Preservation of Cheese and Plain Yogurt by Low-dose Irradiation

Abstract

Irradiation ~reservation of Turkish kashar cheese and plain yogurt was studIed usmg very low doses of Co-60 radiation. No side effects were observed below 0.15 Mrad. The number of bacteria killed was directly related to total dose up to 0.02 Mrad, then the effectiveness of the dose decreased. Coating cheese samples with a sorbic acid solution helped in reducing the number of bacteria by about 10-12% in irradiated samples. The shelf-life of irradiated samples stored at refrigerator temperature was almost the same as that of ones coated with sorbic acid but stored at room temperature. Mold formation in irradiated samples took three to four times longer than in non irradiated ones. With plain yogurt the effect of total dose seemed to be the same as i~ cheese. Irradiation increased the shelf-life of yogurt three-to four-Iold. Preservation by irradiation combined with refrigeration increased the shelf-life about five-fold. Even though dairy technology plays a leading role in the food industry, only limited investigations have been made to preserve milk products by irradiation. Milk products are quite perishable; it is also difficult to irradiate them without changing their quality. Most work on treating food with ionizing radiation has been carried out on meat products, onions, potatoes, spices, fruits, juices, and related products (5), Total doses used in irradiation of these products are usually higher than the permissible dose for milk products (3). In milk, the maximum permissible dose giving no side effects was found to be about 0,05 Mrad (4). Bongirwar and Kumta (1) showed that preservation of cheese was possible at 0.2 Mrad without any off-flavors. Since such a dose is smaller than those used for other foods, milk products can not be preserved for long times by irradiation, However, short term preservation can be successfully accomplished. In addition, one can use irradiation as a supplement to other methods for preservation of milk products. In this research we aimed to determine if radiation at different doses could be used to extend the shelf-life of cheese and yogurt without changing their quality. Since radiation can be safely used only at very low doses, we c.ompared its effects with other methods like refrigeration and use of a sorbic acid solution (0.2 %). Hence preserving cheese and yogurt by refrigeration, sorbic acid and radiation was investigated. EXPERIMENTAL In cheese experiments, Cheddar-type aged kashar cheese was employed. It was about 6 months old. Each cheese sample irradiated had dimensions of 5 x 2.5 x 0.5 em and weighed about 5 g. Each slice was first wrapped with cellophane foil, and then irradiated. The yogurt used in the experiments was made in tubes using fresh milk from a state farm (AOe). No additives were used. The radiation source was Gamma Cell 220 (manufacturer: Atomic Energy of Canada Ltd.), Samples were placed at the center of the cylindrical Co-60 source. The distance of the samples from the source was about 15 em. The irradiation time was varied from 1 to 20 min. Dose rates used were 0.00026,0.0021, and 0.00561 Mrad/min, while the maximum total dose used was 0.112 Mrad. The upper limit for the maximum total dose was determined from a preliminary study. It was observed that above 0.15 Mrad both cheese and yogurt showed pronounced fading of color and some off-flavors. So the maximum total dose was kept at about two-thirds of 0.15 Mrad, Surviving bacteria were determined by using the agar plate method (2), tryptone glucose yeast agar and incubation at 35 C for 48 h. In each set of experiments, a control sample was employed. Effects of dose rate, total dose, sorbic acid solution, and refrigeration on killing bacteria, and on the shelf-life of cheese and yogurt were investigated. RESULTS AND DISCUSSION The effect of dose rate on microbial count of cheese is shown in Fig. 1. The microbial counts were made immediately after irradiation of the cheese. The percent of surviving bacteria decreased exponentially at doses up to 0.001 Mrad/min. Any further increase in the dose was not effective in killing the bacteria. This means that if more than a sufficient number of photons hit the sensitive regions of bacteria, only a certain fraction is necessary to kill them and the rest are not used. Therefore 0.001 Mrad/min is the upper dose rate above which some surplus radiation is not used for killing the bacteria. Moreover, this excess radiation can cause a loss in the nutritional value of cheese. The decrease in number of surviving bacteria after 0.001 Mrad/min (or a total dose of 0.02 Mrad) still shows the dependence of killing rate (i.e. number of bacteria killed per unit time per unit dose) on the probability of collision of photons with bacteria. This dependence is probably due to destruction of less sensitive regions of bacteria at high radiation intensity. Figure 2 gives the percent increase of bacteria in cheese samples after 1 week of storage at room temperature. The percentages were determined from the number of bacteria at the end of 1 week of storage and the number immediately after irradiation. The percent increase in the number of bacteria was quite small at doses above 0.02 Mrad. The upper curve in Fig. 2 looks like a straight line with a small almost constant slope