In the above-described processes, three key factors, time, temperature and humidity, should be scientifically and rationally controlled and regulated. It is best to avoid temperature and humidity fluctuations to maintain the product quality. Sweet potato bread and bread stuffed with sweet potato is presented in Figures 11.2.2 and 11.2.3 respectively.
Figure 11.2.2 Sweet potato bread.
Figure 11.2.3 Bread with sweet potato stuffing.
11.2.3 Sweet Potato Cookies
Cookies, one of the convenience foods, is widely popular among consumers. When adding sweet potato to the flour, a convenience food with unique flavor can be prepared. The cookies with the incorporation of sweet potato can be prepared by the method shown in Figure 11.2.4.
Figure 11.2.4 Technological process for sweet potato cookies.
The operating procedure includes:
1. Sweet potato pretreatment: Sweet potato is placed into the steamer for 30 min. When steamed, sweet potatoes are taken out for cooling, peeling and stirring until smooth.
2. Dough preparation: Open shortening in flour mixing machine, add phospholipid and whisk together; add sugar, egg, dissolved baking soda, ammonium bicarbonate, salt, milk powder, vanillin and water, then whisk thoroughly. In the following, add sweet potato batter and stir until emulsified. Then evenly add wheat flour and milk powder into a flour mixing machine, and stir until smooth. The dough preparation temperature should be controlled between 22 and 28 °C.
3. Cookie extruding and shaping: Place the prepared mixed batter into a cloth bag with a pastry tip. Squeezing the batter onto clean iron pans with the right hand clenching the upper opening, and the left hand holding the pastry tip at the lower end of the cloth bag. Squeeze the batter onto the iron pans to form the circular cookies with a diameter of 2 cm and 1 cm thick.
4. Baking: Place the bake ware with the cookies into a baking oven, for 5–6 min at 230–250 °C. Spray with a little water before placing the cookies into the oven, which will improve water absorption on the surface of the cookie and increase starch pasting, to obtain a smoother surface.
5. Cooling and packaging: Remove the cookies from the oven and cool to 38–40 °C.
The following precautions may be observed during the processing of sweet potato cookies:
Firstly, the added amount of sweet potato has a significant impact on the product property. As shown in Figure 11.2.5, the flavor and feature of sweet potato cookies were restricted when the added amounts of sweet potato were limited, while adding an extra amount of sweet potato would make the bread dough less elastic, tough, disorganized, and with uneasy extrusion. Therefore, by experimental analysis, the most appropriate added amount ratio of flour to sweet potato was 7: 3.
Figure 11.2.5 Impact of amount of added sweet potato on cookie property (cited from Li et al., 2008).
In addition, the order of adding ingredients during dough preparation should be addressed. Before dough preparation, all the ingredients except wheat flour should be mixed until emulsified, then add the wheat flour and prepare the dough to reach the stage of moderate stiffness. Easily clumped ingredients such as milk powder or flour need to be sifted before use. It also needs to be noted that monoglyceride or lecithin should be added if fat and dairy products are abundant. The best formulation of sweet potato cookies is (Figures 11.2.6 and 11.2.7) given in Table 11.2.1.
Figure 11.2.6 Sweet potato cookies.
Figure 11.2.7 Purple sweet potato cookies.
Table 11.2.1 Formulation of sweet potato cookies
S. No. | Ingredients | Amount (g)
1 | Wheat flour | 70
2 | Sweet | potato | 30
3 | Shortening | 30
4 | Soft white sugar | 35
5 | Whole milk powder | 5
6 | CTlLU | 4
7 | Vanillin | appropriate amount
8 | Monoglyceride | 0.8
9 | NaHCO3 | 0.4
10 | NH4HCO3 | 0.2
11.2.4 Purple Sweet Potato Cakes
Purple sweet potato cake has a relatively high nutritional value and health benefits. It is also considered to have the efficiency of dietotherapy and meets people’s currently novel demands for food. Although the price is higher than ordinary wheat flour cake, the purple sweet potato cake exhibits big market potential because of the improved living standards of the people and greater awareness of health issues.
In the preparation of purple sweet potato cake, the primary step is to prepare the purple sweet potato flour, by following the methodology given in Figure 11.2.8.
Figure 11.2.8 Ready to use sweet potato flour.
The operating procedure includes:
1. Flour preparation: Sift wheat flour through 100-mesh sieve. Weigh purple sweet potato flour and wheat flour according to required amounts. Prepare 100 g mixed flour, which is ready for use.
2. Whipping eggs: Whip mixed egg whites and sugar for 15 min until the mixture is frothy, color turns to ivory white, sugar is totally dissolved, and the volume increases 2–3 times.
3. Batter preparation: Add prepared mixed flour, egg yolk, 10 g milk powder, water and 8 g butter into the prepared egg whites (milk powder needs to be dissolved in water before use). Mix well until smooth.
4. Filling molds: Pour prepared batter into mold, pre-coated with a thin layer of oil film.
5. Baking: Bake for 15 min at 175–180 °C.
The best ingredient formulation of purple sweet potato cake is given in Table 11.2.2. As a result, the color of the surface of the trial product is dark purple, which could be caused by non-enzymatic browning and may slightly affect the appearance of the product, although the color of the cross-section of the cake is bright purple and very attractive. By using microwave baking instead of far-infrared baking, the color of both the surface and the cross-section of the cake exhibit attractive prunosus. This method can be used to improve the appearance of the product. But it should be noted that the far-infrared baked product has a better smell than the microwave baked one, and also has a smoother taste. The purple sweet potato cake not only has a smooth and soft taste, but also presents a unique purple sweet potato smell and an attractive prunosus color, so that it becomes as a new member of the cake family. Sweet potato cake and purple sweet potato cake are presented in Figures 11.2.9 and 11.2.10 respectively.
Table 11.2.2 The formulation of purple sweet potato cake
S. No. | Ingredients | Amount (g)
1 | Purple sweet potato flour | 90
2 | Mixed flour | 100
3 | Egg | 200
4 | Sugar | 80
5 | Water | 15
Source: Sun et al. (2013)
Figure 11.2.9 Sweet potato cake.
Figure 11.2.10 Purple sweet potato cake.