Blanched beans

DSC thermograms of blanched bean that have been submitted to an immersion + freezing process (Mbean = 36.5 mg). 

Figure 1. Structural changes of navy beans at various stages of processing la, dry bean lb, soaked/blanched bean lc, canned bean (scanning electron photographs S = starch granule P = protein bodies M = middle lamella)...

With some products, particularly snap beans, there may be some reason to question the validity of a negative catalase reaction as a criterion of adequate blanching, especially in the light of the work reported by Bedford and Joslyn (3). In the case of peas, however, it seems to be entirely adequate. There is urgent need for investigation and the development of an adequate method for testing adequacy of blanching in finished products. 

To study the cell wall alterations diiring processing in detail, total pectic fractions were separated by extraction of AIR from fresh, blanched and sterilized beans with acetate buffer, CDTA and NajCOj. Analysis showed that sterilization caused a lai e increase in the amount of buffer soluble pectins (Figure 1). 

Degree of methylation (DM%) of total pectin of carrot alcohol insoluble sohds (AIS) was decreased from 60.73% for fresh carrots to 48.70, 44.62 and 43.83% for canned carrots preheated at 65°C for 15. 30 and 60 min, respectively (Fig. 2). Similar levels of demethylation were also reported in potato (Bartolome and Hoff, 1972), in carrots (Lee et al., 1979) and in snap beans (Adams and Robertson, 1987) when they were blanched at low temperature between 65°C and 70°C. 

Bourne, M. C. (1987). Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. 

Stolle-Smits, T., Donkers, J., VanDijk, C., Derksen, J., Sassen, M. M. A. (1998). An electron microscopy study on the texture of fresh, blanched and sterilized green bean pods (Phaseolus vulgaris L.). Lebensm. -Wiss. TechnoL, 37,237-244. 

Preparation of dry beans involves preliminary hydration followed by various heat treatments to obtain a tender, palatable product. Water and heat play an important role in chemical reactions, heat transfer and chemical transformations, such as protein denaturation and starch gelatinization. Inadequate water uptake may result in insufficient heat transfer to inactivate antinutritional factors and result in reduced cookability. In general, beans with an initial moisture content between 12 and 18%, are soaked to hydrate the seed to a moisture content of 53 to 57% and subsequently blanched, cooked or canned. This cooking step, if done for an optimal time, renders the seed nontoxic, improves digestibility, develops acceptable flavor and softens the seed coat and cotyledon. 

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