Research Article
Influence of High Pressure Processing on Carrot Texture and Tissue
Herbin University of Commerce, China
lijuan Han
Heilongjiang and TV University, Herbin, China
INTROUCTION
In recent years, High Pressure Processing (HPP) has a great worldwide in the food industry. Compared to other methods, HPP offers several advantages: reducing process time; minimal heat penetration/heat damage problems; freshness; well retaining flavor and no vitamin C loss. In contrast to traditional thermal processing, the functionality-alterations of HPP are minimal (Tewari et al., 1999). Therefore, more and more people pay attention to this technology. At present, some products have appeared, such as syrup, confiture, etc (FDA, 2000).
Recently, many studies were taken place to investigate an application potential of high pressure in processing and preservation of fresh vegetables (Xu et al., 2003 ). In 2000, the American Department of Defense once sponsored $2.3 million, three-year effort to develop the use of ultrahigh-pressure technology to produce high-quality and shelf-stable food products, such as soups, potatoes and cheese products-for both military and civil markets (Fresher, 2000). Vegetables after HPP, pathogenic microorganisms had been I nactivated and their nutritional and sensory characteristics were almost completely retained. Furthermore, their breath was weakened. Therefore, the fresh vegetables after HPP can be preserved for longer time (Shulai, 2003).
However, HPP can sometimes affect the vegetables qualities, because the tissues of vegetables are so tender that they can be easily damaged (Prestamo, 1998). Therefore, to get the practicable technology, it is necessary to observe the tissue and cells of vegetables after HPP in order to know their changes under high pressure. In this study, carrot, as an example of root vegetables (stem vegetables and fruit vegetables will be introduced in other studies), was treated at diverse pressure for different time. The slices of carrot before and after HPP were made and observed by microscope. The study is important for us to well know the mechanism of HPP and to get the rational parameters of preserving vegetables by HPP.
Materials: The carrot was purchased in local supermarket and was absorbed in prime condition, dimethylbenzene, distilled water, hematoxylin, ethanol, acetic acid glacial, etc.
Instruments and equipments: High Pressure Equipment (DL700-0.55x1.5, Da Long Ultra High Pressure Equipment Factory, Shanghai, China) has a vessel capacity 350 mL, maximum pressure of 700 MPa. Pressure was increased within 2 min and released within 0.5 min. Microscope was connected with camera.
Other common tools: vacuum packaging machine, electronic balance, beakers, measuring cylinders, knife, etc.
Experimental procedure and methods: The carrot was washed by flowing tap water and cut into cubes (2x2 cm). All the samples were vacuumed and sealed with plastic bags and the samples for HPP should be sealed twice in order to be isolated from the pressurization medium (oil). The sealed samples were, respectively processed at ambient temperature at the parameters as follows (Shulai, 2005).
Group one at the pressure of 200, 300, 400, 500 and 600 MPa for 5 min.
Group two at the pressure of 400 MPa for 5, 10, 15, 20 and 25 min.
The slices of carrot before and after HPP were made and observed by microscope (Li, 2000).
The effect of varying pressure values: In general, the cells of vegetables are round. Because of their extrusion and collision, their shape is usually polyhedron, which is composed of eight hexagons and six squares. The cells of carrot untreated by high pressure were typical polyhedron (Fig. 1A), their edges and corners were clear and the single cell was full.
The cellular structure micrographs of carrot treated at 200, 300 MPa for 5 min (Fig. 1B and C) indicated that the cell of such treated samples had swollen glossy appearance that is similar to that of untreated sample; this demonstrated that low pressure for short time could not damage. When the samples were processed at 400 and 500 MPa for 5 min, the cells were still intact and keeping original shape (Fig. 1D and E), but their cubage had reduced a little. This demonstrated that pressure (400-500 MPa) couldnt obviously influence on the cellular structure.
Whereas the pressure up to 600 MPa, we observed that the cellular structure changed and some migration of soluble components occurred (Fig. 1F), the cell damage was clearly shown, the collapsed cells appeared and their glossy appearance had disappeared.
Therefore, the pressure (600 MPa) destroyed carrots cell and tissue.
The effect of different time periods: To some extent, according to the theory of high pressure processing, the high pressure for short time has the same effect on the food materials as the low pressure for long time (Zhang et al., 2000). In order to know the processing time how to affect vegetables texture and tissue, we processed the samples at the pressure of 400 MPa for 5, 10, 15, 20 and 25 min, respectively. The micrographs of their cells and tissue were shown in Fig. 2.
When the processing time was 5 min (Fig. 2) the cells structure had no obvious changes, the appearance of cells was turgor and glossy and the shape of cells was clear. These showed that carrotes could keep well after high pressure treatment at these parameters.
While the processing time prolonged, the cubage of carrot cells got smaller and smaller (Fig. 2C-F). This indicated that the intercellular clearance and the inner space of cells became smaller. In fact, there are lots of liquids (75-90%) and gas which are full of the intercellular and inner space of vegetable cells and the liquids contain water as well as other constituents such as salts, vitamins, enzymes and substrates.
Fig. 1: | The cells and tissue of carrot processed at diverse pressure for 5 min |
Fig. 2: | The cells and tissue of carrot processed at 400 MPa for different time |
Under high pressure for longer time, the liquids will flow and effuse from cell wall and the gas will be compressed. Therefore, the cells got smaller.However, except the sample processed at pressure of 400 MPa for 25 min whose cells wall had a little collapsed and shapes were not completely clear, the other cells shapes were still intact and their appearance was glossy too. Therefore, we concluded that the texture and tissue of the carrot treated at 400 MPa for less than 20 min had no obvious changes.
Comparing pressure with time, we discovered that pressure was the main factor to damage carrot structure and tissue, because ultra high pressure could make the cells wall collapsed to destroy carrot cells and low pressure for long time could just decrease the cells cubage, which would be restored when the pressure released. So, in order to protect vegetable from damaging, low pressure for long time is the better technology parameter.
Although low pressure for long time is benefit to protect vegetable from damaging, if the pressure was too low, the pathogenic microorganisms and enzymes would not been inactivated and the vegetable's breath could not been weakened, thus, the fresh vegetables after HPP could not be preserved for longer time (Shulai, 2003). On the other hand, it was inefficient to process vegetables at lower pressure for a long time.
Taking pressure and timefull account, we concluded that processing carrot at 400 Mpa for 20 min or 500 Mpa for 5 min was the practicable technology.