Handbook of Vanilla Science and Technology

The most practical and most commonly used killing methods are exposure of green beans to the sun, killing by oven heat, or hot water killing (Ranadive 1994). In sun-killing, a method originating from Mexico and practiced by the ancient Aztecs (Balls and Arana 1941a,b), beans are held under dark cloth and exposed to direct sun for several days, until the beans turn brown. In oven killing, the beans are tied in bundles and rolled in blankets and placed in an oven at 60°C for 36 to 48 hours (Childers et al. 1959; Ranadive 1994). Hot water killing consists of placing the green beans in wire baskets and submerging them in hot water (60-70°C) for several minutes. A variation of this method consists of repeated submersion for 10 seconds at a time at higher temperatures (80°C) (Childers et al. 1959). Freezing, by dipping in liquid nitrogen or by holding the beans for a few hours in a freezer (0-80°C), is yet another method of killing (Ansaldi et al. 1990). Our own experience and results by other studies (Dignum et al. 2001a, b) indicate, however, that it is preferable to quick-freeze and then store frozen beans at -70°C or below, in order to preserve the viability of enzymes that upon subsequent thawing drive the curing process.

Jones and Vincente (1949a) evaluated various killing methods with respect to the quality of cured vanilla beans and found that killing by hot water scalding gave the best product, with freezing second, and scratching third. Although the various killing methods achieve the same objective, namely, disruption of tissue organization and creation of contact between enzymes and substrates, survival of different enzyme constellations and, subsequently, variation in flavor profile of cured beans might result from different killing methods. For example, the highest activity of β-glucosidase, peroxidase, polyphenol oxidase, and protease was found in heat-killed beans, whereas killing by freezing resulted in a different profile of enzyme activity (Ranadive et al. 1983; Dignum et al. 2001b, 2002b).

Severe killing conditions, excessive or prolonged heat for instance, may lead to a complete destruction of beneficial enzymes and, subsequently, arrest of enzymatic activity required for catalyzing formation of flavor compounds. We believe that rapid killing by heat or freezing is more beneficial for bean quality because these methods achieve the goal of killing, namely, tissue disorganization, while minimizing deleterious effects on the viability of beneficial enzymes.

6.4.2.2 Sweating

The killing stage is followed by “sweating”, a condition consisting of high humidity and high temperature (45°-65°C) for 7 to 10 days (Balls and Arana 1941a, 1942). During this period, the killed bean develops the characteristic vanilla flavor, aroma, and color. During sweating, high enough moisture content is retained as necessary for enzyme-catalyzed reactions. At the same time, enough moisture is allowed to escape in order to reduce water activity to a level that restricts activity and spoilage by micro-organisms. Broadly speaking, enzyme activity during the sweating stage consists of hydrolytic and oxidative action. Apparently, some non-enzymatic oxidative reactions might also occur during this period. The use of elevated temperatures during this stage is to accelerate enzymatic and perhaps also non-enzymatic processes and is practical exploitation of heat stability of hydrolytic and oxidative enzymes stemming, in part, from the enzyme polypeptide side chain glycation with oligosaccharides (Nishi and Itoh 1992; Varki 1993). Traditionally, this process is carried out in Sweat Boxes, in a closed room but rarely in an oven (Ranadive 1994). High temperatures are also achieved by wrapping killed beans in various cloth materials, by densely stacking killed and warm beans in insulated containers and by re-warming with exposure to the sun for a few hours each day during the sweating period. In some instances, the sweating beans are dipped daily in hot water (Balls and Arana 1941a; Childers et al. 1959; Theodose 1973; Dignum et al. 2002b).

6.4.2.3 Drying and conditioning