Mechanisms of heat damage in proteins. 8. The role of sucrose in the susceptibility of protein foods to heat damage.

1. A high-protein yeast cake, based on ovalbumin and lactalbumin, and containing 200 g sucrose/kg was baked and toasted as previously described by Block, Cannon, Wissler, Steffe, Straube, Frazier & Woolridge (1946). This caused a severe reduction in its protein quality for rats, and in its fluorodinitrobenzene (FDNB)reactive lysine content. The damage appeared to be caused by inversion of sucrose to glucose and fructose during fermentation by the yeast followed by Maillard reactions. Processing an albumin-sucrose mixture in a similar way but without fermentation caused no loss in nutritional value for rats and a small reduction in FDNB-reactive lysine. 2. Sucroselysine solutions were prepared and heated as previously described by El-Nockrashy & Frampton (1967). Contrary to their findings, we detected no loss of lysine after storage for 16 h at 35" or after heating for 2 h at loo", although after heating for 4 h at 121" about 0.7 of the original lysine remained. At an alkaline pH, sucrose hydrolyses only slowly even at 121", and this is catalysed to some extent by the presence of lysine. 3. A 'dry' albumin-sucrose mixture (10-200 mg moisture/g) was heated in sealed glass ampoules under a range of conditions. The loss of FDNB-reactive lysine was stongly dependent on the processing conditions; in particular it was decreased by an increase in pH. After 2 h at 121", lysine damage caused by sucrose was similar to that caused by glucose. Lysine damage due to sucrose was believed to follow on from its breakdown to glucose and fructose. 4. Although the presence of sucrose does not make proteins highly sensitive to Maillard reactions and loss of nutritive value, it must not be considered entirely inert. Our results confirm earlier work indicating that its presence at relatively high levels in oil seeds may be largely responsible for the damage to protein quality observed when they are severely processed.