Vitamins

Killeit (1994) reviewed vitamin retention in extruded foods. More research on the bioavailability of added and endogenous vitamins is needed, particularly in light of fortification programs for folate and other vitamins. Concerns of reduced vitamin levels prompt some manufacturers to apply vitamins post-extrusion as a spray. More recent research has focused on vitamin stability in feeds. Fat-coated ascorbic acid, menadione, pyridoxine and folic acid were retained better than were crystalline forms in extruded fish feed (Marchetti et al, 1999).

Although many extruded foods do not naturally have high levels of lipid-soluble vitamins, stability of these nutrients is a concern for fortified foods. Over 50% of all trans-beta-carotene in wheat flour were destroyed when barrel temperature increased from 125 to 200°C (Guzman-Tello and Cheftel, 1990). The degradation process is not straightforward. Fifteen degradation products of all trans-beta-carotene dispersed in corn starch were recovered after twin-screw extrusion (Marty and Berset, 1988). Retention of retinyl palmitate in tapioca snacks mixed with either fish or protein flour was 52% and 73%, respectively after extrusion (Suknark et al, 2001).

Vitamins D and K are fairly stable during food processing, but are not used in many extruded human foods. Vitamin E and related tocopherols function as both vitamin and antioxidant. Gamma and delta tocopherols underwent greater losses (~40%) during extrusion than did alpha and beta forms (23-28%) (Suknark et al, 2001). Rice bran tocopherol decreased as extrusion temperature increased; bran extruded at 120-140°C lost more tocopherols over a year's storage than did bran extruded at 110 °C (Shin et al, 1997). Less than 20% of vitamin E was retained in extruded and drum-dried wheat flour (Wennermark, 1993). The stability of lipid-soluble vitamins is shown in Table 14.3.

Ascorbic acid (vitamin C) decreased in wheat flour when extruded at higher barrel temperatures at fairly low moisture (10%) (Andersson and Hedlund, 1991). Blueberry concentrate appeared to protect 1% added vitamin C in an extruded breakfast cereal compared to a product containing just corn, sucrose, and ascorbic acid (Chaovanalikit, 1999). When ascorbic acid was added to cassava starch

Table 14.3 Stability of lipid-soluble vitamins during the production of tapioca-peanut flour snacks

Processing step Vitamin content (g/100g), fat- and moisture-free basis

Table 14.3 Stability of lipid-soluble vitamins during the production of tapioca-peanut flour snacks

Processing step Vitamin content (g/100g), fat- and moisture-free basis

Total tocopherols

Retinyl palmitate

Raw material

14.44b

2.74a

Extrusion

10.55c

2.19b

Drying to form

10.36c

1.92c

half-product

Frying

43.18a

2.00c

Different letters within columns indicate statistically significant differences. Adapted from Suknark et al, 2001.

Different letters within columns indicate statistically significant differences. Adapted from Suknark et al, 2001.

to increase starch conversion, retention of over 50% occurred at levels of 0.4-1.0% addition (Sriburi and Hill, 2000).

Thiamine is the water-soluble vitamin most susceptible to thermal processing. Thiamine destruction in extruded wheat flour is a first-order reaction (Guzman-Tello and Cheftel, 1987). Killeit (1994) summarised thiamine losses as ranging from 5 to 100%. Thiamine retention in potato flakes decreased under extrusion conditions of lower moisture and higher barrel temperature; sulfites in the potato flakes may have also contributed to vitamin destruction (Maga and Sizer, 1978). Large losses of thiamine occurred when no water was added during extrusion, but riboflavin (B2) and niacin were not affected (Andersson and Hedlund, 1991). Using low-cost single screw extruders, Lorenz and Jansen (1980) found retention of over 90% for thiamin, riboflavin, vitamin B6 and folic acid in corn-soy blends processed at 171 °C.

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