CHAPTER 2 LITERATURE REVIEW

Transcription

1 CHAPTER 2 LITERATURE REVIEW 2.1. Breadmaking Bread is one of the oldest staple foodstuffs, which is made and eaten in most countries around the world. Bread products have evolved into many different forms, each based on distinctive characteristics, such as sourdough, tortillas, baguettes, brioche, pitas, etc. Owing to its high levels of gluten (which give the dough sponginess and elasticity), common wheat (also known as bread wheat, Triticum aestivum) is the most common grain used for the preparation of bread. However, bread is also made from the flour of other wheat species including durum (Triticum durum), spelt (Triticum spelta) and emmer (Triticum dicoccon), as well as rye (Secale cereale), barley (Hordeum vulgare), maize (Zea mays), and oats (Avena sativa), usually - but not always - in combination with wheat flour. Although common wheat is best suited for making highly-risen white bread, other wheat species are capable of giving a good crumb. Spelt bread (Dinkelbrot) continues to be widely consumed in Germany, and emmer bread was a staple food in ancient Egypt. The character of bread depends heavily on the formation of gluten network which traps gas from yeast fermentation and makes a direct contribution to the formation of a cellular crumb structure which, after baking, confers texture and eating qualities 6

2 quite different from other baked products. There are a few basic steps that form the basis of all breadmaking which can be listed as follows (Cauvain, 2003): The mixing of wheat flour and water, together with yeast and salt, and other specified ingredients in appropriate ratios. The development of gluten structure in the dough through the application of energy during mixing. The incorporation of air bubbles within the dough during mixing. The continued development of the gluten structure created in order to modify the rheological properties of the dough and to improve its ability to expand when gas pressures increase during fermentation. The creation and modification of particular flavour compounds in the dough. The subdivision of the dough mass into unit pieces. A preliminary modification of the shape of the divided piece. A short delay in processing to further modify physical and rheological properties of the dough pieces. The shaping of the dough pieces to their required shape. The fermentation and expansion of the shaped dough pieces during proof. Further expansion of the dough pieces and fixation of the final bread structure during baking. Cooling and storage of the final product before consumption. For a long time, it has been believed that there is no substitute for fresh baked bread. With lapse of time, a crispy crust of a fresh baked product develops a moist and 7

3 leathery texture while the soft crumb becomes firm and dry. The fresh flavour is also lost within hours of baking, which has made conventional bread productions to occur after midnight or early morning in order to provide fresh bread to the customers. Transportation of baked products from large automated bakeries to retail stores has also posed problems (Inoue and Bushuk, 1991). Mixed and moulded frozen dough that could be quickly transformed into fresh baked product was suggested as a solution to the existing problem Frozen Dough Development At the beginning of the twentieth century, experiments were conducted in which dough was made and processed, cooled and kept in a refrigerator overnight, and the dough pieces were warmed in the early morning and then baked (Sluimer, 2005b). By cooling dough below 0 C, the activity of yeast, and thus the fermentation process, was brought practically to a standstill. Around 1930, the first commercial method of dough retarding was developed in France. The dough was made and processed in the daytime, given final proof at 10 C so that the activity of the yeast slows down in order to extend the final proof time to about 10 h, and then baked in the early morning directly from the proofer without a warming step. Retarder-proofers were developed in 1970, where in these cabinets dough pieces were cooled in the evening according to a preset program, kept cooled during the night, and reheated early in the morning. After a certain proof time, the dough pieces were baked. 8

4 The bakers at that time, however, feared that dough pieces would freeze. Such fear is not surprising, considering all the physical changes in the structure of the dough that result from freezing. Part of the moisture freezes, leaving higher concentration of solutes in the rest of the moisture. Together with the proteins, this moisture with higher solute concentration forms a glassy state. The ice crystals result in an extra type of surface in the already complicated disperse system of the dough. Overtime, the bakery industry discovered that frozen dough is in many aspects more stable than chilled dough and thus yields better bread (Sluimer, 2005b). It has increasingly exploited the applications of freezing technology and numerous papers have been published on the frozen storage of dough and the influence of such processes on the quality of the final product. The growing interest of the market toward frozen bakery goods has been driven mainly by the economic advantage of a centralized manufacturing and distribution process as well as the standardization of product quality. These products do not demand specialized workers and have the possibility to make fresh bread available at any time of the day (Matuda et al., 2005). The freezing of dough, however, does have its disadvantages compared to bread made from non-frozen dough. Some of the disadvantages are its variable performance, increased expenses and loss of stability during transportation (Berglund, 1988). The next section will discuss in details the unfavourable effects of sub-zero storage in terms of dough and bread quality. 9

5 2.2. Effects of Frozen Storage on Bread Dough and Bread Quality Several problems arising from the production of bread made from frozen dough have been described. These include: gradual loss of the dough strength; decrease in the retention capacity of CO 2 and longer fermentation time; reduced yeast activity; lowering of loaf volume; and deterioration in the texture of the final product Dough Strength Dough that has been subjected to frozen storage has a reduced dough strength, which in turn causes a decrease in the loaf volume. Such loss of dough strength has been attributed to various factors, such as the release of reducing substances from yeast. The reduction of gluten cross-linking caused by ice recrystallization and the water redistribution provoked by a modification in the water binding capacity of dough constituents may also contribute to the loss of dough strength. Kline and Sugihara (1968) and Hsu et al. (1979) suggested that dough weakening can be attributed to the release of reducing substances, such as glutathione from yeast during freezing. Glutathione weakens the dough by cleaving disulfide bonds in the gluten proteins, an important factor in determining the rheology of gluten. Conversely, other workers (Varriano-Marston et al., 1980; Wolt and D Appolonia, 1984; Autio and Sinda, 1992) have suggested that the structural changes in freeze- 10

6 thawed dough are not associated with the release of reducing substances from yeast cells, but with a lack of gluten cross-linking. In particular, Varriano-Marston et al. (1980) hypothesized that ice crystallization could contribute to the weakening of such gluten protein network, thereby weakening the dough and increasing the proofing time. It was acknowledged that the longer proofing time was also due to the destruction of yeast during freezing, resulting in decreased gas production. They hypothesized further that the structural components of the dough, in particular protein, must also be drastically altered by the recrystallization process Dough Structure Berglund et al. (1991) observed that the formation of ice crystals in nonfermented dough stored for 24 weeks led to a disruption of the gluten matrix rendering a network that was less continuous, more ruptured and separated from starch granules. A less uniform gluten matrix would retain gas poorly and hence these structural features might help explain the decreased loaf volume and increased proof time of frozen dough. The starch granules were also damaged by the formation of large ice masses that were formed during recrystallization, further contributing to the decreased ability of the gluten to retain gas during proofing. This was in line with the findings of Gelinas et al. (1995), which suggested that ice crystallization particularly affected proteins, lowering the gas retention properties of frozen dough. 11

7 Damaged starch causes a linear increase in the water absorption capacity of flour (Tipples, 1969) and with more damaged starch in the dough, it is possible that water is drawn away from the gluten matrix by the starch granules. This was further supported by the findings of Lu and Grant (1999) which showed that the amount of freezable water (fraction of free water that does not bind to gluten during dough formation) in frozen dough increased with storage time in frozen conditions. Furthermore, the increase in the amount of freezable water was higher at storage temperature of -15 C and lower at -25 C (Bot, 2003). Together, the results clearly indicate that there is a redistribution of the total water present in the system during frozen storage. The damage to the dough structure caused by frozen storage can be illustrated through the use of low-temperature scanning electron microscopy (SEM) (Zounis et al., 2002). Dough can be taken as foams in which gas bubbles are entrapped in the starch/gluten matrix. In the electron micrographs of frozen dough, these bubbles are shown as spherical voids. However, the presence of ice crystals formed during freezing, represented by angular voids in the electron micrographs, can disrupt the foam structure. An electron micrograph of an unfrozen dough piece that was examined immediately after mixing revealed a structure with no ice crystals. It was a very dense structure with few spherical voids and with the spherical starch granules firmly embedded in the gluten matrix. In comparison, dough mixed and stored for 1 day at -20 C had a porous structure with more uniformly-sized spherical and angular voids created by yeast fermentation during the rest period before freezing, and ice 12

8 crystals formed during blast freezing and storage. The gluten network in the 1 day sample was more stretched than the control and was continuous with starch granules firmly attached to the visible gluten strands, an observation also made by Berglund et al. (1991) in dough frozen for 24 hours. When stored for 10 weeks at -20 C, the size of the voids increased and became less uniform, with some very large voids observed. The angular voids might represent the formation of ice crystals. The long gluten strands visible in the 1 day sample were also absent in the 10 weeks sample. Further disruption was observed when the dough was stored for 27 weeks at -20 C, with a greater number of large angular voids present and the detachment of starch granules from the gluten. Such results proved that the storage of dough at freezing temperature for several weeks resulted in structural damage caused by water migration and ice crystal growth. There is some evidence from SDS gel electrophoresis measurements that the weakening of the protein structure is brought about by denaturation of the glutenin proteins. Analysis of the protein structure of frozen thawed dough shows a considerable increase in the number of lower molecular weight oligomers which are presumably manifest from the depolymerisation of glutenin. The changes are particularly noticeable after several freeze thaw cycles (Kennedy, 2000). Ribotta et al. (2001) studied the effects of freezing and frozen storage on the aggregative behaviour of glutenins. Glutenin subunits, but not gliadins, are able to establish intra- and intermolecular disulfide bonds. This difference allows the 13

9 formation of a glutenin macropolymer (GMP) which plays a special role in the maintenance of gluten structure. Utilizing electrophoresis analysis of the SDS-soluble protein aggregates extracted from frozen dough, they found that there was a decrease in the amount of glutenin subunits of high molecular mass. This result suggested that the protein matrix of dough underwent depolymerization during storage at -18 C. Furthermore, the depolymerization was found to be enhanced during the period in which the dough was kept in frozen conditions. Water redistribution, ice recrystallization, and an increase in the amount of freezable water may affect gluten structure and may be one of the reasons for the depolymerization of glutenin aggregates of high molecular mass. These phenomena may cause a loss in the gas retention capacity during fermentation, reflected by lower bread volume and an increase in the fermentation time. In comparison, Sharadanant and Khan (2006) utilized SEM and electrophoresis studies to determine the effect of gums on starch and protein characteristics of frozen doughs supplemented with three levels of gum arabic, carboxy methyl cellulose (CMC), κ-carrageenan, and locust bean gum after day 1 and after 4, 8, 12, and 16 weeks of frozen storage. Similar to the findings reported by Zounis et al. (2002), electron micrographs of unfrozen dough showed starch granules securely embedded in the gluten matrix. After 8 and 16 weeks of frozen storage, however, the frozen control dough without the gum additives clearly showed damage to the gluten network, and the starch granules appeared to be separated from the gluten. The addition of locust bean gum and gum arabic produced dough with better capability to 14

10 retain the gluten network compared with the frozen control evaluated after different periods of storage. The SDS-soluble protein content increased while residue protein content decreased as the frozen storage time increased. This observation supports the theory of depolymerization of the larger molecular weight glutenin polymers into lower molecular weight polymers also reported by Ribotta et al. (2001). After each frozen storage period, the control dough without the gum additive had the highest amount of SDS-soluble proteins, while dough with κ-carrageenan and locust bean gum had the lowest amount. The control dough had the lowest amount of residue proteins when compared with the dough treated with gums, while κ-carrageenan treated dough had the highest amount of residue proteins, followed by doughs with locust bean gum, gum arabic and CMC Yeast Survival and Gassing Power In frozen dough manufacturing, yeast survival and gas retention are major problems (Hino et al., 1987). It has been shown that freezing and frozen storage of dough caused significant losses in the number of viable cells, with about half of the original cells being rendered unviable after 90 days of frozen storage (Ribotta et al., 2003a). These results are in agreement with Lorenz and Kulp (1995), which suggested that freezing yeast in a dough system increased the susceptibility to cell damage compared with direct freezing of yeast because the yeast in the dough system was under osmotic pressure. The yeast cells were also in a state of active fermentation, with their plasma membrane thinner than when they were in dormant state, making it more susceptible 15

11 to damage. Organic compounds are also concentrated by freezing of the aqueous phase, possibly causing autolysis of yeast cells (Stauffer, 1993). During dough fermentation, yeast produces CO 2 and flavour compounds. The gasforming ability of yeast depends on the strain, the number of yeast cells, the cell activity, and the amount of fermentable sugars. In wheat flour, the 1% fermentable sugars are insufficient for yeast and need to be enhanced by the additional sugars produced from starch by α- and β-amylase. The gassing power of yeast is also affected by frozen storage. In particular, it is affected by the freezing and thawing rate, frozen storage temperature and duration, and freeze-thaw cycles. Fast freezing reduces the gassing power (Autio and Sinda, 1992; El-Hady et al., 1996) and the number of viable yeast cells (Lorenz, 1974). A higher freezing rate also results in a much higher sensitivity to storage duration than slow freezing rate and the maximum yeast activity should be obtained with a slow freezing rate of C/min (Le Bail et al., 1996) Bread Quality Breadmaking properties such as bread height and specific volume are also strongly influenced by the amount of liquid that was released from the frozen dough during thawing. Seguchi et al. (2003) separated the liquid from thawed bread dough by centrifugation and found that the amount of liquid oozed from the dough was increased by freezing-and-thawing cycle and there was a strong inverse correlation 16

12 between the amount of centrifuged liquid and breadmaking properties. The deterioration of the breadmaking properties of frozen-and-thawed bread dough compared with nonfrozen dough may be due to the decrease in the water binding ability of the dough caused by freezing and the subsequent thawing. The temperature of frozen storage of dough also affects the deterioration of bread quality. As mentioned previously, a possible explanation for the quality loss involves the changes in dough rheology as a result of water transport during storage from the hydrated gluten to the ice phase (Bot and de Bruijne, 2003). During baking, the gluten does not rehydrate and excess water may migrate to the starch paste, thus affecting the yield stress of the starch paste and compromising the baking performance of the dough. This is especially true at temperatures not far below the glass transition temperature (T g ) of the dough. Glass transition is a time-dependent change in physical state from a glassy solid to a rubbery viscous liquid and it occurs over a temperature range (Laaksonen and Roos, 2000). If a frozen dough is stored well below T g, it is expected to be relatively stable over storage time (Slade et al., 1989). Unfortunately, T g is usually not very much higher than the commercially relevant freezer temperature of -18 C (Bot, 2003). Räsänen et al. (1998) measured the subzero properties of frozen dough by dynamic mechanical thermal analysis (DMTA) and showed that the glass transition temperatures measured by DMTA moved to higher temperatures during frozen storage when the optimal water content of dough was used and a reduction in the 17

13 water content eliminated this phenomenon. Frozen storage increased the liquid phase in dough with optimal water content, determined by the Farinograph absorption at 500 BU. The growth of ice crystals during frozen storage resulted in the concentration of polymers and a higher glass transition temperature observed by DMTA. The increase of liquid phase during storage was substantially lower when the water content of dough was decreased and therefore ice crystals growth was minimized Bread Staling Behaviour Bread made from frozen dough exhibited detrimental effects on its properties and texture upon aging. The staling of bakery products typically involves an increase in crumb firmness, loss of flavour and aroma, and loss of crispiness (Cauvain, 1998). Moisture loss and starch retrogradation are two of the basic mechanisms responsible for the firming of bread crumb, since starch is the major constituent in bread crumb. Zobel and Kulp (1996) suggested that bread firming was caused by recrystallization of the starch fraction involving amylopectin chains. However, Martin et al. (1991) suggested that the main reason for bread firming was the formation of hydrogen bonds between gluten and starch granules. Bread dough is rubbery and water is free and available to act as a plasticizer, but during baking part of the water is lost and the rest is linked to the biopolymers present in the system. Water mobility progressively decreases but water molecules are still able to diffuse through bread crumb to the hydrophilic sites like glucose or hydroxyl groups available to form hydrogen bonds. During staling, these hydrogen bonds favour the formation of other bridges between 18

14 chains through the displacement of the intermediate water molecules, which diffuse toward the neighbouring sites, resulting in redistribution of water. Therefore, water mobility contributes to the amylopectin recrystallization and the formation of hydrogen bonds between gluten and starch, which are responsible for bread staling (Davidou et al., 1996). The redistribution of water and ice recrystallization in dough during frozen storage can induce changes in the structure and arrangement of amylose and amylopectin and such changes will be reflected during starch gelatinization and retrogradation. The longer the time the dough remained in frozen conditions, the more pronounced the degree of retrogradation. Bread made from frozen dough also exhibits faster retrogradation of starch in low temperature (4 C) storage in comparison to bread made from nonfrozen dough, causing an increase in bread firmness (Ribotta et al., 2001; Ribotta et al., 2003b). Table 2.1. summarizes the effects of frozen storage on the properties of dough and the quality of bread. Table 2.1. Effects of frozen storage on dough properties and bread quality Property Effects of frozen storage References Ingredients used (Flour basis) Dough strength Loss of dough strength due to: Release of reducing substances Kline and Sugihara Not reported. 19

15 from yeast (1968) Hsu et al. (1979) Not reported. Reduction of gluten cross- Varriano-Marston Not reported. linking caused by ice et al. (1980) crystallization Wolt and D Appolonia Not reported. (1984) Autio and Sinda 1.78% pressed yeast, (1992) 0.17% and 0.33% cold yeast homogenate, 0.05% and 0.1% glutathione, 1.4% NaCl, 60.7% water. Dough Less uniform gluten matrix and Berglund et al. 4% yeast, 4% sugar, structure lower gas retention properties of (1991) 4% shortening, 1.5% dough due to ice crystals salt, 55% water. formation Gelinas et al. 0.9% yeast, 4% sugar, 20

16 (1995) 3% shortening, 2% salt, 100 ppm ascorbic acid, 60 ppm potassium bromate, 59% water. Sharadanant and Khan (2006) 5% yeast, 2.5% sugar, 1.5% shortening, 1% salt, 100 ppm ascorbic acid, water (to 500 BU consistency). Damaged starch causes a redistribution of the total water present in the dough system Tipples (1969) Lu and Grant (1999) Not reported. 5% compressed yeast, 4% shortening, 4% sugar, 1.5% salt, 4% ice water, 100 ppm ascorbic acid, 50 ppm potassium bromate. Bot (2003) 2% salt, 60% 21

17 demineralized water Decrease of high molecular mass Ribotta et al. 3% compressed yeast, glutenin subunits and protein (2001) 1.8% salt, 0.2% depolymerization sodium propionate, 0.015% ascorbic acid, 63% water. Sharadanant and 5% yeast, 2.5% sugar, Khan (2006) 1.5% shortening, 1% salt, 100 ppm ascorbic acid, water (to 500 BU consistency). Yeast Loss of viable yeast cells due to Stauffer (1993) Not reported. survival cell damage and gassing Lorenz and Kulp 4% compressed yeast, power (1995) 4% shortening, 2% sugar, 1.5% salt, water (to 500 BU consistency). 22

18 Ribotta et al. (2003a) 3% compressed yeast, 1.8% salt, 0.2% sodium propionate, 0.015% ascorbic acid, 63% water. Freezing rate dependence Lorenz (1974) Not reported. Autio and Sinda (1992) 1.78% pressed yeast, 0.17% and 0.33% cold yeast homogenate, 0.05% and 0.1% glutathione, 1.4% NaCl, 60.7% water. El-Hady et al. (1996) 4% compressed yeast, 2% fat, 1.5% salt, 1% sugar, 1% skim milk powder, water (to 500 BU consistency). Le Bail et al. 2% compressed yeast, 23

19 (1996) 2.2% salt, 58% water. Bread Increase in the amount of liquid Seguchi et al. 3% compressed yeast, quality oozed from frozen dough during (2003) 5% sugar, 1% salt, thawing, causing deterioration of water (to 500 BU the breadmaking properties consistency). Storage temperature dependence Slade et al. (1989) Not reported. Bread Faster retrogradation of starch, Ribotta et al. 3% compressed yeast, staling causing an increase in bread (2001) 1.8% salt, 0.2% behaviour firmness sodium propionate, 0.015% ascorbic acid, 63% water. Ribotta et al. 3% compressed yeast, (2003b) 1.8% salt, 0.2% sodium propionate, 0.015% ascorbic acid, 64% water 24

20 It can therefore be seen that to obtain a product from frozen dough with a quality comparable to freshly made bread is a complex problem since the final structure is modified by several parameters. In order to improve frozen dough, several techniques have been developed in recent years. These include (1) the isolation of freeze resistant yeasts (Van Dijk et al., 2000); (2) addition of improvers such as emulsifiers and water-binding agents, e.g. hydrocolloids to stabilize the dough network; (3) addition of wheat proteins to increase shelf life (Benjamin et al., 1989); (4) modification of dough composition (Wada and Tsukuda, 1997); (5) use of heat stable enzymes to shorten the fermentation (Larsen and Pedersen, 1996); (6) optimization of mixing, freezing and freeze-thaw cycles (Nemeth et al., 1996). In this research, several different types of dough additives are included in the frozen dough formulation to improve the final quality of the bread Dough Additives The inclusion of dough additives may assist in minimizing freezing damages in order to produce bread with similar quality as that made from non-frozen bread. The additives used in this research include propylene glycol alginate (PGA), sodium alginate (SA), various blends of PGA and SA, diacetyl tartaric acid esters of monoglycerides (DATEM), and sucrose ester (SE). No literature was found on the use of PGA and SA in frozen dough, hence they are considered novel additives in frozen dough application. DATEM and SE were used as reference to form the basis of comparisons in this research since they are commonly used as additives in frozen 25

21 dough. Other types of additives which are commonly used in frozen dough production will also be reviewed Propylene Glycol Alginate and Sodium Alginate Alginate was first described by the British chemist E.C.C. Stanford in 1881 and exists as the most abundant polysaccharide in the brown algae (Phaeophyceae) comprising to up to 40% of the dry matter. It is located in the intercellular matrix as a gel containing sodium, calcium, magnesium, strontium and barium ions (Haug, 1964). It is because of its ability to retain water, and its gelling, viscosifying and stabilizing properties, that alginate is widely used industrially. Ca, Mg and Sr-ALGINATE in algal particles Pre-extraction Neutralisation HCl ALGINIC ACID Wash, filtration Na 2 CO 3 or NaOH SODIUM ALGINATE Precipitation CaCl 2 Ca-Alginate HCl HCl Alginic acid Alginic acid Na 2 CO 3 Na 2 CO 3 SODIUM ALGINATE SODIUM ALGINATE Figure 2.1. Principal Scheme for the isolation of alginate from seaweeds (Phillips and Williams, 2000). 26

22 The extraction of alginate from algal material is schematically illustrated in Figure 2.1 above. Because alginate is insoluble within the algae with a counter-ion composition determined by the ion exchange equilibrium with seawater, the first step in alginate production is an ion-exchange with protons by extracting the milled algal tissue with M HCl. In the second step, the alginic acid is brought into solution by neutralization with alkali such as sodium carbonate or sodium hydroxide to form the water soluble sodium alginate. After extensive separation procedures such as sifting, floatation, centrifugation and filtration to remove algal particles, the soluble sodium alginate is precipitated directly by alcohol, calcium chloride or hydrochloric acid, converted to the sodium form if needed and finally dried and milled (Phillips and Williams, 2000). PGA is an ester of alginic acid in which some of the carboxyl groups are esterified with propylene glycol (11-45%), some neutralized with an appropriate alkali and some remain free (Joint FAO/WHO Expert Committee on Food Additives (JECFA), 1997). It is soluble in water giving a viscous, colloidal solution; soluble in up to 60% aqueous ethanol depending upon degree of esterification. When used as an additive in conventional, non-frozen dough formulation, PGA is believed to form a stable complex with starch which inhibits rupture of the dough when gelatinized. Also, PGA prevents retrogradation of the starch under various conditions of storage by inhibiting amylase from being freed or released from the starch granules (Phillips and Williams, 27

23 2000). Both SA and PGA are internationally recognized as food additives with generally recognized as safe (GRAS) status, with an International Numbering System (INS) number of 401 and 405 respectively. Basically, alginate is a linear co-polymer composed of two monomeric units, i.e. D- mannuronic acid and L-guluronic acid. These monomers occur in the alginate molecule as regions made up exclusively of one unit or the other, referred to as M blocks or G-blocks, or as regions in which the monomers approximate an alternating sequence. The structures of the individual monomers are shown in Figure 2.2. D-mannuronic acid L-guluronic acid Figure 2.2. The alginate monomers ( The D-mannuronic acid exists in the 4 C 1 conformation and in alginate polymer is connected in the β-configuration through the 1- and 4-positions. The L-guluronic acid has the 1 C 4 conformation and is α-1, 4- linked in the polymer. Because of the particular shapes of the monomers and their modes of linkage in the polymer, the geometries of the G-block regions, M-block regions, and alternating regions are substantially different. Specifically, the G-blocks are buckled while the M-blocks have a shape referred to as an extended ribbon, as shown in Figure

24 Figure 2.3. Block shapes in alginates (ISP Alginate Guide, 2003) Commercial alginates are derived from a variety of weed sources. Although the ratio of mannuronic acid to guluronic acid (M:G ratio) can be obtained relatively easily, the detailed molecular compositions of alginates in terms of block lengths and block distributions are much more difficult to determine. As a result, alginates are usually referred to as "high M" or "high G", depending on the proportions of mannuronic acid and guluronic acid they contain. Most commercial products are of the high M type, the best example being the alginate obtained from giant kelp, Macrocystis pyrifera. Laminaria hyperborea provides a high G alginate. In general terms, high G alginates produce strong, brittle gels that are heat stable, while high M alginates provide weaker, more elastic gels that have less heat stability but more freeze/thaw stability. 29

25 Diacetyl Tartaric Acid Esters of Monoglycerides DATEM is used in the baking industry to strengthen the dough by building a strong gluten network. The exact mechanism is not well understood, but DATEM appears to interact with the hydrophobic parts of the gluten, helping the proteins unfold and form cross-linked structures. DATEM consists of mixed glycerol esters of mono- and diacetyltartaric acid and fatty acids of food fats. It can be manufactured either by the interaction of diacetyltartaric anhydride and mono- and diglycerides of fatty acids in the presence of acetic acid, or by interaction of acetic anhydride and mono- and diglycerides of fatty acids in the presence of tartaric acid. Owing to inter- and intramolecular acyl group exchange, the two methods of production result in essentially the same components, the distribution of which depends on the relative proportions of the basic raw materials, on temperature, and on reaction time. The major components are a glycerol molecule with a stearic acid residue, a diacetyltartaric acid residue and a free secondary hydroxyl group (JECFA, 2002). Unlike other commercially used dough emulsifiers, DATEM does not form starch complexes. Its main function is as a softener. Typically the level of usage is between to 0.5% of the total flour weight in most commercial baking ( DATEM has an INS number of 472e and is recognized as a GRAS additive by the US FDA as specified in the Code of Federal Regulations 21CFR It is 30

26 dispersible in cold and hot water, soluble in methanol and ethanol (JECFA, 1997). The chemical structure of DATEM is shown below. Figure 2.4. DATEM structure ( ct%20range/emulsifiers/datem/datem_en.htm) Sucrose Esters Sucrose esters (SE) are mono-, di- and tri-esters of sucrose with food fatty acids, prepared from sucrose and methyl and ethyl esters of food fatty acids or by extraction from sucroglycerides (JECFA, 1997). SE is sparingly soluble in water and soluble in ethanol. SE has an INS number of 473 and is internationally recognized as food additives with GRAS status as specified in the Code of Federal Regulations 21CFR The chemical structure of SE is shown below. 31

27 Figure 2.5. Strucure of sucrose esters ( SE consists of a hydrophilic sugar head and one or more lipophilic fatty acid tails and are therefore classified as emulsifiers. The degree of esterification and the length of the fatty acid chain determine the Hydrophilic - Lipophilic Balance (HLB), i.e. whether the emulsifier is of a hydrophilic (high HLB) or a lipophilic nature (low HLB). Lower degree of esterification and shorter fatty acid chain will give higher HLB value, a characteristic desirable in bakery products. The addition of SE into dough formulation produces bread with fine and soft crumb structure, high volume, extended shelf life, increased dough mixing tolerance, and improved freeze-thaw stability (Barrett et al., 2002) Xanthan Gum Xanthan gum is an extracellular polysaccharide secreted by the bacterium Xanthomonas campestris. It consists of a linear (1 4)-linked β-d-glucose backbone with trisaccharide side chains on every other glucose at C(O)3, containing a glucuronic acid residue linked (1 4) to a terminal mannose unit and (1 2) to a second mannose that connects to the backbone (Sworn, 2000). The viscosity of 32

28 xanthan gum solutions is stabile over a wide range of ph and temperature conditions and the polysaccharide is resistant to enzymatic degradation. Xanthan gum induces cooking and cooling stability of wheat flour dough and improves the freeze-thaw stability of starch-thickened frozen foods (Sanderson, 1981). The addition of xanthan gum into a frozen dough formulation can strengthen the dough by forming a strong interaction with the flour proteins. It also increases water absorption and the ability of the dough to retain gas, increasing the specific volume of the final bread and the water activity of the crumb (Collar et al., 1999; Rosell et al., 2001). The increase in specific volume, as well as high porosity (open structure) and softer crust, however, are obtained only at low concentrations of xanthan gum (0.16% flour basis). Increased xanthan gum concentration resulted in a decrease in specific volume compared to that of the control samples (Mandala, 2005) Guar Gum Guar gum is a polysaccharide which consists of a chain of β-d-mannopyranosyl units joined by (1 4)-linkages. On average, every second residue carries a α-dgalactopyranosyl residue linked to the main chain by α (1 6) linkage (Belitz and Grosch, 1999). Guar gum solutions are highly viscous at low concentrations and useful in thickening, stabilization and water-binding applications. In bakery products, guar gum is used to improve mixing and recipe tolerance, to extend the shelf life of 33

29 products through moisture retention and to prevent syneresis in frozen foods and pie fillings (Maier et al., 1993). In frozen dough, however, the addition of guar gum was found to be disadvantageous. It yielded a product with less desirable properties compared with control samples as it lowered the specific volume and porosity of bread, and produced a rubbery crust with low crust thickness (Mandala, 2005). In contrast, Ribotta et al. (2001) found that the addition of guar gum in frozen dough produced bread with a higher volume, a more open crumb structure with higher percentage of gas cells than those prepared without it. This result was substantiated by Ribotta et al. (2004), who observed that guar gum improved the volume and texture of bread made from frozen dough, but the negative effect of frozen dough storage on the dynamic rheological parameters and microstructural damage was not avoided. Clearly, more research is needed to verify which of these contradictory results is true and to elucidate the mechanism responsible. Perhaps both explanations are true under particular conditions. A possible reason for the conflicting results may be the low level of guar gum (0.16% and 0.65%) used by Mandala (2005), which was less than half the amount incorporated into the frozen dough (1.5%) by Ribotta et al. (2001). Hence, any improving effect of the gum may have been insufficient to counteract the detrimental effects of the sub-zero temperature storage. 34

30 Hydroxypropylmethylcellulose In hydroxypropylmethylcellulose (HPMC), the etherification of hydroxyl groups of the cellulose by methoxyl and hydroxypropyl groups increases its water solubility and also confers some affinity for the non-polar phase in doughs. Hence, in a multiphase system like bread dough, this bifunctional behaviour allows the dough to retain its uniformity and to protect and maintain the emulsion stability during breadmaking. HPMC forms interfacial films at the boundaries of gas cells conferring some stability to the cells against gas expansion and other changes in processing condition (Bell, 1990). When the temperature rises during baking, HPMC forms gels by interacting with the hydrocolloid chains creating a temporary network (Sarkar and Walker, 1995). This imparts some strength to the dough during expansion and protects against volume loss. This gel also acts as a barrier against the moisture content decrease but such barrier property does not remain after cooling; therefore it provides better texture and softness without conferring any adverse effect on the palatability of the bread. Barcenas et al. (2004) examined the effects of HPMC as a bread improver on partially baked frozen dough (dough was baked at 165 C for 7 min prior to freezing and frozen storage) and found that the specific volume of the final bread containing HPMC was not significantly affected by the duration of frozen storage. HPMC was also able to produce final bread with higher moisture content than the control and to maintain almost constant moisture content throughout the 42 d frozen storage. This 35

31 result was supported by the findings of Collar et al. (1998) and Dziezak (1991), which suggested that HPMC had the ability to increase the water absorption and maintain the moisture content of the products containing it. It was also observed that HPMC increased the softness of breadcrumb made from part-baked frozen dough compared to the control and was not affected by the frozen storage time, thereby improving the bread texture. HPMC retarded staling of the final bread and the rate of crumb hardening was independent of the frozen storage time (Barcenas et al., 2004). Since HPMC is hydrophilic, it could bind available water in the system, decreasing the possibility of formation of complexes between the polymers present in the bread. In cointrast to proteins or starch polysaccharides, HPMC molecules do not aggregate at low temperatures. Haque et al. (1993) suggested that at low temperatures the HPMC chains do not associate because their hydrophobic substituents are surrounded by sheaths of structured water that inhibits intermolecular associations between the polymer chains. Thus the presence of HPMC does not lead to redistribution of water in the dough and as a consequence the formation of bridges between gluten and starch will not be favoured and in turn bread staling will be partially prevented. 36

32 κ-carrageenan κ-carrageenan is a sulphated polysaccharide extracted from certain red algae. Specifically, it is a high molecular weight linear polysaccharide comprising repeating galactose and 3,6-anhydrogalactose units, both sulphated and non-sulphated, joined by alternating (1 3)-α- and (1 4)-β- glycosidic links. κ-carrageenan contains approximately 25% ester sulfate and 34% 3,6-anhydrogalactose (Imeson, 2000). When used as a dough additve, κ-carrageenan has an ability to improve the specific volume of the bread due to its interactions with gluten proteins (Leon et al., 2000). Sharadanant and Khan (2006) showed that the presence κ-carrageenan in frozen doughs significantly lowered the amount of SDS-soluble proteins and increased the amount of residue proteins compared with control dough. However, in frozen dough applications, κ-carrageenan forms rigid gels that are not stable to freeze-thaw cycles. Hence, the specific volume of the final bread produced from frozen dough is decreased with increasing frozen storage period. Furthermore, κ-carrageenan molecules can form interactions with each other without competing with gluten proteins and starchy polysaccharides for the water available in the system. Leon et al. (2000) showed that in the presence of κ-carrageenan, the moisture content of the final bread was higher than that of the control, although the water activity was lower. These results agree with the ability of hydrocolloids to increase water absorption and maintain the moisture content of the product to which it is added, 37

33 while reducing the water activity due to competition for wtere with bread polymers such as proteins and starch. However, Barcenas et al. (2004) showed that a higher hardening rate of the final bread containing κ-carrageenan was obtained at longer frozen storage time when compared to that of the control or bread containing HPMC. Sharadanant and Khan (2003a) showed that 1 and 3 % κ-carrageenan decreased the amount of freezable water, increased the maximum resistance to extension, and produced a detrimental effect on frozen dough by increasing the proof time. Although its addition caused an increase in loaf volume, κ-carrageenan gave bread with an inferior appearance as indicated by the lower L color values of the chromameter and decreased the crumb quality (Sharadanant and Khan, 2003b). Hence, κ-carrageenan is not an appropriate improver for frozen dough as it promotes higher hardening rate and favours staling compared with dough without the additive, and does not improve the baking performance Other Hydrocolloids In recent years, there is increasing research on the use of other hydrophilic gums such as CMC, gum arabic and locust bean gum to improve the quality of frozen dough and the final baked product. 38

34 CMC is a cellulose derivative with carboxymethyl groups (-CH 2 -COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It dissolves rapidly in cold water and is mainly used for controlling viscosity without gelling. CMC solutions tend to be both highly viscous and stable, but the viscosity will drop during heating (BeMiller and Whistler, 1996). Gum arabic is an exudate of acacia trees. It is a heterogeneous material containing two fractions; 70% polysaccharide chains with little or no nitrogenous material and 30% protein structures. It has high solubility, low viscosity and high compatibility with high concentrations of sugar (BeMiller and Whistler, 1996). Gum arabic readily dissolves in water to give clear solutions ranging in colour from very pale yellow to orange-brown and with a ph of ~4.5. The highly branched structure of the gum gives rise to compact molecules with a relatively small hydrodynamic volume and as a consequence gum solutions become viscous only at high concentrations (Williams and Phillips, 2000). Locust bean gum is the ground endosperm of seeds with galactomannan as the main component. It has a high viscosity and is rarely used alone, but in combination with other gums such as CMC, carrageenan, xanthan and guar gum (BeMiller and Whistler, 1996). The structure of locust bean gum is reported to consist of a linear chain of β-d-mannopyranosyl units linked 1, 4 with single-membered α- Dgalactopyranosyl units occurring as side branches. The galactopyranosyl units are linked 1, 6 with the main chain (Braun and Rosen, 2000). 39

35 Sharadanant and Khan (2003a, 2003b) investigated the effects of various levels of hydrophilllic gums such as CMC, gum arabic and locust bean gum on the quality of dough frozen for up to 16 weeks and on the characteristics of bread. They reported a decrease in proof time and an increase in maximum R/E for the addition of 1 and 3% CMC and 1-3% locust bean gum compared with the control, suggesting the ability of the gums to improve the quality of frozen dough by reducing freeze-thaw damage. However, addition of 3% gum arabic gave values similar to the control while the addition of 2% CMC and 1 and 2% gum arabic gave values lower than the control. The addition of gums also increased the specific loaf volume of the bread significantly, with locust bean gum producing the highest loaf volume followed by gum arabic and CMC. External appearance of the bread and its internal characteristics such as texture, grain, cell wall structure, color and softness were also improved. Bread firmness was significantly reduced by the addition of locust bean gum, followed by gum arabic and CMC. Asghar et al. (2005) reported similar findings with CMC and gum arabic in dough frozen up to 8 weeks. Bread characteristics were analyzed after every 15 days for specific loaf volume, external and internal characteristics. Specific loaf volume increased significantly with the addition of different levels of gums compared with the frozen control. Although the external and internal characteristics of bread deteriorated with storage time, addition of gum arabic and CMC improved the characteristics of bread as compared to the control after each storage period. 40

36 Ascorbic Acid Oxidants are required to improve the structure and final loaf volume of bread as well as to increase the dough strength. Due to the death of the yeast cells during frozen storage, reducing substances (particularly glutathione) are formed, leading to a reduction in gluten strength as a result of weakened disulfide bridges that are essential in the stabilization of gluten network (Kline and Sugihara, 1968; Hsu et al., 1979; Stauffer, 1993). Hence, more oxidants are required to compensate this reducing action in the frozen dough production. Ascorbic acid (AA) has been widely used as an oxidant in the baking industry. It is an oxidizing agent that strengthens the gluten network by creating disulfide bonds (Nakamura and Kurata, 1997). It also gives large increases in oven rise and bread score (Yamanda and Preston, 1992). The amount used for good dough processing is ppm, based on flour weight, and it depends on the desired effects on the quality of baked goods (El-Hady et al., 1999). Kenny et al. (1999) demonstrated that the addition of 100 ppm AA into dough frozen for 8 weeks produced bread with significantly higher volume than the control, with loaf volume for both formulations decreasing with frozen storage time. The crumb firmness value of bread with AA was also lower than the control, with a gradual increase in the firmness with frozen storage time, which was probably related to the decrease in volume. The difference in firmness between the control and bread with 41

37 AA and the superiority of AA when compared to sodium stearoyl lactylate (SSL) and DATEM became more pronounced with increasing frozen storage time. Furthermore, extensigraph measurements showed that stronger dough with higher R 5cm values than the control was produced with the addition of AA, while the reduction in resistance to extension and extensibility, commonly associated with freezing and thawing, became less pronounced. When combined with potassium bromate (KB), the ability of AA to inhibit freezing damage is more pronounced. This is shown by the higher maximum resistance ratio, i.e. the ratio of the maximum resistance of frozen dough to the maximum resistance of nonfrozen dough, for the AA+KB dough compared to that of the AA dough. The decrease in loaf volume of bread with increasing duration of dough frozen storage is also less for the AA+KB dough than for the AA dough (Inoue and Bushuk, 1991). In other words, the combination of AA and KB, compared with AA alone, strengthens the dough more efficiently and improves the baking potential of frozen dough. These findings were further supported by El-Hady et al. (1999). It should be noted however, that in 1992 the Joint FAO/WHO Expert Committee on Food Additives considered the use of KB as flour improver or flour treatment agent to be not acceptable (JECFA Evaluation, 1995). This was due to its possible carcinogenic effect on human health (IARC, 1986, 1999). Several states in the USA and Canada as well as many countries in Europe followed suit based on the WHO report. The United Kingdom had earlier prohibited the use of KB with the release of 42

38 The Potassium Bromate (Prohibition as a Flour Improver) Regulations The US Food and Drug Administration, however, believes that 50 ppm or less of KB as an improver in white flour and 75 ppm or less in whole wheat flour are safe (CFSAN, 2005) Sodium Stearoyl Lactylate Sodium stearoyl lactylate (SSL) is a surfactant reported to maintain volume and softness in fresh and frozen dough products (Varriano-Marston et al., 1980; Wolt and D Appolonia, 1984; Armero and Collar, 1996). Kenny et al. (1999) demonstrated that the addition of SSL into dough pieces frozen for 2, 5, and 8 weeks produced bread with significantly higher loaf volume than bread without the improver. The volume, however, was still lower than that of bread containing AA or DATEM, implying that both AA and DATEM are better dough improvers than SSL. Results from the extensigraph measurements indicated that SSL produced stronger dough with higher resistance to extension (R/E) values when compared to the control, although the values were still lower than that of DATEM and AA. The reduction in resistance to extension and extensibility associated with freezing and thawing of dough was also less prominent in dough containing SSL than the control (Kenny et al., 1999). 43