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Vitamin retention in extruded food products
ARTICLE IN PRESS JOURNAL OF FOOD COMPOSITION AND ANALYSIS Journal of Food Composition and Analysis 19 (2006) 379–383 www.elsevier.com/locate/jfca
Short Communication
Vitamin retention in extruded food products Nelofar Athar, Allan Hardacre, Grant Taylor, Suzanne Clark, Rebecca Harding1, Jason McLaughlin New Zealand Institute for Crop and Food Research Limited, Private Bag 11600, Palmerston North, New Zealand Received 28 May 2004; received in revised form 26 July 2004; accepted 2 March 2005
Abstract Crisp extruded snack food like products were produced from a range of cereal products using a short barrel, single screw snack food extruder. The retention of B group vitamins during extrusion processing was compared for the different cereal grains and under different extrusion conditions. This work showed that short barrel extruders used for snack food production retain between 44% and 62% of the B group vitamins. This is considerably higher than the 20% retention for maize reported previously for long barrel extruders. The stability of the vitamins was similar, with riboflavin and niacin having the highest stability. Pyridoxine was stable in maize, but less so in oats and the maize+pea ingredients. Thiamin was the least stable during extrusion. It is concluded that short term high-temperature cooking of extruded snacks allows the retention of higher levels of heat labile B vitamins than the longer time and lower temperature cooking methods used in modern snack food extruders. r 2006 Published by Elsevier Inc. Keywords: Extrusion; Nutrient retention
1. Introduction Extrusion cooking is a relatively modern, hightemperature, short-time processing technology that was invented in 1940s to manufacture snack foods. This technique has gained ground in human food and animal feed industries world-wide, primarily for the processing of cereal grains. The best known products are low density foamed corn and rice breakfast and snack foods that are widely available. Similar products made from legumes are available in Asian countries. Extrusion cooking, particularly in the snack food industry, is a complex process that differs from conventional processing by using high shear rates and high temperatures (4150 1C) for very short periods Corresponding author. Tel.: +64 6 351 7066/6146; fax: +64 6 351 7050. E-mail address: [email protected] (N. Athar). 1 Present address: Fonterra Cooperative Group, Private Bag 11029, Palmerston North, New Zealand.
0889-1575/$ - see front matter r 2006 Published by Elsevier Inc. doi:10.1016/j.jfca.2005.03.004
(seconds). A wide range of thermo-mechanical and thermo-chemical processes are involved, including shear, Maillard reactions, protein denaturation and hydrolysis. These processes result in the physical, chemical and nutritional modification of food constituents (Harper, 1981; Linko et al., 1981; Jowitt, 1984; Zeuthin et al., 1984). Cereal grain products are among the most important sources of B group vitamins in the Western diet, and for this reason there is considerable interest in retaining the nutritional benefits of cereals during processing. Extrusion processing is increasingly used to process the ingredients in muesli bars, breakfast cereals and snack foods, the source of a significant proportion of the cereal grain, and hence B group vitamins, in Western diets (Cheftel, 1986). Several studies have assessed the effects of extrusion cooking on the retention of B group vitamins (Cheftel, 1986; Camire et al., 1990; Killeit, 1994). During the extrusion of crispbread products (Cheftel, 1986) at a specific mechanical energy (SME) from 0.09 to 0.13 kWh/kg, and retention times of 0.5 to
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1 min at 178 1C, the levels of B group vitamins decreased. From 38% to 65% of thiamine remained, 85% of riboflavin, 80% of niacin and 71–83% of pyridoxine. Thiamine and pyridoxine were the most thermo-labile and levels decreased linearly with temperature. Camire et al. (1990) obtained similar results, with thiamin losses increasing with barrel temperature, and with shear and temperature as screw speed was increased. There was some evidence that thiamine was more sensitive to heat than riboflavin and that riboflavin was more sensitive to shear. Many of the studies reported using twin-screw extruders for their ability to operate over a wide range of conditions. However, many of the most widely consumed expanded snack foods are processed in short barrel, low residence time extruders, similar to the machine used in this work. In this paper, the retention of B group vitamins during extrusion processing of whole oat grain and maize and pea grits is compared for a short barrel, low residence time extruder at different temperatures. This information is useful for estimating the loss of important vitamins during the manufacture of extruded snack products from a diverse range of ingredients.
2. Material and methods 2.1. Cereal ingredients Commercially available, specification 220 maize (corn) grits were purchased from Corson Grain Ltd (Box 1046, Gisborne, NZ). The maize grits are degermed before manufacture and for this reason contain about 75% starch, about 8% protein and less than 0.5% oil. Dried green peas (variety Rex, Crop and Food Research) were coarsely roller milled and sieved in our laboratory to produce a pea grit product of similar size to the spec. 220 maize. The pea grits contained approximately 43% starch, about 22% protein and 1.3% oil. An unmilled naked oat variety (CRO 59) was used as the whole grain. Whole oats contain approximately 62% starch, about 11% protein and 4% oil. They therefore contain far more oil than the other grain products, and this is likely to alter their processing properties. Because pea grits did not extrude well in the equipment available due to their high protein content, they were blended 50/50 with maize grits. In all cases the ingredients contained about 12% moisture as used. 2.2. Extrusion processing The food products used in this study were made on a Dorsey, single-screw, short barrel (90 mm) snack food extruder. The extruder was fitted with a 4-start screw and a 2-hole die with 5 mm apertures. The ingredients were fed into the extruder in the form of a dry granular
material at the rate of 75 g min1 using a mass flow feeding device. The extruder was operated at barrel temperatures between 130 and 160 1C. No water was added, but steam generated from the moisture in the hot starchy melt formed from the maize and maize+pea ingredients under high shear and temperature caused the extrudate to expand as it left the die to form a lowdensity starch-based foam. The plastic foam set rapidly as it left the die, forming a light, crispy, rigid material similar to many well known snack foods. During the extrusion process, moisture was lost from the grain products. Typically about 5% of the initial moisture is lost, and the extrusions contain about 7% moisture. The high oil content of the oats prevented the formation of a well-structured foam, and this particular extrudate was hard and dense. As the extrudate emerged from the die, it was cut into 20–30 mm long snack-like extrusions with a rotating knife. Energy usage during extrusion varied between the grits used in the experiment. For the maize and maize+pea grits, the current drawn by the motor driving the extruder screw was set from 12–14 A, corresponding to a power of from 7.1 to 8.2 kW, which in turn corresponded to a SME input of about 0.095 kWh kg1 of ingredients. The high oil content in the oats reduced this considerably to about 0.012 kWh kg1 of grain. The values for the maize and maize+pea ingredients were similar to those quoted by Cheftel (1986) but in comparison to the 0.5–1 min residence times used in that study, the short barrel extruder used in this work had a residence time of about 5 s. 2.3. B group vitamins 2.3.1. Vitamin analysis The samples of extrudate were ground to a homogenous state using a food processor. Analysis of four B group vitamins (thiamin, riboflavin, niacin and pyridoxine) was performed using HPLC by a commercial analytical laboratory (AgriQuality New Zealand). Thiamin analysis. A portion of the sample was acid autoclaved at 121 1C, followed by enzymatic digestion to release protein-bound vitamin and break any thiamin– phosphate bonds. The extract was then assayed by ionpair reversed phase HPLC with a buffered mobile phase (methanol–citrate, pH 2.4). The thiamin was oxidized to thiochrome by post-column reaction with hexacyanoferrate (III) and detected by fluorescence (Gehring et al., 1995). Riboflavin analysis. A portion of the sample was acid autoclaved at 121 1C, followed by enzymatic digestion to release protein-bound vitamin and break any riboflavinphosphate bonds. The extract was assayed by reversedphase, ion-pair HPLC techniques and detected by fluorescence detection (Egberg and Potter, 1975).
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Niacin analysis. A portion of the sample was heated with dilute hydrochloric acid to extract the vitamin and hydrolyze the niacinamide to niacin. The extract was cooled, made to a known volume and subjected to HPLC analysis, using reversed-phase, ion-pair techniques and UV detection (Woollard, 1984). Pyridoxine analysis. A portion of the samples was dephosphorylated by enzymatic hydrolysis for the analysis of pyridoxine. Pyridoxamine is transformed into Pyridoxal by reacting with glyoxylic acid in the presence of Fe2+, which is then reduced to pyridoxine by the action of sodium borohydride in alkaline medium. The extract was then assayed for pyridoxine by reversed-phase, ion-pair HPLC with fluorescence detection (Bitsch and Moller, 1989; Reitzer-Bergaentzle and Marchioni, 1993). 2.3.2. Vitamin retention Samples of the cereal ingredients were analyzed for vitamin content before and after extrusion. To allow for any changes in vitamin concentration, the retention factors for the products were calculated using the true retention method (Bergstro¨m, 1998). The equation used for calculating true retention is True retention ð%Þ ¼ ðVitamin content of extruded cereal g extrudateÞ= ðVitamin content of raw cereal g ingredientsÞ 100: 2.3.3. Experiments Three experiments were performed. In the first, the vitamin retention of extrusions made from maize grits, a 50/50 mixture of maize+pea grits, and whole oat were compared during extrusion at 152 1C. In the second experiment, the effect of barrel temperature on the retention of vitamins during the extrusion of maize grits was compared. For the third experiment, extrusion was carried out at a barrel temperature of 160 1C and maize germ material and/or amylose starch were added to maize grits. The germ material was added to increase natural levels of the B group vitamins. Amylose starch was added to increase the levels of slowly digestible fibre in possible snack products. Four formulations were used: 100% maize, 90% maize with 10% germ material, 80% maize with 20% amylose starch added, and finally 70% maize, 20% amylose, and 10% germ material. For each formulation and temperature, three replicates of each material were produced. However for cost reasons, the three replicates were pooled and two subsamples of each extrusion analysed for B group vitamins. Statistical analysis of the results was not therefore possible. However, similar materials were measured in each of the three experiments and this can be used to gauge the consistency of the data presented.
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3. Results and discussion Apart from the absolute values for vitamin concentrations presented for comparison in Table 1, these data are reported as percent retention. By comparing individual duplicates and the data for the maize grits across the three experiments it appears that the error variation in the values is about 710%. This is high but expected when low levels of vitamins are involved and may be conservative when compared with the 38–65% losses of thiamin reported by Cheftel (1986). 3.1. Experiment 1: extrusion of three cereals The levels of selected B group vitamins measured in the ingredients before extrusion are shown in Table 1. Extrusion resulted in only slight changes in protein, fat and carbohydrate levels and no changes in the levels of eight mineral elements (calcium, iron, magnesium, phosphorus, potassium, sodium, zinc and copper; data not shown). However, major changes were observed in the levels of important B group vitamins including thiamin and pyridoxine that are regarded as the most heat sensitive (Cheftel, 1986). Since retinol was not detected in any of three cereal samples prior to extrusion, it was decided not to analyse for retinol in post-extrusion samples. There was, of course, variation in the level of B group vitamins between the cereal ingredients with the maize grits having the lowest overall vitamin content and the maize+pea grits mixture having the highest levels. The retention of thiamin, riboflavin, niacin and pyridoxine varied depending on the cereal type (Table 2). Overall, the retention of riboflavin and niacin was highest, with the exception of 100% retention of
Table 1 Vitamin levels in the cereals before extrusion Vitamins (mg/100 g)
Oats
Maize
Maize+peas
Thiamin Riboflavin Niacin Pyridoxine
0.11 0.06 1.35 0.10
0.09 0.04 0.63 0.04
0.22 0.08 1.85 0.06
Table 2 Vitamin retention (% retained) for B group vitamins after extrusion Vitamins (% retained)
Oats
Maize
Maize+peas
Thiamin Riboflavin Niacin Pyridoxine
23 100 100 35
44 86 75 100
61 70 60 18
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pyridoxine in maize. Retention was poorest for thiamin and for pyridoxine in the oats and maize+pea extrusions. The retention of vitamins in each cereal during the extrusion process did not appear to be related to initial levels of the vitamins; for example, the maize+pea mixture had the lowest retention of niacin, yet the highest absolute level of niacin before extrusion. Differences between the oats, maize and maize+pea ingredients are interesting because of the large differences in the SME applied to these two groups. Surprisingly, the low SME applied to the oats during extrusion resulted in very low retention of thiamin and pyridoxine compared to maize alone. A similar low value was obtained for the maize+pea mixture also. This suggests that thiamin and pyridoxine in oats and pyridoxine in peas may be more labile during extrusion than in maize. One of the duplicate pre-extrusion samples used to determine the retention of thiamin after extrusion for the maize sample had a value much higher than similar values obtained from experiments 2 and 3. Ignoring this value would return a thiamin retention value of 50%, much closer to the 62–79% obtained in experiments 2 and 3. 3.2. Experiment 2: effect of extrusion temperature Maize grits did not expand fully during extrusion at temperatures below 130 1C and some grits remained intact in the extruded product, whereas the extrudates were overcooked and unacceptably brown in colour at barrel temperatures greater than 160 1C. Hence, the experiments were carried out at temperatures of 130, 140, 150, and 160 1C. This temperature range had little differential effect on the retention of any of the vitamins. Thiamin and pyridoxine, reported earlier to be the most thermosensitive, decreased during extrusion along with niacin. However, they decreased by the same amount at all temperatures. In contrast, other studies (Beetner et al., 1974, 1976; Harper, 1979; Camire et al., 1990; Killeit, 1994) have shown that the percentage of thiamine retained decreased from 40% to 60% to between 50% and 20% as temperatures increased from 140 to 190 1C. These data represent a huge range in variation and suggest that variables other than temperature may play a role. One study (Bjo¨rck and Asp, 1983) found that an increase in barrel temperature increases riboflavin retention possibly due to a decrease in shear resulting from decreases in melt viscosity at higher temperatures. In contrast with the study, other researchers did not use a low residence time extruder. Killeit (1994) noted that increasing throughput, and therefore decreasing residence times, improved the retention of the B vitamins. Our work supports this and suggests that over the useful temperature operating range of this extruder,
Table 3 Vitamin retention for B group vitamins at different extrusion temperatures Vitamins (% retained)
130 1C
140 1C
150 1C
160 1C
Thiamin Riboflavin Niacin Pyridoxine
62 100 83 86
62 100 69 86
67 67 64 67
62 100 73 100
Table 4 Vitamin retention of B group vitamins in extruded maize with additives of maize germ material and/or amylose starch Vitamins (% retained)
100% maize
90% maize, 10% germ
80% maize, 20% amylose
70% maize, 20% amylose, 10% germ
Thiamin Riboflavin Niacin Pyridoxine
79 100 90 78
86 100 86 74
83 100 100 74
70 82 94 92
the loss of B vitamins was independent of temperature (Table 3). 3.3. Experiment 3: vitamin retention in maize with different additives The addition of maize germ material and amylose starch to maize, either separately or together, did not affect vitamin retention in the extrudates nor did the addition of the germ material increase the levels of thiamin in the extrudate. In effect, the results for the additives experiment were similar to those of the temperature experiment and the results for maize only in experiment 1 (Table 4).
4. Conclusion This work showed that for low retention times in a short barrel extruder, the retention of B group vitamins was unaffected by temperature and other ingredients. This provides evidence that the short barrel extruders used for snack food production retain a reasonably high level of the B group vitamins. Compared with other published work that used long barrel extruders with high retention times and found only 20% retention of the heat labile thiamin in maize extrudates, thiamin retention in the maize extrudates in this work was not less than 62% for experiments 2 and 3 and 44% in experiment 1. Pyridoxine is also regarded as heat labile. However, there was little evidence of this for maize grits (Experiments 1 and 2).
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Overall, trends for the stability of the vitamins were similar, with riboflavin and niacin having the highest stability. Pyridoxine was stable in maize but less so in oats and the maize+pea ingredient. Overall, thiamin showed the greatest reductions during extrusion. It was not evident what the mechanism for reduction of vitamins was. Clearly, heat labiality was at least in part related to the grain variety used. Equally it is clear that the extrusion process had an effect. However, increasing the temperature of the extrudate did not increase the loss of vitamins. It is, therefore, suggested that there may be initial loss of a less stable fraction and that the vitamin component retained may be a more stable or protected form.
Recommendation Based on the indicative results of these exploratory experiments, we plan to re-run similar experiments with more replicates. References Beetner, G., Tsao, T., Frey, A., Harper, J., 1974. Degradation of thiamin and riboflavin during extrusion processing. Journal of Food Science 39, 207–208. Beetner, G., Tsao, T., Frey, A., Lorenz, K., 1976. Stability of thiamin and riboflavin during extrusion processing of triticale. Journal of Milk and Food Technology 39, 244–245. Bergstro¨m, L., 1998. Nutrient Losses and Gains in the Preparation of Food. Livsmedelsverket, National Food Administration, Sweden.
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Bitsch, R., Moller, J., 1989. Analysis of PYRIDOXINE vitamers in foods using a modified high-performance liquid chromatographic method. Journal of Chromatography 463 (1), 207–221. Bjo¨rck, I., Asp, N.-G., 1983. The effects of extrusion-cooking on nutritional value. A literature review. Journal of Food Engineering 2, 281–308. Camire, M.E., Camire, A., Krumhar, K., 1990. Chemical and nutritional changes in foods during extrusion. Critical Reviews in Food Science and Nutrition 29, 35–57. Cheftel, J.C., 1986. Nutritional effects of extrusion cooking. Food Chemistry 20, 263–283. Egberg, D.C., Potter, R.H., 1975. An improved automated determination of riboflavine in food products. Journal of Agricultural and Food Chemistry 23, 815–820. Gehring, T.A., Cooper, W.M., Holder, C.L., Thomson, H.C., 1995. Liquid chromatographic determination of thiamin in rodent feed by postcolumn derivatization and fluorescence detection. Journal of AOAC International 78, 307–309. Harper, J.M., 1979. Food extrusion. Critical Reviews in Food Science and Nutrition 11, 155. Harper, J.M., 1981. Extrusion of foods, vols. 1 and 2. CRC Press, Boca Raton, FL. Jowitt, R. (Ed.), 1984. Extrusion-cooking Technology. Elsevier Applied Science Publishers, London. Killeit, U., 1994. Vitamin retention in extrusion cooking. Food Chemsitry, 149–155. Linko, P., Colonna, P., Mercier, C., 1981. High temperature, short time extrusion-cooking. Advances in Cereal Science and Technology 4, 145–235. Reitzer-Bergaentzle, A., Marchioni, M., 1993. HPLC determination of vitamin PYRIDOXINE in foods. Food Chemistry 48, 321–324. Woollard, D.C., 1984. New ion-pair reagent for the HPLC separation of B-group vitamins in pharmaceuticals. Journal of Chromatography 301, 470–476. Zeuthin, P., Cheftel, J.C., Eriksson, C., Jul, M., Leniger, H., Linko, P., Varela, G., Vos, G. (Eds.), 1984. Thermal processing and quality of foods. Elsevier Applied Science Publishers, London.
Short Communication
Vitamin retention in extruded food products Nelofar Athar, Allan Hardacre, Grant Taylor, Suzanne Clark, Rebecca Harding1, Jason McLaughlin New Zealand Institute for Crop and Food Research Limited, Private Bag 11600, Palmerston North, New Zealand Received 28 May 2004; received in revised form 26 July 2004; accepted 2 March 2005
Abstract Crisp extruded snack food like products were produced from a range of cereal products using a short barrel, single screw snack food extruder. The retention of B group vitamins during extrusion processing was compared for the different cereal grains and under different extrusion conditions. This work showed that short barrel extruders used for snack food production retain between 44% and 62% of the B group vitamins. This is considerably higher than the 20% retention for maize reported previously for long barrel extruders. The stability of the vitamins was similar, with riboflavin and niacin having the highest stability. Pyridoxine was stable in maize, but less so in oats and the maize+pea ingredients. Thiamin was the least stable during extrusion. It is concluded that short term high-temperature cooking of extruded snacks allows the retention of higher levels of heat labile B vitamins than the longer time and lower temperature cooking methods used in modern snack food extruders. r 2006 Published by Elsevier Inc. Keywords: Extrusion; Nutrient retention
1. Introduction Extrusion cooking is a relatively modern, hightemperature, short-time processing technology that was invented in 1940s to manufacture snack foods. This technique has gained ground in human food and animal feed industries world-wide, primarily for the processing of cereal grains. The best known products are low density foamed corn and rice breakfast and snack foods that are widely available. Similar products made from legumes are available in Asian countries. Extrusion cooking, particularly in the snack food industry, is a complex process that differs from conventional processing by using high shear rates and high temperatures (4150 1C) for very short periods Corresponding author. Tel.: +64 6 351 7066/6146; fax: +64 6 351 7050. E-mail address: [email protected] (N. Athar). 1 Present address: Fonterra Cooperative Group, Private Bag 11029, Palmerston North, New Zealand.
0889-1575/$ - see front matter r 2006 Published by Elsevier Inc. doi:10.1016/j.jfca.2005.03.004
(seconds). A wide range of thermo-mechanical and thermo-chemical processes are involved, including shear, Maillard reactions, protein denaturation and hydrolysis. These processes result in the physical, chemical and nutritional modification of food constituents (Harper, 1981; Linko et al., 1981; Jowitt, 1984; Zeuthin et al., 1984). Cereal grain products are among the most important sources of B group vitamins in the Western diet, and for this reason there is considerable interest in retaining the nutritional benefits of cereals during processing. Extrusion processing is increasingly used to process the ingredients in muesli bars, breakfast cereals and snack foods, the source of a significant proportion of the cereal grain, and hence B group vitamins, in Western diets (Cheftel, 1986). Several studies have assessed the effects of extrusion cooking on the retention of B group vitamins (Cheftel, 1986; Camire et al., 1990; Killeit, 1994). During the extrusion of crispbread products (Cheftel, 1986) at a specific mechanical energy (SME) from 0.09 to 0.13 kWh/kg, and retention times of 0.5 to
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1 min at 178 1C, the levels of B group vitamins decreased. From 38% to 65% of thiamine remained, 85% of riboflavin, 80% of niacin and 71–83% of pyridoxine. Thiamine and pyridoxine were the most thermo-labile and levels decreased linearly with temperature. Camire et al. (1990) obtained similar results, with thiamin losses increasing with barrel temperature, and with shear and temperature as screw speed was increased. There was some evidence that thiamine was more sensitive to heat than riboflavin and that riboflavin was more sensitive to shear. Many of the studies reported using twin-screw extruders for their ability to operate over a wide range of conditions. However, many of the most widely consumed expanded snack foods are processed in short barrel, low residence time extruders, similar to the machine used in this work. In this paper, the retention of B group vitamins during extrusion processing of whole oat grain and maize and pea grits is compared for a short barrel, low residence time extruder at different temperatures. This information is useful for estimating the loss of important vitamins during the manufacture of extruded snack products from a diverse range of ingredients.
2. Material and methods 2.1. Cereal ingredients Commercially available, specification 220 maize (corn) grits were purchased from Corson Grain Ltd (Box 1046, Gisborne, NZ). The maize grits are degermed before manufacture and for this reason contain about 75% starch, about 8% protein and less than 0.5% oil. Dried green peas (variety Rex, Crop and Food Research) were coarsely roller milled and sieved in our laboratory to produce a pea grit product of similar size to the spec. 220 maize. The pea grits contained approximately 43% starch, about 22% protein and 1.3% oil. An unmilled naked oat variety (CRO 59) was used as the whole grain. Whole oats contain approximately 62% starch, about 11% protein and 4% oil. They therefore contain far more oil than the other grain products, and this is likely to alter their processing properties. Because pea grits did not extrude well in the equipment available due to their high protein content, they were blended 50/50 with maize grits. In all cases the ingredients contained about 12% moisture as used. 2.2. Extrusion processing The food products used in this study were made on a Dorsey, single-screw, short barrel (90 mm) snack food extruder. The extruder was fitted with a 4-start screw and a 2-hole die with 5 mm apertures. The ingredients were fed into the extruder in the form of a dry granular
material at the rate of 75 g min1 using a mass flow feeding device. The extruder was operated at barrel temperatures between 130 and 160 1C. No water was added, but steam generated from the moisture in the hot starchy melt formed from the maize and maize+pea ingredients under high shear and temperature caused the extrudate to expand as it left the die to form a lowdensity starch-based foam. The plastic foam set rapidly as it left the die, forming a light, crispy, rigid material similar to many well known snack foods. During the extrusion process, moisture was lost from the grain products. Typically about 5% of the initial moisture is lost, and the extrusions contain about 7% moisture. The high oil content of the oats prevented the formation of a well-structured foam, and this particular extrudate was hard and dense. As the extrudate emerged from the die, it was cut into 20–30 mm long snack-like extrusions with a rotating knife. Energy usage during extrusion varied between the grits used in the experiment. For the maize and maize+pea grits, the current drawn by the motor driving the extruder screw was set from 12–14 A, corresponding to a power of from 7.1 to 8.2 kW, which in turn corresponded to a SME input of about 0.095 kWh kg1 of ingredients. The high oil content in the oats reduced this considerably to about 0.012 kWh kg1 of grain. The values for the maize and maize+pea ingredients were similar to those quoted by Cheftel (1986) but in comparison to the 0.5–1 min residence times used in that study, the short barrel extruder used in this work had a residence time of about 5 s. 2.3. B group vitamins 2.3.1. Vitamin analysis The samples of extrudate were ground to a homogenous state using a food processor. Analysis of four B group vitamins (thiamin, riboflavin, niacin and pyridoxine) was performed using HPLC by a commercial analytical laboratory (AgriQuality New Zealand). Thiamin analysis. A portion of the sample was acid autoclaved at 121 1C, followed by enzymatic digestion to release protein-bound vitamin and break any thiamin– phosphate bonds. The extract was then assayed by ionpair reversed phase HPLC with a buffered mobile phase (methanol–citrate, pH 2.4). The thiamin was oxidized to thiochrome by post-column reaction with hexacyanoferrate (III) and detected by fluorescence (Gehring et al., 1995). Riboflavin analysis. A portion of the sample was acid autoclaved at 121 1C, followed by enzymatic digestion to release protein-bound vitamin and break any riboflavinphosphate bonds. The extract was assayed by reversedphase, ion-pair HPLC techniques and detected by fluorescence detection (Egberg and Potter, 1975).
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Niacin analysis. A portion of the sample was heated with dilute hydrochloric acid to extract the vitamin and hydrolyze the niacinamide to niacin. The extract was cooled, made to a known volume and subjected to HPLC analysis, using reversed-phase, ion-pair techniques and UV detection (Woollard, 1984). Pyridoxine analysis. A portion of the samples was dephosphorylated by enzymatic hydrolysis for the analysis of pyridoxine. Pyridoxamine is transformed into Pyridoxal by reacting with glyoxylic acid in the presence of Fe2+, which is then reduced to pyridoxine by the action of sodium borohydride in alkaline medium. The extract was then assayed for pyridoxine by reversed-phase, ion-pair HPLC with fluorescence detection (Bitsch and Moller, 1989; Reitzer-Bergaentzle and Marchioni, 1993). 2.3.2. Vitamin retention Samples of the cereal ingredients were analyzed for vitamin content before and after extrusion. To allow for any changes in vitamin concentration, the retention factors for the products were calculated using the true retention method (Bergstro¨m, 1998). The equation used for calculating true retention is True retention ð%Þ ¼ ðVitamin content of extruded cereal g extrudateÞ= ðVitamin content of raw cereal g ingredientsÞ 100: 2.3.3. Experiments Three experiments were performed. In the first, the vitamin retention of extrusions made from maize grits, a 50/50 mixture of maize+pea grits, and whole oat were compared during extrusion at 152 1C. In the second experiment, the effect of barrel temperature on the retention of vitamins during the extrusion of maize grits was compared. For the third experiment, extrusion was carried out at a barrel temperature of 160 1C and maize germ material and/or amylose starch were added to maize grits. The germ material was added to increase natural levels of the B group vitamins. Amylose starch was added to increase the levels of slowly digestible fibre in possible snack products. Four formulations were used: 100% maize, 90% maize with 10% germ material, 80% maize with 20% amylose starch added, and finally 70% maize, 20% amylose, and 10% germ material. For each formulation and temperature, three replicates of each material were produced. However for cost reasons, the three replicates were pooled and two subsamples of each extrusion analysed for B group vitamins. Statistical analysis of the results was not therefore possible. However, similar materials were measured in each of the three experiments and this can be used to gauge the consistency of the data presented.
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3. Results and discussion Apart from the absolute values for vitamin concentrations presented for comparison in Table 1, these data are reported as percent retention. By comparing individual duplicates and the data for the maize grits across the three experiments it appears that the error variation in the values is about 710%. This is high but expected when low levels of vitamins are involved and may be conservative when compared with the 38–65% losses of thiamin reported by Cheftel (1986). 3.1. Experiment 1: extrusion of three cereals The levels of selected B group vitamins measured in the ingredients before extrusion are shown in Table 1. Extrusion resulted in only slight changes in protein, fat and carbohydrate levels and no changes in the levels of eight mineral elements (calcium, iron, magnesium, phosphorus, potassium, sodium, zinc and copper; data not shown). However, major changes were observed in the levels of important B group vitamins including thiamin and pyridoxine that are regarded as the most heat sensitive (Cheftel, 1986). Since retinol was not detected in any of three cereal samples prior to extrusion, it was decided not to analyse for retinol in post-extrusion samples. There was, of course, variation in the level of B group vitamins between the cereal ingredients with the maize grits having the lowest overall vitamin content and the maize+pea grits mixture having the highest levels. The retention of thiamin, riboflavin, niacin and pyridoxine varied depending on the cereal type (Table 2). Overall, the retention of riboflavin and niacin was highest, with the exception of 100% retention of
Table 1 Vitamin levels in the cereals before extrusion Vitamins (mg/100 g)
Oats
Maize
Maize+peas
Thiamin Riboflavin Niacin Pyridoxine
0.11 0.06 1.35 0.10
0.09 0.04 0.63 0.04
0.22 0.08 1.85 0.06
Table 2 Vitamin retention (% retained) for B group vitamins after extrusion Vitamins (% retained)
Oats
Maize
Maize+peas
Thiamin Riboflavin Niacin Pyridoxine
23 100 100 35
44 86 75 100
61 70 60 18
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pyridoxine in maize. Retention was poorest for thiamin and for pyridoxine in the oats and maize+pea extrusions. The retention of vitamins in each cereal during the extrusion process did not appear to be related to initial levels of the vitamins; for example, the maize+pea mixture had the lowest retention of niacin, yet the highest absolute level of niacin before extrusion. Differences between the oats, maize and maize+pea ingredients are interesting because of the large differences in the SME applied to these two groups. Surprisingly, the low SME applied to the oats during extrusion resulted in very low retention of thiamin and pyridoxine compared to maize alone. A similar low value was obtained for the maize+pea mixture also. This suggests that thiamin and pyridoxine in oats and pyridoxine in peas may be more labile during extrusion than in maize. One of the duplicate pre-extrusion samples used to determine the retention of thiamin after extrusion for the maize sample had a value much higher than similar values obtained from experiments 2 and 3. Ignoring this value would return a thiamin retention value of 50%, much closer to the 62–79% obtained in experiments 2 and 3. 3.2. Experiment 2: effect of extrusion temperature Maize grits did not expand fully during extrusion at temperatures below 130 1C and some grits remained intact in the extruded product, whereas the extrudates were overcooked and unacceptably brown in colour at barrel temperatures greater than 160 1C. Hence, the experiments were carried out at temperatures of 130, 140, 150, and 160 1C. This temperature range had little differential effect on the retention of any of the vitamins. Thiamin and pyridoxine, reported earlier to be the most thermosensitive, decreased during extrusion along with niacin. However, they decreased by the same amount at all temperatures. In contrast, other studies (Beetner et al., 1974, 1976; Harper, 1979; Camire et al., 1990; Killeit, 1994) have shown that the percentage of thiamine retained decreased from 40% to 60% to between 50% and 20% as temperatures increased from 140 to 190 1C. These data represent a huge range in variation and suggest that variables other than temperature may play a role. One study (Bjo¨rck and Asp, 1983) found that an increase in barrel temperature increases riboflavin retention possibly due to a decrease in shear resulting from decreases in melt viscosity at higher temperatures. In contrast with the study, other researchers did not use a low residence time extruder. Killeit (1994) noted that increasing throughput, and therefore decreasing residence times, improved the retention of the B vitamins. Our work supports this and suggests that over the useful temperature operating range of this extruder,
Table 3 Vitamin retention for B group vitamins at different extrusion temperatures Vitamins (% retained)
130 1C
140 1C
150 1C
160 1C
Thiamin Riboflavin Niacin Pyridoxine
62 100 83 86
62 100 69 86
67 67 64 67
62 100 73 100
Table 4 Vitamin retention of B group vitamins in extruded maize with additives of maize germ material and/or amylose starch Vitamins (% retained)
100% maize
90% maize, 10% germ
80% maize, 20% amylose
70% maize, 20% amylose, 10% germ
Thiamin Riboflavin Niacin Pyridoxine
79 100 90 78
86 100 86 74
83 100 100 74
70 82 94 92
the loss of B vitamins was independent of temperature (Table 3). 3.3. Experiment 3: vitamin retention in maize with different additives The addition of maize germ material and amylose starch to maize, either separately or together, did not affect vitamin retention in the extrudates nor did the addition of the germ material increase the levels of thiamin in the extrudate. In effect, the results for the additives experiment were similar to those of the temperature experiment and the results for maize only in experiment 1 (Table 4).
4. Conclusion This work showed that for low retention times in a short barrel extruder, the retention of B group vitamins was unaffected by temperature and other ingredients. This provides evidence that the short barrel extruders used for snack food production retain a reasonably high level of the B group vitamins. Compared with other published work that used long barrel extruders with high retention times and found only 20% retention of the heat labile thiamin in maize extrudates, thiamin retention in the maize extrudates in this work was not less than 62% for experiments 2 and 3 and 44% in experiment 1. Pyridoxine is also regarded as heat labile. However, there was little evidence of this for maize grits (Experiments 1 and 2).
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Overall, trends for the stability of the vitamins were similar, with riboflavin and niacin having the highest stability. Pyridoxine was stable in maize but less so in oats and the maize+pea ingredient. Overall, thiamin showed the greatest reductions during extrusion. It was not evident what the mechanism for reduction of vitamins was. Clearly, heat labiality was at least in part related to the grain variety used. Equally it is clear that the extrusion process had an effect. However, increasing the temperature of the extrudate did not increase the loss of vitamins. It is, therefore, suggested that there may be initial loss of a less stable fraction and that the vitamin component retained may be a more stable or protected form.
Recommendation Based on the indicative results of these exploratory experiments, we plan to re-run similar experiments with more replicates. References Beetner, G., Tsao, T., Frey, A., Harper, J., 1974. Degradation of thiamin and riboflavin during extrusion processing. Journal of Food Science 39, 207–208. Beetner, G., Tsao, T., Frey, A., Lorenz, K., 1976. Stability of thiamin and riboflavin during extrusion processing of triticale. Journal of Milk and Food Technology 39, 244–245. Bergstro¨m, L., 1998. Nutrient Losses and Gains in the Preparation of Food. Livsmedelsverket, National Food Administration, Sweden.
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Bitsch, R., Moller, J., 1989. Analysis of PYRIDOXINE vitamers in foods using a modified high-performance liquid chromatographic method. Journal of Chromatography 463 (1), 207–221. Bjo¨rck, I., Asp, N.-G., 1983. The effects of extrusion-cooking on nutritional value. A literature review. Journal of Food Engineering 2, 281–308. Camire, M.E., Camire, A., Krumhar, K., 1990. Chemical and nutritional changes in foods during extrusion. Critical Reviews in Food Science and Nutrition 29, 35–57. Cheftel, J.C., 1986. Nutritional effects of extrusion cooking. Food Chemistry 20, 263–283. Egberg, D.C., Potter, R.H., 1975. An improved automated determination of riboflavine in food products. Journal of Agricultural and Food Chemistry 23, 815–820. Gehring, T.A., Cooper, W.M., Holder, C.L., Thomson, H.C., 1995. Liquid chromatographic determination of thiamin in rodent feed by postcolumn derivatization and fluorescence detection. Journal of AOAC International 78, 307–309. Harper, J.M., 1979. Food extrusion. Critical Reviews in Food Science and Nutrition 11, 155. Harper, J.M., 1981. Extrusion of foods, vols. 1 and 2. CRC Press, Boca Raton, FL. Jowitt, R. (Ed.), 1984. Extrusion-cooking Technology. Elsevier Applied Science Publishers, London. Killeit, U., 1994. Vitamin retention in extrusion cooking. Food Chemsitry, 149–155. Linko, P., Colonna, P., Mercier, C., 1981. High temperature, short time extrusion-cooking. Advances in Cereal Science and Technology 4, 145–235. Reitzer-Bergaentzle, A., Marchioni, M., 1993. HPLC determination of vitamin PYRIDOXINE in foods. Food Chemistry 48, 321–324. Woollard, D.C., 1984. New ion-pair reagent for the HPLC separation of B-group vitamins in pharmaceuticals. Journal of Chromatography 301, 470–476. Zeuthin, P., Cheftel, J.C., Eriksson, C., Jul, M., Leniger, H., Linko, P., Varela, G., Vos, G. (Eds.), 1984. Thermal processing and quality of foods. Elsevier Applied Science Publishers, London.