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Paper No. 05-009
SUPERHEATED STEAM: ITS NOT JUST ABOUT
DRYING
C. Pronyk
Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 5V6.
S. Cenkowski
Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 5V6
W.E. Muir
Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada, R3T 5V6
Written for presentation at the CSAE/SCGR 2005 Meeting
Winnipeg, Manitoba June 26 - 29, 2005
Abstract
Superheated steam has been known as a drying medium for over 100 years. From the
beginning acceptance of the technology has occurred because of the properties of superheated steam that make it attractive for drying as well as processing applications. Our research program in the Department of Biosystems Engineering at the University of Manitoba and others have looked to the processing benefits of superheated steam to create snack food products (instant noodles, tortilla chips, potato chips, pork bundles), affect changes in product quality and performance, and sterilize grains (bacteria and mycotoxins). Superheated steam has proven itself to be beneficial not only as a drying medium, but also as a processing medium.
INTRODUCTION
The knowledge and practice of drying in superheated steam has been known for over a hundred years (Hausbrand 1912). Even from its early beginnings, superheated steam was being utilised for the alternative benefits it provides in addition to its abilities to act as a drying medium. There are many inherent properties of superheated steam that make it attractive for not only drying, but for many processing applications as well. In order to make a superheated steam system economical, the exhaust steam is often collected and either used elsewhere, or the latent heat is recovered by condensing the steam. By condensing and collecting the exhaust steam, dust and expensive or toxic compounds may be collected instead of releasing them into the environment Kudra and Mujumdar 2002). Drying with superheated steam produces an oxygen free environment, which eliminates the possibility of fire or explosions in the system and can lead to improved product quality by eliminating scorching. The lack of oxygen can also eliminate oxidative reactions from occurring within the product. Processing in superheated steam allows for concurrent blanching, pasteurization, sterilization, and deodorization of products in addition to any drying that is occurring (Deventer and Heijmans 2001). Food products can also become partially cooked with potentially beneficial changes in textural properties. Often before industry will accept a new technology it must present benefits greater than what is currently provided by the accepted technology. Superheated steam has continually provided these benefits through superior drying
characteristics but through its use as a processing medium to improve product quality. This paper will explore some of those benefits in greater detail.
EARLY USE OF SUPERHEATED STEAM AS A PROCESSING MEDIUM
One of the first industrial uses of superheated steam drying was to dry brown coal. In Austria the Fleissner process was developed in the late 1920s to provide a dry coal from a low-grade high-moisture content (55-65%) coal (Potter and Beeby 1994). Some work was also accomplished in the USA for the drying of lignite using the Fleissner process (Lavine et al. 1930; Cooley and Lavine 1933). In the Fleissner process, a large portion of the liquid leaves the product in a physico-chemical process within a vessel filled with saturated steam under high pressure. The pressure is reduced and a portion of the remaining moisture flashes to superheated steam and leaves the product. The high moisture content of brown coal reduces its usefulness by increasing transportation costs and hindering combustion and gasification. Conventional drying at rapid rates in hot air would dry the outer surface before the centre causing the outer surface to shrink prematurely, crack, and cause the lump to disintegrate (Bainbridge and Satchwell 1947). Drying by the Fleissner process reduces the amount of disintegration, produces better weathering
properties than raw or air-dried coal, and steam drying does not increase the reactivity of the coal to oxygen, thus reducing the risk of spontaneous combustion (Lavine et al. 1930; Bainbridge and Satchwell 1947). Use of superheated steam for drying brown coal produced benefits in addition to drying that were not available to hot air dried coal.
Another early use of superheated steam was seen in the lumber industry were it still maintains a large acceptance by industry. Kauman (1956) reported that there were superheated steam kilns
for the drying of lumber operating on the West Coast of the United States as early as 1908. In addition a man named Tiemann developed a “high velocity, low superheat” kiln during the First World War. These early kilns did not gain widespread acceptance due to inefficiencies in the systems design. After the Second World War, kilns that run with superheated steam or air-superheated steam mixtures gained industrial acceptance in Germany and later spread outside of the country (Kollmann 1961). Today, besides for the increased drying rates, drying of lumber in superheated steam at sub-atmospheric to high pressures may provide additional benefits. These include: removal of resins (high pressure) or minimal removal of resins (low pressure), reduced internal stresses, stabilised dimensions, improved permeability, no oxidative discoloration (oak and beech), splits and checks can be avoided, staining and mould are eliminated (Ishikawa et al. 2004; Kollmann 1961; Kudra and Mujumdar 2002).
SNACK FOODS
Instant noodles
Since our program in the Department of Biosystems Engineering at the University of Manitoba began, a variety of food products have been dried and processed in our laboratory including potatoes (Tang and Cenkowski 2000), sugar-beet pulp (Tang et al. 2000), noodles (Markowski et al. 2003), and distillers’ and brewers’ spent grain (Pronyk et al. 2004c; Tang and Cenkowski 2001; Tang et al. 2004). From the beginning, quality of the dried products was seen as an inseparable issue in drying and processing and as such was incorporated into our program.
In Asia a large part of wheat consumption is in the form of noodles (Miskelly 1993). Instant noodles were introduced in 1958 and have found a large market in North America with Canadian sales of M$71/year (Bailey 2002). A potential method for drying noodles is with superheated steam at temperatures between 100 and 150ºC. With high temperature processing using superheated steam, Asian noodles become partially cooked, creating an instant noodle and eliminating the necessity of additional cooking in oil (Markowski et al. 2003). There are health concerns in North American society over the fat content in foods and particularly the presence of trans fatty acids from partially hydrogenated and hydrogenated oils used in the frying process. Trans fatty acids have been associated with an increased risk of coronary heart disease in epidemiologic studies (Willett and Ascherio 1994).By eliminating the oil incorporated during frying from the noodles, a healthier product would be created that would be desirable to consumers.
Early results from Markowski et al. (2003) showed that breaking strength is adversely affected by an increase in processing temperature (Fig. 1) and noodles brown at temperatures above 130ºC. A study of the starch by differential scanning calorimetry also showed that starch is being modified which affects its ability to swell when cooked. When noodles were processed at 110ºC and 120ºC in superheated steam textural parameters of recovery, adhesiveness, and gumminess were
improved. However, these improvements are overwhelmed by deleterious effects on the key textural parameters of maximum cutting stress, resistance to compression, and surface firmness. These results meant that a noodle with a soft, mushy texture is produced that would not be accepted by consumers even though the drying characteristics are favourable. However, they did not compare their processed noodles to commercially available instant noodles or control samples to see if these results were similar to properties of other instant noodle products. The scope of the
project was limited to the study of dried noodles and not to instant noodles which would be expected to exhibit different textural properties. The potential to create commercially acceptable instant noodles with superheated steam is plausible as studies of impingement drying of potato chips using superheated steam showed that the textural characteristics of superheated steam processed samples were more similar to the commercial oil fried chips than those dried with hot air (Caixeta et al. 2002).
Fig. 1. Breaking strength of noodles dried in steam at 110, 120, 130, and 140ºC and exposed to various residence times. Data points are mean values of five tests and vertical bars indicate standard deviations. Straight lines show trends in breaking strength changes when noodles were exposed to same steam temperature but for a different residence time.
Tortilla chips
Li et al. (1999) processed tortilla chips in impinging jets of superheated steam and hot air at temperatures of 115, 130, and 145ºC and with convective heat transfer coefficients of 100, 130 and 160 W/(m ºC). The effect of superheated steam processing on product quality was evaluated
based on parameters such as shrinkage, crispness, starch gelatinization, and microstructure. Higher steam temperatures caused less shrinkage and a higher modulus of deformation
(crispness). However, at higher steam temperatures there was less starch gelatinization and the pasting properties showed that the tortilla chips had an increased ability to absorb water. Analysis of the microstructure of the superheated steam processed tortilla chips revealed that higher steam temperatures resulted in more pores and a coarser appearance. When compared to hot air,
superheated steam processing resulted in higher drying rates and more starch gelatinization at equivalent temperatures and convective heat transfer coefficients.
Potato chips
Caixeta et al. (2002) processed potato chips in impinging jets of superheated steam and hot air at temperatures of 115, 130, and 145ºC and with convective heat transfer coefficients of 100 and 160 W/(m ºC). The purpose of the study was to test the feasibility of producing low-fat potato chips with the desired texture and flavour characteristics. In general potato chips dried at the higher temperatures and convective heat transfer coefficients in superheated steam showed less shrinkage, lower bulk density, higher porosity, and darker color. When compared to the air-dried samples superheated steam processed potato chips had more shrinkage, higher bulk density, lower porosity, and lighter color at the same conditions. Superheated steam processed potato chips also retained more vitamin C and were closer in texture to the commercial potato chips than the air dried samples.
Zousoon (pork bundles)
Zousoon is a pan fried pork product of intermediate moisture (2-12%) found in Taiwan and China (Huang et al. 2004). A study conducted by Huang et al. (2004) looked into using superheated steam as a way to process pork bundles in an oxygen free environment to inhibit lipid oxidation. Samples were pan-fried or processed in superheated steam at 150ºC then packaged in cans and stored for 1 y. Trained panellists evaluated the stored sample after 1 y for “off” odours and found that the pan-fried samples generated an strong odour whereas the superheated steam processed samples developed only a slight odour. Lipid analysis showed that superheated steam was found to be effective in suppressing lipid oxidation and had greater stability compared to pan-fried zousoon.
CHANGES IN MATERIAL PROPERTIES
Oat groat viscosity
Heat treatment is vital part of oat processing, wherein oats are exposed to an elevated temperature for a period of time so that the oats are made suitable for incorporation in food systems. The main goals are to extend shelf life by inactivating enzymes which cause rancidity, reduce bacterial counts to acceptable levels, and to produce a desirable toasted aroma and flavour in the oats. Heat treatment of oats also causes the starch to gelatinize, a physiochemical process wherein starch
granules that are subjected to sufficiently high temperatures imbibe water and swell, thus forming a gel of increased viscosity.
Our group at the University of Manitoba in conjunction with Can-Oat Milling in Portage la Prairie, MB, have been investigating conditioning methods that will modify the viscosity of whole oat flour to provide a better “cook out” in the extrusion of oat flour containing products, such as Cheerio-like hoop breakfast cereals. Preliminary work with superheated steam produced favourable viscosities compared to the current kiln conditioning system (Kesler 2005).
Unconditioned oat groats were obtained from Can-Oat Milling and tempered in the lab to the desired moisture content of 14%. The groats were processed in superheated steam at 120ºC with a velocity of 0.35 m/s for 5, 7.5, and 10 min. Processed samples were analysed for viscosity by Rapid Visco Analyzer (RVA) test and inactivation of the enzyme peroxidase. The enzyme peroxidase was used as an indicator for successful heat treatment as it is the most heat stable enzyme within the oat. Results showed that superheated steam processing increased the viscosity of the groats and inactivated enzymes at a processing time of 7.5 min or greater (Table 1).
Differences in viscosities between each processing time were found to be not statistically significant (P > 0.05). The increase in viscosity was not as great as the kiln dried groat probably due to incomplete gelatinization seen by the higher moisture contents.
Table 1. Peroxidase test results, moisture content, and viscosity of oats processed under various conditions
Processing Conditions Peroxidase Test Results Moisture Content (% wb) Viscosity (cP)
Mean S.D.
Unprocessed Control Positive 12.5 550 66
5 min Positive 11.5 5900 200
7.5 min Negative 11.6 6120 36
10 min Negative 11.6 5800 305
Kiln Dried Negative 7.1 7250 120
Miscellaneous improvements
In addition to the product improvements in coal and lumber dried in superheated steam there are many other products that have benefited from processing in superheated steam. When dried in superheated steam, paper can see increases in the burst index, TEA index, elastic modulus, and up to 30% in tensile strength (Kiiskinen and Edelmann 2002). Foods may become more porous due to the increased heat transfer rates in superheated steam causing the moisture in a product to flash into steam creating many pores (Yoshida and Hyodo 1966, Kudra and Mujumdar 2002). Synthetic fibres spun in superheated steam produced a stronger and finer fibre (Yoshida and Hyodo 1963). Products may have odours removed, as with distillers spent grain where acetic acid can be stripped away by the superheated steam, thus giving the spent grain an aroma like baked goods instead of a sour smell (Tang and Cenkowski 2001).
FOOD SAFETY
Currently, our research is looking into decontamination of food products with superheated steam in addition to drying. Food products can often be contaminated with spores and mycotoxins, many of which are resistant to heat. To ensure the safety of our food supply they must be reduced or eliminated from the final product through processing procedures. It is hypothesised that the high heat capacity of a superheated steam environment in processing may be successful in reducing or eliminating spores and mycotoxins in food products.
Spores
Studies on eliminating Bacillus stearothermophilus, a thermophilic, spore-forming
microorganism associated with flat sour spoilage of low acid canned foods stored at elevated temperatures, have been initiated by our group (Zmidzinska et al. 2004). Spores of this bacterium are very heat resistant and are often used to monitor sterilisation processes of moist heat (Brown 1994, Spicher et al. 1999). The spores were mixed with fine sand (105 to 212 µm in diameter) and were exposed to superheated steam at temperatures of 105, 130, 145, 160, and 175ºC and a steam velocity of 0.35 m/s. D-values (time required to destroy 90% of initial population) calculated from the experiments of treating spores in sand have shown that spores at a lower concentration level of approximately 103 cfu•g-1 were easier to eliminate at lower superheated steam temperatures (105 and 130ºC) than the spores at a higher concentration level of
approximately106 cfu•g-1 (Table 2). Higher operating temperatures (145, 160, and 175ºC) caused faster elimination of the spores at the higher inoculum level. The first 5 min of processing were
most effective for most processing temperatures for reducing levels and if an intermittent cooling period was allowed the killing effect was improved compared with continuous processing.
Table 5. D-values of Bacillus stearothermophilus treated in superheated steam
Temperature (ºC)
D-value (min) for 103 (cfu•g-1) inoculum level
D-value (min) for 106 (cfu•g-1) inoculum level[a]
105 23.5 -130 65.9 -145 63.0 29.0 160 9.3 2.1 175 2.2 1.5 [a]
D-values at 105 and 130ºC could not be calculated even after 80 min of processing
0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16
Processing Time (min)
DON Concentration (ppm)
110ºC
135ºC
160ºC 185ºC
Fig. 2. DON concentration in wheat kernels processed in superheated steam at various temperatures.
Mycotoxins
Spent grains from alcohol production can be contaminated with mycotoxins from Fusarium moulds. The most common mycotoxin in Canada, deoxynivalenol (DON), is stable through
fermentation so must be reduced to low levels if contaminated spent grains are to be used as animal feed (Bennett and Richard 1996). Our study looked at the application of superheated steam to reduce or eliminate the level of DON in naturally contaminated whole kernels of wheat (Pronyk et al. 2004a, 2004b). Wheat kernels were processed in superheated steam at temperatures of 110, 135, 160, and 185ºC at a velocity of 0.65 m/s for periods of 2 to 15 min. Results showed that the greatest reduction of DON occurred at the highest steam temperatures and longest processing times (Fig. 2). Significant reductions occurred at 160 and 185ºC (P < 0.05) when compare with control samples. When compared with results of processing whole barley kernels in hot air (Yumbe-Guevara et al. 2003) the reduction in DON levels was greater and occurred at lower temperatures in superheated steam. Levels of DON were reduced by 52% at 185ºC and 6 min of processing. Further experimentation with saturated steam has shown that only thermal degradation is taking place and not any solubilisation as had been hypothesised.
SUMMARY
Superheated steam is a versatile processing medium that may create desirable changes in a product, in addition to any drying that may occur. New applications in the snack food industry to produce healthier low fat products and to ensure the microbial safety of our food supply are being studied by various researchers. These benefits may speed industrial acceptance of superheated steam drying and processing.
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