Blanching Flashcards
What is blanching and why is it used?
Blanching is a mild heat treatment, not a preservation method, usually performed between preparation and processing. The fruits or vegetables are heated rapidly to a predetermined temperature and time and after either cooling rapidly or passing immediately to the next stage of process.
What is the major purpose and effects of blanching in fruits and vegetables?
Blanching causes cell death and physical - metabolic chaos in the cell. The heat destroys the enzymes and damage the cytoplasmatic and other membranes making them permeable (open) to water and solutes and the turgor pressure is lost. So, we have loss of nutrients from the tissue and the cell constituents are no longer compartmentalized and are free to move and interact inside the cell. Some weight loss from the tissue is inevitable as both water and solutes are lost from the cells.
Why is it important to inactivate the enzymes?
The enzymes can cause reduction of the quality in the processed product. In frozen foods, deterioration can happen in any delay before the process, during freezing, storage or defrosting. The similar can happen in dehydrated foods during processing, storage or rehydration.
Is blanching important prior to heat sterilization?
Blanching prior to heat sterilization is less important us the heat from the sterilization will inactivate all the enzymes but the time to reach the high temperatures is sometimes longer. So, to maintain better quality, enzymes must be destroyed before the sterilization process (ex. Canning).
Which enzymes are important to deactivate during blanching?
We need to ensure the destruction of quality-changing enzymes that can worsen the color, texture, production of odors or flavors and the ones that break down nutrients. Peroxidase and less often catalase are used as indicator enzymes for the effectiveness of blanching. We choose peroxidase because is easy to measure and it is the most heat resistant enzyme of these quality changing enzymes even though small amount of enzymes can survive the peroxidase destruction.
How blanching can help in vacuum canning?
Blanching removes gases from plant tissues and especially intercellular gas and it helps before canning to achieve vacuum in the containers and preventing the expansion of air in the can during process. It reduces the strain on the cans and the risk of faulty cans or seams. Also, by removing the O2 we avoid oxidation of the product and corrosion (διάβρωση) of the can. Also, in green vegetables by removing gases and surface dust the color is brightened.
How blanching helps with shrinking and softening?
Blanching can help with shrinking and softening of the tissues, resulting in fuller containers, reducing the need of tinplate (επίστρωση λευκοσιδήρου) in canning and can ease the filling of cans. It is important though to control the time and temperature of the process to not have overprocessing and excessive loss of texture. This texture loss can also be avoided by adding calcium chloride instead of blanching water due to the formation of calcium pectate complexes.
How blanching helps with cleaning and decontaminating?
Blanching can remove pesticide leftovers or radionuclides from the surface of vegetables and reduce toxic constituents. Also, we can achieve good reduction on microorganisms which is useful in frozen and dried foods where surviving organisms can grow during defrosting or rehydration and it is useful before sterilization if large numbers are present before the processing.
What are the most common loses during blanching and what is the solution?
All water-soluble materials like minerals, sugars, proteins and vitamins can leach out and lead to nutrient loss and also some nutrient loss (vitamin C) can occur due to thermal instability and less by oxidation. Vitamin C is the easiest nutrient to lose because is water-soluble, thermally unstable and easy to oxidate during storage. So, the aim is to minimize the leaching and thermal destruction of vitamin C and eliminating the oxidation. This can be achieved with steam blanching that has lower losses of nutrients because leaching is not that important.
What factors have to be considered before blanching?
The blanching conditions must be evaluated for the raw material specifically and have a balance between maintaining quality and avoiding over-processing. So, the factors to consider while designing a blanching system are:
Fruit and vegetable properties like thermal conductivity determined by type, variety and maturity.
The requirements of blanching such as achieving a central temperature, level of peroxidase inactivation, maintain certain amount of vitamin C. Size
Size and shape of pieces.
Method of heating and temperature of blanching medium.
What are the common conditions for blanching?
1-15 min at 70-100°C is normal.
What is the basic equipment and methods for blanching?
The two main ways of blanching are through saturated steam or hot water. Cooling may be with water or air but water cooling can increase leaching losses and absorption of water leading to weight gain. Air cooling can cause weight lose by evaporation but is better for nutrients.
How the blanching is accomplished?
Usually, the material is transported on a mesh belt through a steam tunnel but the uniformity of heating and the cleaning is poor if the food is unevenly distributed but the wastewater is less in steam.
What are the benefits of fluidized bed blanching?
Offers individual quick blanching, where each piece is heated rapidly with thin layers of steam, held in a deep bed to allow temperature equilibration and cooled by air.
What are the two ways of hot water blanching?
In reel blanchers, the food enters a slowly rotating mesh drum, partially submerged in hot water and the heating time is control by the rotation speed.
In pipe blanchers the food is in contact with hot water recirculating through a pipe and the heating time is determined by the length of the pipe and the velocity of water.
