Stalling staling of bakery products

Prepared Foods, March, 1999 by Stanley Cauvain

Storage techniques and ingredients are keys to minimizing staling of frozen bakery products.

Whether prepared in a home kitchen or an in-store bakery, nothing beats the texture and aroma of fresh baked goods. Creating such products from scratch in either place, however, is becoming less common due to time and money constraints.

Frozen baked goods offer both home and commercial users a viable alternative, but it is left to food manufacturers to maximize these products' quality through formulation and processing alterations. The following article offers advice toward this end. Adapted from "Improving the Control of Staling in Frozen Bakery Products," by Stanley Cauvain, Campden & Chorleywood Food Research Assoc., UK, it first appeared in Trends in Food Science & Technology (1998, Vol. 9, pp. 56-61).

Frozen in Time

Multiple factors contribute to staling, including moisture loss, starch retrogradation and loss of crumb cohesion. (See [ILLUSTRATION FOR CHART OMITTED], p. 71.) However, starch retrogradation - the reformation of starch molecules into an ordered, crystalline state - is perhaps the primary physical change associated with staling.

The frozen state itself slows staling. However, as bread's storage temperature nears freezing ([approximately] -8 [degrees] C), the rate of crumb firming from starch retrogradation increases. It is estimated that the very act of freezing and thawing is equivalent to about 24 hours of staling at ambient temperatures ([approximately] 20 [degrees] C) since bread passes through its maximum staling temperature (4 [degrees] C) twice. In contrast, the maximum staling rate for cake is much higher: 25 [degrees] C. Cake also freezes at the lower temperature of [less than] -15 [degrees] C due to its much higher proportion of soluble solids, primarily in the form of sugars.

To follow some of the effects that frozen storage has on stability, two thermodynamic states termed "glassy" and "rubbery" must be considered.

The molecules in a window's glass do not form crystals as they are cooled, but rather enter a glassy state that is a highly viscous liquid where molecular movement is extremely restricted and flow is "stopped." Similarly, as a food's matrix is frozen, water freezes out as ice crystals, leaving an increasingly concentrated, viscous liquid. Eventually molecular movement also stops. The temperature at which this happens is called the glass transition point (Tg). When window glass or a food is warmed above its Tg, its viscosity greatly decreases and it is said to become "rubbery."

For maximum storage stability, a product must be stored at temperatures below its Tg where the viscosity of the unfrozen water fraction is very great. Researchers (H. Levine and L. Slade, 1990) have highlighted the important role that viscosity plays in restricting starch molecules' freedom to recrystallize, which thereby inhibits staling. A product's Tg depends on its composition and can be measured using differential scanning calorimetry.

Fresh Strategies

Tactics to increase frozen storage stability (R. George, 1993) are as follows:

* Optimize freezing rates to minimize unfrozen water in the product matrix.

* Adjust frozen storage temperatures to below a product's Tg.

* Reformulate to increase a product's Tg to practical freezer temperatures.

Unfortunately, due to their low conductivity, bread products do not freeze rapidly. In addition, "crusty breads" such as French baguettes are subject to "shelling," a condition in which the crust separates from the crumb, regardless of how optimal the freezing conditions. This condition, also observed in "part-baked" products during their second baking, is due to the crust's and crumb's considerable difference in moisture content, which leads to different expansion and contraction rates during freezing and thawing.

Cake quality deteriorates during frozen storage due to crumb weakening from ice crystal formation, starch recrystallization, and moisture migration and redistribution within the crumb.

Above 0 [degrees] C, decreasing storage temperature has two opposite effects on cake firming. It increases starch recrystallization, which tends to increase firming, and decreases moisture migration, which tends to decrease firming.

Below 0 [degrees] C, faster freezing rates have a significant effect in maintaining the integrity of defrosted cake crumb and in slowing moisture migration. Faster rates favor smaller ice crystal formation, which minimizes internal structure and cell wall damage.

Faster freezing rates also increase ice crystal nucleation that impacts the amount of unfrozen water in the matrix (and thus a product's Tg). As a food is frozen, properties such as pH, viscosity and freezing point in its remaining unfrozen matrix change due to the freeze-concentration effect. Lower temperatures would tend to slow reaction rates on one hand, but the increased concentration of reactive soluble components tends to increase reaction rates on the other.

As the temperature is lowered below Tg, the movement of these soluble reactive components is inhibited to the point at which the water is kinetically metastable. Studies have confirmed that cakes can be stored for very long periods if storage temperature is below their Tg.