Is Added in

Article pubs.acs.org/EF

Effects on Pellet Properties and Energy Use When Starch Is Added in the Wood-Fuel Pelletizing Process Magnus Ståhl,* Jonas Berghel, Stefan Frodeson, Karin Granström, and Roger Renström Department of Energy, Environmental and Building Technology, Faculty for Technology and Sciences, Karlstad University, SE-651 88 Karlstad, Sweden ABSTRACT: The production and use of wood-fuel pellets have increased significantly worldwide in recent years. The increased use of biomaterials has resulted in higher raw material prices, and there are no signs that indicate a decrease in raw material competition. Additives can be used for different purposes. Partly, they are used to facilitate the use of new raw materials to increase the raw material base, and partly, they are used to decrease the energy use in the pelletizing process. They are also used to increase durability or shelf life. Consequently, it is necessary to do research that systematically investigates the consequences of using additives. In this work, it is investigated how various percentages of different kinds of starch influence pellet properties, including shelf life and energy use in the pelletizing process. Four different starch grades were used: native wheat starch, oxidized corn starch, native potato starch, and oxidized potato starch. The pellets were produced in a small industrial pellet press located at Karlstad University, Karlstad, Sweden. The result shows that starch increases the durability of the pellets. Oxidized starches increase the durability more than native starches, and the best results were obtained by adding oxidized corn starch. The durability did not decrease with storage time when the pellets were stored indoors during 7 months. The oxidation process was not consistently altered by the addition of starch. The energy consumption of the pellet press decreases when starch is added. Again, the oxidized corn starch showed the best result; when 2.8% of corn starch was added, the average energy consumption was reduced by 14%.

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INTRODUCTION The production and use of wood-fuel pellets have increased significantly worldwide in recent years. New production units with a capacity of up to 900 000 tons/year are emerging, for example, in Russia, the Baltic states, Canada, United States, and Sweden. Presently, there are about 630 wood-fuel pellet plants around the world, with a production capacity of more than 30 million tons/year.1 The increased use of biomaterial during the past decade has resulted in higher raw material prices, and there are no signs that indicate a decrease in raw material competition. In addition, the raw material cost is one of the main cost factors to pellet producers.2−4 An important producer issue, therefore, is choosing the right raw material. Raw Material Base. In answer to the intensified competition for raw material, the pellet industry tries to diversify their raw material base. New materials from the forest as well as from the agriculture industry are likely to come into use. Pellets of cull tree and short-term thinning material have similar characteristics to those of the present pellets, except for a higher ash content.5 Energy crops and lignin are used and could come into increased use if the ethanol production increases, with lignin being one of its byproducts.6 A comparison made between Salix, reed canary grass, hemp, straw, screenings, rape-seed meal, rape cake, and distiller’s waste found that the Salix and reed canary grass were of the greatest interest, because they have competitive raw material cost advantages and acceptable fuel properties and can be mixed with sawdust in large-scale burners.7 Additives. New raw materials may influence pellet properties in an undesirable manner. If additives can counter such developments, it would be enough motivation to use them © 2012 American Chemical Society

(according to SS-EN 14961-1:2010, the term additive is used for amounts of pressing aid up to a maximum amount of 20 wt % of pressing mass). When it comes to household users, the pellets compete with district heating and heat pumps. These sources do not typically need any maintenance, and therefore, it is important that disturbances connected with the use of pellets are reduced to a minimum. A common problem for household users is insufficient pellet durability.8 When it comes to pellet producers, the energy consumption is an important part of the production costs. This means that all additives are warranted that reduce disturbances for the household users through improved durability and/or reduce the producer costs through decreased energy consumption. The additives have to be accepted as a renewable substance. Furthermore, the additives are often more expensive than the dominant raw material. This implies that the additives must lead to benefits in the production chain and not upon any account cause problems for the household user. Consequently, research that investigates the influence of additives has to cover the main part of the chain from raw material to delivered heat. In this paper, the effects in the chain from wet raw material to delivery at the consumers are examined. Starch as an Additive. Starch fits the criteria of being renewable, having properties that make it a likely candidate to be a useful additive. Starches and modified starches have been used in adhesives, binding, film forming, foam strengthening, gelling, moisture retaining, stabilizing, and thickening applicaReceived: December 16, 2011 Revised: February 16, 2012 Published: February 18, 2012 1937

dx.doi.org/10.1021/ef201968r | Energy Fuels 2012, 26, 1937−1945

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tions.9 Native starch is the basic product obtained when cereals or tubes are separated into their starch, protein, and fiber components. The modification chosen for this investigation was oxidation. Oxidation of starch, performed using sodium hypochlorite, is a common process in the starch industry. It depolymerizes the starch molecules, and at the same time, carboxyl and carbonyl groups are introduced on the starch molecules. According to Solam GmbH, the perceived benefit results not from the added groups but from the depolymerization. The degradation of the starch affects the behavior of the granules upon gelatinization and thereby its properties as an adhesive. Oxidation is known to improve the tack and adhesive properties of the starches.10 Starch is unique among the carbohydrates in that it occurs naturally as discrete particles.11 Such granules are relatively dense and insoluble and hydrate only slightly in cold water. If dispersed, they produce low-viscosity slurries. Starch suspended in cold water is essentially unable to act as an adhesive. The thickening effect of starch is realized when the granules are heated in the presence of water.11 Starch is formed from two polymers: one mostly linear polysaccharide called amylose and one large highly branched polysaccharide called amylopectin. The various starches differ in the proportion of amylose to amylopectin: regular corn starch contains 25−28% amylose; wheat contains 25−29% amylose; and potato contains 18−21% amylose.11 Certain hybrids of wheat and corn have starch with much different amylose/amylopectin ratios.12 Starch granules are made up of concentric alternating layers of crystalline regions interspaced with amorphous layers.9 As granules absorb water, they swell, losing crystallinity and leaching amylose.10 Starch gelatinization is the uptake and swelling that occurs when starch and water is heated together. The reason for choosing oxidized starch is that the gelling behavior is quite different from a native starch. The oxidized starch granules do not swell in one piece as a native starch does; they divide themselves into many small parts during the gelatinization. All starches contain small amounts of lipids and proteins. The lipid content is approximately 0.8% for corn, 0.1% for potato, and 0.9% for wheat, calculated per dry substance. Potato starch is unique in that it contains phosphate groups attached to some hydroxyl groups, which gives it a charge that increases its hydrophilicity and contributes to the rapid swelling of potato starch in warm water.9 Starch is already used on some markets to achieve reduced operating costs and better durability. Obernberger and Thek13 found that 7 of 23 producers of pellets (mostly in Austria) used starch as a biological binding agent to reduce the operating cost and achieve higher abrasion resistance. The starch content among the pellet producers varied between 0.16 and 1.25 wt % [wet basis (wb)]. Nielsen14 showed by laboratory measurements that, when 5% of potato starch was added, the strength of the pellets increased. Neither abrasion resistance nor strength is included as properties in the European standard (SS-EN 15210), which makes papers using these properties somewhat difficult to interpret. In this paper, we treat both properties as closely related to durability, which is included in the European standard. The use of starch as an additive could also affect the properties of agricultural products. Razuan et al.15 tested pelletization of palm kernel cake and used three different starches as the binder: corn starch, tapioca starch, and potato starch. Corn starch was the most effective of the starch binders; the tensile strength of the pellets improved with up to 10 wt %.

Further additions of the three starches, up to 20 wt %, made the pellets deteriorate in terms of tensile strength, even though density increased. In single-pellet compression equipment, Finney et al.16 tested suitable binders, such as caustic soda and starch. They found that maize-based starch, i.e., corn starch, as much as doubled pellet tensile strength. The tensile strength is not defined in the European standard but can be considered to be related to durability. When starch is added, the temperature during the pelletizing process seems to be important. Finney et al.16 tested elevated temperatures to evaluate the softening of starch present and found that an increased production temperature (45−75 °C) improved the overall pellet quality, measured as durability, tensile strength, and density, although an additional temperature increase (of up to 125 °C) did not further improve the quality. Quality Improvements. As mentioned earlier, the introduced additive has to improve the properties that are of importance for the household user. In a problem inventory, it was concluded that crumbled pellets, high amounts of fines, or incorrect equipment setups cause most of the problems that pellet users experience.8 Ståhl and Wikström8 found in their inventory that the effects of fines or crumbled pellets on the pellets caused most problems during usage in combination with incorrect equipment. The amount of fines or crumbled pellets is defined as requirements in a standard. Since 2010, there is a new European standard (SS-EN 15210) that determines the mechanical durability of the pellets. In this standard, the demands on durability of the pellets must be equal to or above 97.5%. In Sweden, the household user usually buys the pellets in sacks of 16 kg. More and more of the sacked pellets are bought on the Internet. An investigation of how nine of the big pellet producers in Sweden present their pellets in terms of properties on their Internet sites found that only two of the producers presented values on the durability of their pellets, although all companies stated that their pellets fulfill the criteria in the European standard.17 The investigated pellet producers with websites were Neova (www.neova.se), SCA (www.sca.com), Agrol (www.agroenergi.se), Stora Enso (www.pellets.storaenso. com), Skellefteå Kraft Biopellets (www.skekraft.se), BooForssjö Energi (www.booforssjoenergi.se), Bioenergi Luleå (www. bioenergilulea.se), Laxå Pellets (www.laxapellets.se), and Rindi (www.rindi.se). This is important, because durability, if reported to the public, could help consumers choose the pellets that are most suitable to their burners. Should the presentation of pellets be more complete, additives that increase pellet durability would be even more appreciated. Storage. Storage of produced pellets is common because of seasonal demand variation and wood-fuel trade. Known problems that appear during storage include emissions of noxious gases, spontaneous self-heating, and decreased durability. It is important to make sure that additives do not aggravate these storage issues.18 The dominating compounds released from softwood pellets are terpenes, aldehydes, and carbon monoxide.19,20 Terpenes are components of the resin produced by conifers, and they are released from both conifer trees and conifer wood (i.e., softwood). Aldehydes, mostly hexanal, belong to the most significant class of odorous compounds formed because of oxidation of unsaturated lipid compounds, such as fatty acids and resin acids. Carbon monoxide is formed when organic materials, such as wood, are stored with little access to oxygen. 1938

dx.doi.org/10.1021/ef201968r | Energy Fuels 2012, 26, 1937−1945

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Table 1. Properties of Starches (Data Delivered from Solam GmbH) granule size (μm) average (weight)a average (number)b granules/g (×106) density (kg/m3) gelatinization temperature (°C) gelling behavior a

native wheat starch

oxidized corn starch

native potato starch

oxidized potato starch

0.5−45 25 8 2600 ∼600 52−85 ∼60 the granules swells in one piece during gelatinization

2−30 15 10 1300 ∼600 62−80 ∼65 the granules divide themselves into many small parts upon gelatinization

5−100 45 30 100 ∼700 58−65 ∼60 the granules swells in one piece during gelatinization

5−100 45 30 100 ∼700 58−65 ∼60 the granules divide themselves into many small parts upon gelatinization

The average diameter, as used in the starch business. bThe median size, as used in the starch business.

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Fatty acid oxidation has been identified to cause spontaneous pellet self-heating, which has caused fires in silos. Several research groups have seen that oxidation is an important factor.21,22 Another problem is that high levels of volatile aldehydes emitted from pellets can be responsible for a pungent smell,23 which can cause consumer dissatisfaction. There is also a health perspective, because exposure to the aldehyde hexanal causes discomfort to the eyes and nose and results in headaches at a concentration of 10 ppm.24 Hexanal has been detected in domestic pellet storage rooms; the emissions varied with ambient temperature and peaked after 2 months of storage in the midst of the warm season.19 Oxidation of unsaturated fatty acids is a complex self-catalyzing free-radical chain reaction, a process commonly referred to as the fats going rancid.25 Radicals can be produced by light photons or metal ions or the spontaneous reaction of oxygen.26 When the oxidation has started, it self-catalyzes and continues until all of the radicals have been neutralized.25,26 Energy Use. The pellet plants use electricity in the pelletizing process. According to Ljungblom,1 there are 630 wood-fuel pellet plants globally, with a production capacity of more than 30 million tons/year. This large scale of production means that even small reductions in electricity use in the pelletizing process lead to significant cost reductions. Nielsen14 claims that the capacity of the pellet press is limited by the maximum power consumption of the pellet press. This could mean that a reduction in electricity use for the pelletizing process either leads to cost reduction, increased capacity, or both. Materials or mixes of materials that require more electricity to pelletize lead to a lower production capacity and higher maintenance costs. Hence, it is important to prove that additives or mixes of materials that are introduced on the market significantly reduce the electricity used by the pellet machine. In this work, it is investigated how various percentages of starch influence some properties of interest to the producer and the household user, i.e., additive influence on the energy use of pellet production, pellet length, bulk density, pellet density, and durability. The producer tests the pellet properties before sacking. Most pellets, however, are stored for a couple of months up to a year before being combusted. During this time, some properties change. Therefore, we test the pellets after storage and introduce indicators for shelf life in this study. Four different kinds of starch are used in the tests. The optimum starch mixes is not investigated here nor are the combustion properties.

EXPERIMENTAL SECTION

Sawdust. The raw material used for the production of pellets was fresh sawdust of Norway spruce (Picea abies) produced at a local sawmill that uses frame saws. The wet sawdust was dried in air in a belt dryer at a low inlet temperature of 75 °C until it reached 11% (wb). The sawdust was ground using a 5 mm sieve. The dried sawdust was conditioned from 11 to 12.1 ± 0.1% (wb) in the diagonal mixer by adding water to an appropriate moisture content. The sawdust was then stored in the diagonal mixer (by which the materials were completely mixed after 5 min) for 2 days to reach a uniform moisture content. The amount of extractives in the sawdust was 2% [dry basis (db)]. Starch. Four different starch grades were used in this study: native wheat and potato starch and oxidized corn and potato starch. Solam GmbH supplied the starches, and the commercial names of the products are Solpearl P, Solpearl W, Solsize P TSC, and Solsize M TSC. The selected starches are commonly used products. Native starches from potato and a cereal, such as wheat or corn, were compared to oxidized starches. The granules of the different starch raw materials have different sizes and shapes (see Table 1). Because of the different sizes and shapes, the number of granules present in 1 g of starch differs. This could influence the spreading in the wood material and the number of possible “gluing” points in the pellet. Note that oxidation does not affect the granule size or gelatinization temperature. Accordingly, native corn starch has a granule size of 2−30 μm and a gelatinization temperature of 62−80 °C,9 as does oxidized corn starch (see Table 1). Production Equipment. The pellets were produced in a production unit located at the Department of Energy, Environmental and Building Technology at Karlstad University, Karlstad, Sweden (see Figure 1). It consists of (1) a diagonal mixer, (2) a conveyor screw, (3) an inlet feeder where conditioning takes place (if needed), (4) an Amandus Kahl C33-390 pelletizing press with a flat die and a maximum output of 300 kg/h (a description of the machine can be downloaded from www.akahl.de/akahl/en/products/biomass_

Figure 1. Pellet production unit at Karlstad University. 1939

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[2,2,4-Trimethylpentene was chosen because its large molecules were thought unlikely to penetrate the wood. The pellets were weighted before the test and then lowered into the cylinder one by one. Between every pellet, the increase in volume was pipetted to detect the pellet volume (using a pipet with a volume of 0.5 ± 0.01 mL). The pellet density was decided on the basis of the pellet weight (including moisture) and volume, and the result is presented as the average density], (5) mechanical durability determined according to SS-EN 1521030 presented as the percentage of pellets, (6) extractive content determined by Soxhlet extraction of 7−9 g of crushed pellets with 50 mL of acetone for 8 h (The extractive matter was then dried according to SS-EN 14774-1,28 and the amount of extractives was related to the pellet dry substance. The coefficient of variation for this method was 5%), and (7) hexanal content analyzed using a static headspace gas chromatographic method (SHS−GC) as described in ref 31, except that 100 μL of the headspace gas was injected and a gas chromatography−flame ionization detector (GC−FID) was used instead of gas chromatography−mass spectrometry (GC−MS) [The GC−FID was a Clarus 480 fitted with a capillary column (J&W Scientific, DB5-MS, 30 m × 0.25 mm). The temperature was programmed as 40 °C for 2 min, 15 °C/min, 70 °C for 5 min, 15 °C/ min, and 200 °C for 7−25 min. The temperature of the injector was 200 °C, and helium was used as the carrier gas. Hexanal of analytical grade was obtained from Sigma-Aldrich. The coefficient of variation for this method has been determined as 11%].31 If the statistical error is not stated in the standard, it is commented on in the data/results in each section. Generally, the standard deviation is used when there is a sufficient amount of data present, or else the error of measurement is used. For the storage data, a coefficient of variation is reported. For pellet length, bulk density, and durability, the significance of the results are tested with a t test, where α = 0.05. From the measured load current, the average power consumption and average energy consumption for alternating currents were calculated according to the method described in a previously published study conducted at Karlstad University.32 The pellets were stored in a controlled laboratory setting to avoid undue influence of the temperature and humidity fluctuations. They were kept indoors in unclosed black plastic bags at 18 °C and at about 55% humidity, and they were protected from direct light. For pellets with 0 or 3% starch, the hexanal content was analyzed as long as the oxidation continued and the extractive content was tested 3 times during the first 75 days and also after 7 months. The durability of pellets with 0, 1, and 3% of starch added was tested both immediately after production and after 7 months of storage. The pellets produced from each test were sent to Solam GmbH, who conducted two studies on the pellets. In the one study, they used a stereomicroscope from Olympus. The pellets were divided with a scalpel and then coated with an iodine solution before a photo was taken in the microscope. The other study treated the physical changes of the starch in the pellets. The size determination of the granules for the various pellets was made using the Malvern laser technique. The size is determined from the volume of the particles, and thus, the diameter can be calculated.

pelleting/), and (5) a volumetric feeder for additives and a cooling tower. In the pelletizing press, the sawdust is pressed through the flat die by pan grinder rollers. The die has a 9 hole radius with 52 holes on each row, which sums up to a total of 468 holes. It has a working width of 75 mm, a hole diameter of 8 mm, a total thickness of 50 mm, a relief depth of 20 mm, and hole inlet diameter of 10.2 mm. The inlet tapers 17°, and it has no cutting blade. The open area of the die is 64%. The effective compression length in this study was chosen to be 30 mm. The additive was supplied to the inlet screw feeder (3) through a volumetric feeder (5). The volumetric feeder consists of a hopper, a gearbox, and an agitator that rotates above the screw to maintain a constant additive flow and to prevent bridging. The volumetric feeder throughput depends upon the screw speed and kind of additives. This means that the volumetric feeder has to be calibrated. The output for the additive was determined as a function of the frequency of the screw. Production Process. The continuous feed pellet machine was run until stationary conditions were obtained. Before every new test, there was a break-in period of 5 min with the current additive to ensure stationary conditions. Every test run lasted for 5 min. The feed control for the dried sawdust at 12.1% (wb) was set at a fixed rpm (4.0 Hz), corresponding to approximately 85 kg of sawdust/h. The additive flow was subsequently increased through the volumetric feeder, going from 0.75 to 3.0% based on the weight percentage of pressing mass dry bases. The additive moisture content for used assortments was 12.7% for native wheat starch, 12.5% for oxidized corn starch, 13.6% for native potato starch, and 11.9% for oxidized potato starch. The moisture content (wb) was determined according to SS-EN 14774-1. The moisture content for starch was measured by taking 4 samples of each assortment; altogether 12 samples were taken. During the test production of pellets, samples of approximately 300 g of dried sawdust were taken every 15 min, which resulted in a total of 14 samples. The moisture content of the cooled pellets was examined by taking 1 sample from each 5 min test, i.e., a total of 20 samples. Storage. To study the effects of the starch on the pellets during storage, the pellets were stored for 7 months. It was examined to what extent storage affects pellets with and without starch with reference to the extractive content, amount of hexanal formed, and durability. Of these properties, durability is the only one included in the European standard.17 The extractive content is defined as in the standard SCANCM 49:03 and, thus, refers to acetone-soluble matter. The amount of acetone-soluble matter in wood chips provides a measure of the content of wood extractives, which includes, e.g., fatty acids, resin acids, fatty alcohols, sterols, diglycerides, triglycerides, steryl esters, and waxes. Phenolic compounds, such as lignans, are also included. The shelf life was gauged by analyzing the amount of hexanal formed during storage, because the formation of hexanal signifies the oxidation of fats. Measurements. During the tests, die temperature, screw frequency, current consumption of the pelletizing machine, and pressure from the rollers on the die (“die pressure”) were measured every 10 s. The die temperature was measured using Pt-100; the error in the measurements is ±0.5 °C. The current load was measured with an accuracy of ±1%. The pressure from the rollers was measured with an accuracy of ±1.25 bar. The die temperature and the pressure from the rollers indicate stationary and stable conditions. The produced pellets were cooled to ambient room temperature and sieved, before being analyzed. The analysis was performed by testing and comparing the produced pellets with the quality parameter settings in the Swedish and/or European standards,17,27 complemented with additional tests. The tested parameters were (1) moisture content (%, wb) for sawdust, starch, and pellets determined according to SS-EN 14774-1,28 (2) average length (mm) determined by measuring the length of two samples of at least 20 randomly chosen cooled pellets using a calliper according to SS 187120,27 (3) bulk density (kg m−3) determined according to SS-EN 15103:201029 by measuring the weight of a 5 L bucket filled with cooled pellets, (4) pellet density (kg m−3) determined using a 100 mL graduated cylinder filled with 2,2,4-trimethylpentene and five randomly selected pellets

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RESULTS The addition of starch in this test was 0.7−3.1% on dry solids (% ds). The die pressure and the die temperature showed very small variations during all of the test series, namely, 116 ± 0.6 bar (mean ± standard deviation) and 107.9 ± 0.2 °C for native wheat starch, 119 ± 0.4 bar and 107.6 ± 0.2 °C for oxidized corn starch, 122 ± 0.4 bar and 107.0 ± 0.2 °C for native potato starch, and 125.0 ± 0.4 bar and 107.9 ± 0.2 °C for oxidized potato starch, respectively. A stable environment implies that we can conclude that the tests for which starch grades were used were performed during stable conditions. 1940

dx.doi.org/10.1021/ef201968r | Energy Fuels 2012, 26, 1937−1945

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Table 2. Results from Pellet Tests and Measured Data during Pellet Productiona test run

native wheat starch

oxidized corn starch

native potato starch

oxidized potato starch

amount of starch (%) 0.0 0.7 1.1 2.0 3.0 0.0 0.8 1.0 1.9 2.8 0.0 0.8 1.0 2.1 3.1 0.0 0.8 1.0 2.1 3.1

load current (A)

pellet MC (%, wb)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

6.9 7.1 7.0 7.6 8.2 6.7 7.1 8.7 8.9 8.5 7.0 6.9 7.3 7.5 7.2 7.0 7.4 7.6 8.6 9.4

25.3 24.9 24.5 24.3 23.6 24.7 23.7 23.2 22.7 22.2 24.3 24.3 24.5 24.5 24.2 24.4 24.0 23.8 22.9 22.7

1.3 0.7 0.6 0.9 0.7 0.9 0.7 0.8 0.8 0.5 1.3 0.6 0.8 1.0 1.0 1.1 1.0 0.8 0.7 0.8

average pellet length (mm) 6.8 7.3 7.5 8.2 8.3 6.4 9.0 9.9 13.9 13.9 6.7 7.6 7.8 7.6 9.7 6.6 8.3 9.9 11.2 13.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.2 2.7 2.6 3.9 3.9 1.8 4.3 4.7 6.9 6.2 2.1 3.2 3.4 3.6 4.9 2.3 3.5 6.4 5.7 7.0

bulk density (kg/m3) 638 655 653 664 666 631 662 659 658 650 623 639 641 646 654 620 648 643 653 654

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

9.5 5.3 8.4 0.3 3.3 8.2 4.4 0.7 0.2 2.5 5.8 5.7 2.9 6.6 4.8 14.5 6.0 6.8 3.0 4.7

pellet density (g/cm3) 1.38 1.30 1.34 1.37 1.32 1.31 1.36 1.33 1.32 1.34 1.25 1.29 1.29 1.31 1.28 1.31 1.36 1.35 1.29 1.29

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.06 0.07 0.02 0.06 0.12 0.12 0.06 0.06 0.08 0.08 0.11 0.07 0.10 0.13 0.04 0.12 0.11 0.12 0.07 0.05

durability (%)

material flow (kg of ds/min)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.27 1.30 1.30 1.32 1.32 1.32 1.36 1.34 1.36 1.39 1.34 1.33 1.37 1.39 1.38 1.37 1.34 1.37 1.34 1.38

87.00 90.55 90.57 91.99 93.79 85.17 93.57 95.09 96.77 98.10 84.85 86.49 88.05 88.40 89.78 83.86 90.23 90.60 95.03 96.63

1.03 0.15 0.31 0.95 0,01 0.57 0.82 0.42 0.20 0.34 0.15 0.24 0.11 1.04 0.87 0.52 0.05 0.09 0.11 0.24

a

MC = moisture content. For the amount of starch, the standard deviation is