**8. Applications of microwave blanching foods**

Blanching with hot water after the microwave treatment compensates for any lack of heating uniformity that may have taken place, and also prevents desiccation or shriveling of delicate vegetables. And while microwave blanching alone provides a fresh vegetable flavor, the combination with initial water or steam blanching provides an economic advantage. This is because low-cost hot water or steam power is used to first partially raise the temperature, while microwave power, which costs more, does the more difficult task of internally blanching the food product.

10 The Development and Application of Microwave Heating

**7. Blanching using microwave processing** 

very seldom industrially used.

(Arroqui et al., 2002).

The possibly high and nearly homogeneous heating rates, also in solid foods (heat generation within the food) and the corresponding short process times, which helps preserving a very high quality yield advantages of microwave compared to conventional techniques. The crucial point in both processes is the control and the knowledge of the lowest temperatures within the product, where the destruction of microorganisms has the slowest rate. Due to the difficult measurement or calculation of temperature profiles it is still

Blanching is an important step in the industrial processing of fruits and vegetables. It consists of a thermal process that can be performed by immersing vegetables in hot water (88-99 0C, the most common method), hot and boiling solutions containing acids and/or salts, steam, or microwaves. Blanching is carried out before freezing, frying, drying and canning. The main purpose of this process is to inactivate the enzyme systems that may cause color, flavor and textural changes, such as peroxidase, polyphenol-oxidase, lipoxygenase and pectin enzymes. The efficiency of the blanching process is usually based on the inactivation of one of the heat resistant enzymes: peroxidase or polyphenoloxidase.

Blanching has additional benefits, such as the cleansing of the product, the decreasing of the initial microbial load, exhausting gas from the plant tissue, and the preheating before processing. A moderate heating process such as blanching may also release carotenoids and

However, this operation has also some inconvenient effects such as losses in product quality (texture and turgor), environmental impact, and energy costs. Leaching and degradation of nutritive components, such as sugars, minerals and vitamins, may occur when blanching with water or steam. The blanching process should assure enzyme inactivation while minimizing the negative effects, taking into account the interdependence of every aspect

The use of microwaves for food processing has increased through the last decades. Some of the advantages compared with conventional heating methods include speed of operation, energy savings, precise process controls and faster start-up and shut-down times (Kidmose and Martens, 1999). Microwave blanching of fruits and vegetables is still limited. Some of the advantages compared with conventional heating methods include speed of operation and no additional water required. Hence there is a lower leaching of vitamins and other

Blanching with hot water after the microwave treatment compensates for any lack of heating uniformity that may have taken place, and also prevents desiccation or shriveling of delicate vegetables. And while microwave blanching alone provides a fresh vegetable flavor, the combination with initial water or steam blanching provides an economic advantage. This is

soluble nutrients, and the generation of waste water is eliminated or greatly reduced.

make them more extractable and bioavailable (Arroqui et al., 2002).

**8. Applications of microwave blanching foods** 

A still further advantage is that microwave blanching enables a finish blanching of the center sections more quickly and without being affected by thick or non-uniform sections. Uniformity is also more rapidly accomplished in microwave ovens of the continuous tunnel types in contrast to the customary non-uniformity in institutional or domestic ovens (Smith and Williams, 1971).

The spraying of cold water at the end of the blanching process allows a better nutrient retention than the immersion of the food in cold water. Sub-atmospheric pressure, when applied to the steam blanching process, reduces the amount of oxygen and therefore results in a lower degradation of vegetable pigments and nutrients. Pressurized steam reduces blanching time. Optimal conditions of time, temperature, vapor pressure and microwave power depend on the particular vegetable that is being processed and must be empirically determined.

The knowledge of precise microwave power per weight of food that is needed to inactivate a particular enzyme should be sufficient to achieve a successful blanching and to avoid adverse effects. When the process temperature is not adequate, the enzymatic deteriorative action may prevail or even increase in some cases. Figure 2 shows the activity of mushroom polyphenol oxidase in a phosphate buffer 0.05M solution. The samples were previously treated in a microwave oven at specific times, using different potency levels: high, medium and low, which correspond to 770, 560 and 240 watts, respectively.

**Figure 2.** Mushroom tyrosinase as affected by microwaves.
