Effect of gamma irradiation on microbiological and nutritional properties of the freeze-dried berries

Freshly harvested ripe fruits were purchased from the local green market in Belgrade, Serbia. The fruits were washed and dried at room temperature. The weight of each berry sample was 100 g. Dry samples of fruit were lyophilized.

Lipohylized berries were packed into sterilized zip bags and irradiated with different gamma radiation doses in the facility for radiation sterilization and conservation of the Vinca Institute of Nuclear Sciences.

For the fruit lyophilization, the Home Freeze Dryer, Harvest Right, the USA were used. The freezing was done at −42°C, and after freezing, the ice was vacuum pumped out in 24 h. The vacuum pressure during freeze-drying was 20 Pa.

After the lyophilization process, the following values of berry powders were measured (Table 1).

During gamma irradiation, high-energy photons, generated from the radioactive source Co-60, penetrate through the product and destroy the DNA chains in living organisms that thrive in the product (insects, molds, yeasts, and bacteria). Radiation doses of 3 kGy, 5 kGy, and 7 kGy were used. The average dose rate was 10 kGy/h.

For the measurement of absorbed radiation dose, we used the ethanol-monochlorobenzene (ECB) dosimeter system [9, 10]. Measuring equipment for ECB is the instrument OK-302/2 type oscillotitrator of Radelkis (Budapest, Hungary). Since the oscillotitrator's measurement [11] is highly temperature-dependent [12], all dosimeters are heated to 20°C.

The initial contamination and the number of microorganisms, total molds, and bacteria in the samples were examined in the accredited microbiology laboratory, using the methods Ph. Eur. 7.0 (2.6.12. – microbiological examination of nonsterile products (total viable aerobic count)) [13]. Microbiological analyses were performed before irradiation and after gamma irradiation with radiation doses of 3 kGy, 5 kGy, and 7 kGy.

The samples of freeze-dried berries before and after gamma irradiation were analysed to determine their content of total fat, water, protein, carbohydrates, sugars, dietary fiber, and energy values. The total fat content was determined by Weibull–Stoldt-Standard application [14]. The berries’ water content was analysed using an electronic water content analyser that works on drying with air circulation. Protein content was determined using a standard ISO procedure, ISO 1871:2009 [15], and the Kjeldahl method [16]. Determination of total carbohydrate and sugars present in freeze-dried berries was performed using the phenol sulfuric acid method [17]. An enzymatic-gravimetric method was used to determine the content of dietary fiber in freeze-dried berries samples.

Some studies have shown that with the irradiation treatment, the berries lose weight [18]. Weight changes were not observed in our case, which shows that the weight lost during irradiation depends on the sample's water content. In this case, all the water was eliminated by the lyophilization process.

To eliminate total microorganisms, molds, and bacteria from freeze-dried berries, they were treated with gamma irradiation doses of 3 kGy, 5 kGy, and 7 kGy. The measured absorbed radiation doses were (3 ± 0.12) kGy, (5.0 ± 0.22) kGy, and (7.1 ± 0.18) kGy. The uniformity of the delivered dose was 3.6%.

The content of the total number of microorganisms was translated into a radiation dose curve, which represents a logarithmic variation of the total number of microorganisms as a function of the absorbed radiation dose (Fig. 1).

Fig. 1

Figure 1 shows that the radiation dose of 7 kGy is enough to eliminate the total number of microorganisms below the permitted limit. Similar results are shown in the literature [19].

Table 2 shows that the radiation dose of 5 kGy is enough to eliminate the total molds below the permitted limit, as reported in the previous research [20].

None of the six tested bacteria (Salmonella sp., Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus cereus, Sulfitoreducing clostridia) were found in the samples, either before or after irradiation.

Several studies have confirmed that doses up to 10 kGy do not cause any toxicological hazards or nutritional and microbiological problems in fresh berries [21]. To determine the gamma irradiation influence on the nutritional properties of freeze-dried berries, we performed an analysis in the non-irradiated samples and the samples irradiated with the irradiation dose of 3 kGy, 5 kGy, and 7 kGy. The data are shown in Tables 3–7.

It can be inferred that the changes in most nutritional were insignificant and within the limits of measurement error. The exception is the total fat content in the samples. Its values vary and do not follow any trend; so, we can assume that their detection method is quite unreliable at such a low amount of total fat content in the samples.