African maize porridge: a food with slow in vitro starch digestibility
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Food Chemistry 72 (2001) 347±353
www.elsevier.com/locate/foodchem
African maize porridge: a food with slow in vitro starch digestibility
B. van der Merwe a,*, C. Erasmus b, J.R.N. Taylor a
a
Department of Food Science, University of Pretoria, Pretoria, 0002, South Africa
b
CSIR Food Science and Technology, PO Box 395, Pretoria, 0001, South Africa
Received 18 April 2000; accepted 19 July 2000
Abstract
The incidence of diabetes is very low in rural, traditionally living, South African Black people, but higher in the urbanised Black
population. One factor, that could have contributed to the increased prevalence of diabetes, is the change in diet from maize por-
ridge to bread. An in vitro method was used to determine the starch digestibility of African maize porridge compared to other
cereal foods. Maize porridge had a much lower in vitro starch digestibility than white bread (P348 B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 White wheat bread ¯our and whole kernel rolled oats porridge. It consisted of 38.39 g maize meal, 0.71 g salt were bought from a local supermarket. The rolled oats and 125 g water. One quantity of porridge was cooked were milled with a laboratory hammer mill to a particle per day. The cultivar was chosen randomly and two size
B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 349
GI 0:862HI 8:198 porridge, on the other hand, has a dense structure with
no air holes (Fig. 2b and c). Maize porridge consisted of
2.6. Light microscopy a matrix with endosperm grit particles suspended in it.
The matrix consisted of gelatinised starch granules,
A cube of cold porridge was cut and ®xed with glu- whereas the starch granules in the endosperm grit par-
taraldehyde. The sample was rinsed three times with ticles were enclosed in endosperm cells. After digestion
phosphate buer (0.075 mol dmÿ3, pH 6.9). It was then of the porridge with pepsin and alpha-amylase, there
dehydrated with ethanol and imbedded in LR White were still some intact and partially digested starch
resin. Ultra-thin sections (0.4 mm) were made and granules left in cells (Fig. 2d).
stained with Toluidine Blue O. It is suggested that the porous structure of bread would
greatly increase the surface area of the sample. The
2.7. Statistical analyses enzyme solutions (pepsin and alpha-amylase) would ®ll
the pores and have a large contact surface area with the
Signi®cant dierences between means were obtained substrates (protein and starch). With maize porridge, the
with Tukey's honest signi®cant dierence test. P-values enzymes would only be in contact with the outer surface
of350 B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 Fig. 2. Light micrographs of (a) white bread, (b) maize porridge low magni®cation, (c) maize porridge higher magni®cation and (d) maize porridge after digestion with pepsin and alpha-amylase (ah=air hole, m=amorphous area, c=cellular area, i=intact starch granule, e=empty space in cell, sg=starch granule). (type 3) in food products (Muir, Birkett, Brown, Jones fact that pure maize starch is more digestible than pure & O'Dea, 1995). wheat starch (Faulks & Bailey, 1990) could have con- The dierence in particle size between bread ¯our and tributed to the higher digestibility of maize ¯our porridge maize meal did not contribute to the dierence in compared to wheat ¯our porridge. The low starch digestibility between white bread and maize porridge. digestibility of oat ¯our porridge is in apparent contra- The starch digestibility of porridge made from cultivar diction to the ®ndings of Liljeberg, Granfeldt and C maize ¯our with a particle size similar to that of BjoÈrck (1996) who observed that oatmeal porridge had wheat bread ¯our was the same as that of cultivar C a high GI. However, Wood, Braaten, Scott, Riedel, maize meal porridge. This ®nding agrees with Nelles, Wolynetz and Collins (1994) showed that little more Dewar and Taylor (1999), who found that decreasing than a standard serving of commercial rolled oats con- the particle size of maize grits had no signi®cant eect tains enough beta-glucans to reduce postprandial blood on the amount of starch that was solubilised after glucose levels. The mechanism for this is thought to be digestion with malt enzymes when the maize grits had related to the ability of beta-glucans to increase viscosity. been cooked under well-stirred conditions. Zhang, Doehlert and Moore (1997) found that both To compare cereals that were prepared and cooked in particle size and heat treatment aect the viscosity of the same way, the digestibilities of maize, wheat and oat oat slurries. The oatmeal porridge of Liljeberg et al. ¯our porridges were determined (Fig. 3). White bread (1996) and the oat ¯our porridge of the present study was more digestible than all the ¯our porridges. Maize diered in terms of both. ¯our porridge was more digestible than wheat and oat The fact that wheat ¯our porridge was much less ¯our porridges. The reason for the large dierence in digestible than wheat bread (less digestible even than digestibility between bread and the porridges is probably maize porridge), indicates that the preparation method due to the porosity of bread, as referred to earlier. The had a great eect on starch digestibility. This is in
B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 351 agreement with workers (Granfeldt et al., 1992; Holm, 3.2. Factors aecting maize porridge starch digestibility BjoÈrck, ASP, SjoÈberg & Lundquist, 1985) who studied the eect of processing on the digestibility of cereal There were signi®cant (P
352 B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 Fig. 5. In vitro starch digestibility of short ( ), standard (~) and long ( ) hotplate-cooked maize porridge made from cultivar C maize meal (at each time, means not sharing the same letter are signi®cantly [P < 0.05] dierent). the cultivars R2 5 0.92). A signi®cant positive correla- less distorted than with the standard time and, there- tion (R2=0.75; P=0.05) between rate of starch digest- fore, less susceptible to enzyme digestion. ibility (slopes of ®tted lines) and endosperm hardness The reduced digestibility with increased cooking time was obtained, which is shown in Fig. 4. may be explained as follows: increasing the cooking The compositions of the maize meals, from dierent time would increase the degree of starch granule dis- cultivars, were similar and dierences could not be related ruption, especially because of the increased number of to endosperm hardness. The hard cultivars had more lar- times that the porridge was stirred during the extended ger particles than the soft cultivars but, as mentioned, cooking time. This could have led to more starch mole- reducing the particle size of maize meal (
B. van der Merwe et al. / Food Chemistry 72 (2001) 347±353 353
tics, University of Pretoria) for advice and assistance in Muir, J. G., Birkett, A., Brown, I., Jones, G., & O'Dea, K. (1995).
statistical analysis of data. Food processing and maize variety aects amounts of starch escap-
ing digestion in the small intestine. American Journal of Clinical
Nutrition, 61, 82±89.
Nelles, E. M., Dewar, J., & Taylor, J. R. N. (1999). Eect of the par-
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