Glucose6P

Figure 1. Trehalose metabolism by Saccharomyces cerevisiae. Structure of trehalose (a-D-glucosyl-1 ^ 1-a-D-glucose) is shown in the upper panel. Transport is mediated by a symport system, but synthesis from glucose-6-phosphate (P) may be the major route for trehalose accumulation (lower panel). Trehalose not only alleviates stress, but it can also serve as a carbon source following its hydrolysis to glucose by the enzyme trehalase.

of arginine and an increase in freeze tolerance and gassing power compared to the parent strain.

Despite the promising experimental results with genetically modified strains, such strains are not used commercially for frozen bread manufacture. Instead, the industry relies more on manipulating manufacturing conditions to enhance yeast performance. It is essential, for example,

Box 8—7. Frozen Doughs and Yeast Cryotolerance (Continued)

that naturally cryotolerant strains or mutant strains derived from wild type strains be used (Codón et al., 2003). Of course, increasing the amount of yeast is also important to compensate for any loss of activity. Finally, minimizing the extent of fermentation prior to freezing appears to improve yeast viability after thawing.

References

Codón,A.C.,A.M Rincón, M.A. Moreno-Mateos, J. Delgado-Jarana, M. Rey, C. Limón, I.V Rosado, B. Cubero, X. Peñate, F.Castrejon, and T. Benítez. 2003. New Saccharomyces cerevisiae baker s yeast displaying enhanced resistance to freezing.J.Agri. Food Chem. 51:483-491. Shima,J.,A. Hino, C.Yamada-Iyo,Y. Suzuki, R. Nakajima, H.Watanabe, K. Mori, and H.Takano. 1999. Stress tolerance in doughs of Saccharomyces cerevisiae trehalose mutants derived from commercial baker s yeast.Appl. Environ. Microbiol. 65:2841-2846. Shima,J.,Y. Sakata-Tsuda,Y. Suzuki, R. Nakajima, H.Watanabe, S. Kawamoto, and H.Takano. 2003. Disruption of the CAR1 gene encoding arginase enhances freeze tolerance of the commercial bakers yeast Saccharomyces cerevisiae. Appl. Environ. Microbiol. 69:715-718.

manufacturer. Internal appearance is also based, in part, on color, specifically, the absence of streaks or uneven color.The size, frequency, and distribution of air cells are important determinants of bread quality, especially if there are large, irregular-shaped holes that are often considered as defects.

In contrast to the appearance properties, which are difficult to assess objectively, methods do exist to measure texture properties. Instrumental devices, such as the Instron Universal Testing Machine, can be used to measure crumb softness, resistance, and compressibility. Other compression-type instruments also exist, such as the Baker Compres-simeter and the Voland Stevens Texture Analyzer, which can be used to measure firming changes and staling rates. Various spec-trophotometric methods, including near infrared and infrared spectroscopy can detect changes in starch crystallization and conformation, and are especially useful for measuring retrogradation and staling rates. However, X-ray diffraction is now generally considered the most definitive technique for detecting changes in crystallization.

Bread flavor is no less easy to describe or quantify. It can also be difficult to separate flavor from texture and other sensory attributes. In general, bread should have a fresh, some what yeasty flavor and aroma, with acidic and wheaty notes. Salt also provides an important flavor in bread, as do the alcohols, aldehydes, and other organic end products produced during yeast metabolism.

Many of the important flavor compounds of bread are formed during baking and are derived from caramelization and Maillard reactions; the Maillard reaction alone results in more than 100 volatile aroma compounds. Importantly, these reactions also generate pigments and are responsible for crust color formation. Although both types of reactions require heat, are non-enzymatic, and use sugars as the primary reactant, they differ in that caramelization reactions occur at higher temperatures than Maillard reactions and only reducing sugars react in the Maillard reaction. The latter also requires primary amino acids. The Maillard reaction is probably predominant during baking, since the temperatures achieved generally are not high enough to induce extensive caramelization.

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