Lectin activity in terms of phytohemagglutinating activity (PHA)
in light red kidney beans was effectively reduced (less than onetenth
residual lectin detected in extruded samples) under all
extrusion parameters used in this study (Fig. 1). The PHA activity in
non-extruded red kidney beans was 0.55 ng/ml, which was determined
in the final dilution mixture. There was a significant difference
(p 0.05) between non-extruded dry beans and all extruded
bean samples. This showed that extrusion temperatures used in
this study were high enough to significantly reduce lectins.
However, when comparing PHA activity across extruded samples
only, there were no significant differences resulting from for feed
rate (p ¼ 0.687), moisture (p ¼ 0.391) and screw speed (p ¼ 0.975).
The explanation for this phenomenon is that lectins in extruded
samples were inactivated at a temperature lower than the product
temperature at die end. Alonso et al. (2000) reported a complete
inactivation or non-detectable levels of PHA activity in kidney
beans as a result of extrusion processing, where a relatively higher
temperature of 155 C/150 C was employed. Our results are
consistent with those of Coffey et al. (1992) who reported
a complete inactivation of lectins in red kidney beans in 2 h at 93 C,
under wet heat conditions.Though the residence time in extrusion
was much shorter, the significantly higher temperature (up to
120 C) facilitated lectin inactivation.
Other methods of processing that have been used successfully to
inactivate lectins in beans include: cooking for 60e130 min at
relatively lower temperature of 89 C (Bonorden & Swanson, 1992);
pre-soaking kidney bean for 16 h before cooking in boiling water
(Bender & Reaidi, 1982); soaking, autoclaving and toasting lima
beans (Adeparusi, 2001); and autoclaving of beans at 121 C for
5 min (Carvalho & Sgarbieri