Chemistry Reference
In-Depth Information
transformation product, lactulose, lactose does not contain furanoid ring
structures (Clamp et al., 1961); the formation of such structures is precluded
by the (1
!
4) linkage. In solution, lactose exists as an equilibrium mixture of
- and
-lactose. A third crystalline form, anhydrous
-lactose, may be
prepared from crystalline
-lactose hydrate by heating. Although
-lactose
hydrate and
-lactose are stable at room temperature, anhydrous
-lactose is
unstable and is converted either to
-hydrate (in the presence of water) or to
-lactose (by heating to temperatures greater than 93.58C). The proportions
of
- and
-lactose in an equilibrated solution at room temperature are
37
and
63%, respectively. The solubility of lactose in water is low (17.8 g dl
-1
at
258C) compared with other sugars, and crystallization is often a problem in
concentrated systems.
In addition to its high concentration in dairy products, lactose is still the
most widely used excipient in tablet manufacture. Since the majority of drugs
have amine functional groups, the role of Maillard reactions between drug
molecules and lactose in tablet discoloration is well known. The potential of
such reactions in the solid state to limit the shelf-life, efficacy and bioavail-
ability of drug formulations has been studied extensively (Kumar and Banker,
1994; Qiu et al., 2005a). Apart from storage conditions, milling (which influ-
ences surface area of reactants) high compression pressure and reactants in the
amorphous state have been shown to promote Maillard reactions in tablets
(Qiu, et al., 2005b). Whereas browning inhibitors, such as inorganic bisul-
phites, have been used commonly to stabilize pharmaceutical formulations
where Maillard reactions are known to cause a problem, the addition of
browning inhibitors is not generally an option for dairy products. However,
the detrimental effects of Maillard reactions can be minimized through careful
manipulation of processing and storage conditions.
Maillard reactions in dairy products are generally detrimental to the
organoleptic, nutritional and functional qualities of the product and are
therefore undesirable. However, some dairy processes exploit a controlled
Maillard reaction during manufacture. For example, Maillard browning
reactions are an integral part of the manufacture of products such as dulce
de leche in Latin America (also known as doce de leite in Portuguese, confiture
de lait in French or simply 'milk jam' in English) and khoa in India in which
milk is heated in the presence of sucrose to produce brown products with a
pleasant flavour (Pavlovic et al., 1994; Srinivasan and Gopalan, 1994). Mail-
lard reactions also play an important role in the generation of flavour during
the manufacture of ghee, or clarified butter, and milk chocolate crumb
(Minifie, 1989). Chocolate crumb manufacturing optimally requires high
total solids (90-94%), pH 5.5-8.0 and a temperature of 72-778C for 4-8 h,
ideal conditions for Maillard browning.
