Dulce de leche is a dairy-based confectionery product, widely consumed in Argentina and other Latin American countries as filling or topping for desserts, cakes and biscuits (Hansen, 1978; Bouzas, 1997). It is prepared by heat concentration of whole milk with added sucrose until it reaches 700 g total solids/kg dulce de leche. Sucrose is usually partly replaced by other sugars (not more than 400 g/kg of total sugars added according to Normas Mercosur, 1996) to avoid crystallization. Sodium bicarbonate is added to prevent coagulation of proteins as the pH of the mixture decreases and the isoelectric pH of proteins increases during heating.

Maillard browning is the main reaction of milk proteins during the manufacture of dulce de leche. This results in an attractive flavour and colour, but also leads to less desirable changes, such as loss of nutritional value and formation of toxic compounds (Rizzi, 1994; Friedman, 1996a). The natural reactants are free amino groups of milk proteins and lactose. However, as the addition of other reducing sugars is an accepted practice, the consequences may be quite different depending on the formulation used. In addition, other reactions leading to crosslinkages between protein chains contribute to the reduction in nutritional value (Mauron, 1990; Friedman, 1996b; Finot, 1997). The reduction of nutritional quality of milk proteins is attributed to the lower digestibility and the biological inactivation or destruction of essential amino acids. Lysine is the essential amino acid most affected as its free ε-amino group is very reactive (Hurrell, 1990; Mauron, 1990; Finot, 1997).

Several studies have investigated the physical and rheological characteristics (Pauletti et al. 1984, 1996), sensory properties (Sabioni et al. 1984; Buera et al. 1990; Hough et al. 1990), sugar composition and microbiological stability (Ferramondo et al. 1984) of dulce de leche. However, there is little information about the loss in available lysine during manufacture (Pavlovic et al. 1994).

The aim of this study was to examine the effects of the nature and quantity of various sugars commonly used in the manufacture of dulce de leche on the loss of available lysine.