Flour Quality: Flour Aging - The effects on flour quality and baking performance.Uzwil, 05.07.2012 Today, more and more wheat is processed directly after the grain has been harvested. This creates some challenges to the milling and baking industry in terms of ensuring consistent raw material quality, because flours from newly harvested wheat have a lower baking quality.
Dr. Manfred Dirndorfer explains the effects of this development on the quality of flours and shows how flours with quality issues can be upgraded by using maturing agents. In times of wheat supply shortages and highly volatile prices, wheat is often processed immediately after the grain has been harvested, which creates some challenges for the milling and baking industry with regard to consistent quality.
Newly harvested wheat generally has a lower milling and baking performance. However, after the wheat kernels have been stored for some weeks, the bran is easier to separate from the endosperm, flour extraction increases, and ash content drops. Aged hard wheat flour has a higher water absorption, better mixing tolerance, and greater gas retention capabilities and produces bread with a greater loaf volume.
A directly ground flour that has not aged for a certain time period cannot develop the gluten as expected, resulting in a final bread loaf with less volume and a weaker gluten structure. Yeast doughs made from aged flour are easier to handle than those made from un-aged flour, because dough with a stronger gluten is less sticky and less likely to tear when stretched. This, in turn, allows for a higher volume and finer crumb on the baked bread.
The relationship between physical dough properties and bread-making quality during flour aging was investigated by two Japanese domestic bread-making spring wheat cultivars and a winter wheat cultivar. The specific loaf volumes increased for four weeks after milling in all cultivars. Farinograph stability increased for two weeks after milling and showed higher correlation coefficients to specific loaf volumes.
These improvements have been related to post-harvest maturation and physical changes in the wheat kernel. On the other hand, directly milled flour does not perform as well as flour that has been kept in storage for a certain time period. To address this issue, additional flour storage capacity is needed for a natural aging process, or the millers and bakers will have to add ingredients and flour additives to the flour.
Natural aging occurs when freshly milled “green” flour is exposed to air for several days or weeks. One way of (bio-)chemically impacting gluten functionality is by letting the flour oxidize with oxygen in the air (“natural aging”) for approximately 10 days. With modern milling and distribution processes, flour is sufficiently aged when it arrives at the customer’s site (3 days to a week at least), but it might be up to 3 weeks for the very large (50 kg) almost-airtight bags professional bakeries use.
Flour is naturally aged when it comes into contact with the oxygen in the air. The gluten network gets strengthened by oxidizing gluten-forming proteins, allowing them to create a stronger gluten. Flour components change during aging. The unique properties of wheat reside primarily in its gluten-forming storage proteins. Prominent reactions include sulfhydryl (SH) oxidation and SH-disulfide (SS) interchange, leading to SS crozs-links which strengthen the gluten network.
Natural aging has some constraints. First, it requires time, often several days up to weeks. During this time, the flour takes up valuable silo capacity and might support mould growth or become infested with insects or rodents. Natural aging can also be inconsistent, and it is not as effective as many bleaching and maturing agents.
Maturing agents are additives that change the baking properties of flours by strengthening the wheat gluten network. Maturing agents are added to flour by the millers or are available in many dough conditioners that are added by the bakers. Only very small amounts – parts per million – of maturing agents are needed to increase the performance of the flour.
Maturing agents that strengthen gluten simulate natural aging. They oxidize portions of glutenin and gliadin molecules, altering them so that more bonds are formed during gluten development. The more bonds, the stronger, drier, and more cohesive the dough. When gases expand during final proof and oven spring, the aged gluten network has a better elasticity. The loaf volume is higher, and the bread crumb is less coarse. In general, maturing agents that strengthen the gluten network do not whiten flour. However, many are more effective than natural aging at strengthening gluten.
Ascorbic acid (vitamin C) is one of the most popular ingredients used in flour improvement. The mixing of flour leads to a decrease in the low molecular thiol glutathione and an increase in cysteine concentration. Adding ascorbic acid reduces the concentration of both thiols to a minimum at an incorporation level of ~ 100 ppm (parts per million). Furthermore, the concentrations of high molecular weight thiols in the glutenins of flours from different wheat cultivars isolated from the dough increased to a maximum using a comparable application rate of ascorbic acid.
Dr. Manfred Dirndorfer
Senior Consultant Flour Improvers and Bakery Ingredients
Bühler Uzwil, Switzerland