Masters Theses

Date of Award

6-1980

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

J. L. Collins

Committee Members

S. L. Melton, H. O. Jaynes

Abstract

The purpose of this study was to develop a product (cake doughnut) utilizing sweet potato as an ingredient and to evaluate certain quality factors including chemical composition, physical and sensory attributes, and caloric content of the product.

Three types of sweet potato were prepared: sweet potato flour, baked, and steam cooked sweet potato. Four levels of each type were substituted at 0, 7, 14 and 21% for equal amounts of wheat flour of the doughnut recipe.

Consistency of the dough prepared with sweet potato was adjusted to that of the control dough (0% sweet potato). Reducing the amount of water permitted a change in consistency of the dough. Doughs of similar consistency for all treatments were used to prepare doughnuts for evaluation.

Proximate composition of raw, cured sweet potato flesh and cooked doughnuts was determined. Generally, the amount of crude fat in the doughnuts increased as the level of steam cooked sweet potato was increased. Crude protein decreased as the level of sweet potato was increased due to lower crude protein content in sweet potato than in wheat flour. Other changes were due to the level of sweet potato.

Highest caloric content was found in doughnuts with steam cooked potato.

Doughnuts prepared from steam cooked sweet potato were softer and higher in volume than doughnuts prepared with sweet potato flour or baked sweet potato.

Type and level of sweet potato had no effect on Hunter L values (lightness) for the crust. The crumb became darker as the level of baked and steam cooked sweet potato was increased. Steam cooked sweet potato produced the darkest crumb in doughnuts, while sweet potato flour produced the lightest crumb.

The presence of sweet potato had little or no effect on degree of redness of the crust, but the crumb became more red as the level of sweet potato was increased. Small differences in degree of redness were determined for crust and crumb among types of sweet potato.

Yellow coloration of crust was not affected by type or level of sweet potato. Yellowness of the crumb was affected by type and level of sweet potato. Doughnuts with steam cooked sweet potato were more yellow than doughnuts with baked sweet potatoes since baking destroys some of the carotenoid pigments.

The doughnuts were evaluated by a sensory panel for 13 attributes: 6 for flavor, 4 for texture, and 3 for surface appearance and color. Type of sweet potato affected the sweet, bready, oily, spicy and sweet potato flavor while level of sweet potato affected all of the flavor attributes except salty and oily. As the level of sweet potato was increased, the doughnuts became sweeter. Increasing the level of the three types of sweet potato produced a progressively greater sweet potato flavor in doughnuts. Doughnuts with steam cooked sweet potato were more oily than the control doughnuts. Increased levels of baked sweet potato made the doughnuts sweeter; increased levels of sweet potato flour, more bready and spicy; increased levels of steam cooked, more oily; and increased levels of flour and baked, exhibit a greater sweet potato flavor.

All textural attributes were affected by type and level of sweet potato. Elasticity decreased with increasing levels of baked sweet potato; tenderness decreased with increasing levels of steam cooked sweet potato; and adhesiveness increased with increased" levels of sweet potato flour and baked sweet potato. Doughnuts with steam cooked potato were more elastic; with steam cooked sweet potato flavor, most tender; with sweet potato flour and baked sweet potato, more sticky; and with baked sweet potato, more gummy.

The sweet potato in the doughnuts affected crumb color, and level of sweet potato affected surface appearance and crumb color. With increasing levels of all types of sweet potato, the yellowness of the crumb increased. Baked sweet potato produced doughnuts with darker crust and steam cooked sweet potato produced doughnuts with more yellow crumb.

While doughnuts with sweet potato flour had many desirable attributes, the production cost may make the use of flour uneconomical. Baked or steam cooked sweet potato may be used to produce doughnuts of satisfactory quality. However, a blend of baked and steam cooked sweet potato may be used to take advantage of the higher sweet flavor of the baked and the higher volume development of the steam cooked sweet potato. Since doughnuts with baked sweet potato alone were relatively hard, use of steam cooked sweet potato would counteract this effect, producing a more desirably soft doughnut.

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