This paper summarizes and critiques a 2007 study by Rocha-Guzman et al. examining how pressure cooking affects the antioxidant activity and total phenolic content of three common bean (Phaseolus vulgaris L.) cultivars. The original study separated seed coat, cotyledon, and cooking water fractions to assess changes in polyphenol concentration and free radical scavenging activity before and after autoclave cooking. While the study reported a reduction in phenol content alongside an increase in antioxidant activity, this critique identifies significant methodological weaknesses, including inconsistent extraction protocols, the absence of a dilution curve, and failure to present statistical results — all of which undermine the validity of the authors' conclusions.
Rocha-Guzman, N. E., Gonzalez-Laredo, R. F., Ibarra-Perez, F. J., Nava-Berumen, C. A., and Gallegos-Infante, J. A. (2007). Effect of pressure cooking on the antioxidant activity of extracts from three common bean (Phaseolus vulgaris L.) cultivars. Food Chemistry, 100(1), 31–35.
The above authors were interested in understanding both the anti-nutritional and antioxidant activity of dried beans when consumed. The anti-nutritional components include enzyme inhibitors, lectins, phytates, cyanoglycosides, and polyphenols. The authors chose to focus on polyphenols because many of these compounds also exhibit antioxidant activity.
Prior research had shown that uncooked beans contain significant levels of polyphenols and antioxidants, but the effects of food processing on the concentration of these compounds had yet to be determined. The authors therefore assessed total phenol content and antioxidant activity after commercial-style processing of three different cultivars: Flor De Mayo M38, Bayo Victoria, and Pinto Villa.
The beans were cooked in an autoclave at high temperature, allowed to cool, and then the seed coat was separated from the cotyledon. This approach allowed independent testing of the cooking water, seed coat, and cotyledon. The aqueous phase of the cooking water was removed through lyophilization. Organic compounds were extracted from these three different preparations using 70% acetone. The extracts were then dried in a lyophilizer at low temperature.
Equal-weight samples at different dilutions were tested for total phenol content using a spectrophotometer. Free radical scavenging activity was assessed using DPPH (2,2-diphenyl-1-picrylhydrazyl), a standard method for evaluating antioxidant capacity in food extracts.
Most of the phenol content in dried beans is contained in the seed coat, and the majority of this is lost during cooking to the water and cotyledon tissue. Overall phenol content in the cooked beans was reduced by 90% or more. In contrast, antioxidant activity increased significantly in all three cultivars, and the evidence supported the conclusion that the increase in cotyledon antioxidant activity came from the seed coat via the cooking water.
The authors chose to compare total extracted phenolic compounds and antioxidant activity from the seed coat and cotyledon before and after cooking. This is a straightforward approach that minimized the impact of potential confounding factors. However, the primary difference between the two preparations — aside from the cooking itself — is that the uncooked cotyledon and seed coat were ground into flour prior to acetone extraction. The authors do not mention whether the cooked beans were also ground into a paste before extraction, or whether whole cotyledons were extracted. If the results do represent a comparison between extracts prepared from flour and whole cotyledons, this discrepancy could affect the efficiency of the extraction process and compromise the validity of the comparison.
There may be another flaw in the methods. Both the uncooked and cooked seed coat and cotyledon preparations were extracted with acetone, but the lyophilized cooking water residue was not. This deviation in protocol may have introduced an artifact that prevented accurate, direct comparisons between the cooking water, seed coat, and cotyledon samples.
A serious flaw in this study is the lack of a dilution curve with respect to phenol concentration and antioxidant activity. Although the authors performed four 10-fold dilutions, they reported in the results section that the same antioxidant activity was found at all four dilutions. Additional dilutions should have been performed until antioxidant activity could no longer be detected, thereby generating absolute values that would allow direct comparison between uncooked and cooked beans, the different cultivars, and the cooking water, seed coat, and cotyledon preparations. Given the lack of a dilution curve, these comparisons cannot be meaningfully made.
The authors' conclusion that the concentration of the acetone extracts had no influence on antioxidant activity implies that the activity is concentration-independent. This conclusion suggests that the antioxidant activity in these preparations defies the fundamental laws of physics and chemistry, which require that biological activity vary with concentration. This is a significant logical problem with the study's interpretation of its own data.
"Absent statistical results undermine study conclusions"
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