Experiment Four: Discussion

As potatoes need to be stored for extended periods in order to enable a year-long consumer supply, storage is an important issue for the potato industry. Optimal storage conditions are required in order to maintain potato quality (prevention of rotting, sprouting, or accumulation of reducing sugars or glycoalkaloids) as well as to prevent losses of valuable nutrients. Since potatoes are generally eaten in a cooked form it is important to understand the effects of different cooking and processing methods on the nutritional content of the potato. Hence the purpose of this experiment was to examine the potential for changes in Taewa nutrient composition and nutritional value due to various post-harvest storage or processing and cooking scenarios.

Based on the findings in Sections 2.2.5, 2.3.5 and 2.4.5., the nutrients likely to be of most nutritional consequence in Taewa includes:

Nutrients of benefit:

x Energy

x Resistant Starch

x Fibre (includes total, insoluble and soluble fibre)

x Protein (particularly with regards to the essential amino acid (EAA) content and amino acids likely to be limiting in potato ; methionine, cysteine, leucine or lysine)

x Minerals: potassium (K), phosphorus (P), magnesium (mg), iron (Fe), zinc (Zn) and copper (Cu)

x Vitamins: C, thiamine (B1), B6, niacin and folate

x Phytochemicals: Total phenolic content (TPC), flavonoid (FLV), anthocyanin content (ACY) and antioxidant capacity

Nutrients to limit: x Reducing sugars x Glycoalkaloids

Hence where analysed, the nutrients listed above will be the main focus of discussion with regards to the impact of storage and processing methods.

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2.5.5.1. Nutrient and Phytochemical Changes Due to 6 month Storage

As discussed in Section 1.4.4. post-harvest conditions such as storage temperature, humidity, physical or microbiological damage to the tuber, light or heat degradation can impact on potato nutrient content. In this research five cultivars of potatoes were stored at 5oC for 6 months, in a darkened chiller with 80-90% humidity control.

On a fresh weight basis, energy content (281-373 kJ cf 340-395 kJ/100 g FW after storage); total fibre; insoluble fibre; soluble fibre and total protein were unaffected by the 6 month storage period. Slight gains in total protein were noted after storage (0.2-0.3 g/ 100 g FW in tuber flesh and 0.3-0.5 g/ 100 g FW in tuber skin), which translated into slight gains in total EAA as well as the limiting amino acids cysteine, leucine and lysine in both tuber flesh and skin samples. This is of benefit with regards to potentially providing greater than 12-13%; 8-9% and 10-11% of the required cysteine, leucine and lysine contributions for a 70 kg adult in a 150 g portion of whole potato (Section 2.3.5.1.b.).

Despite a general decrease appearing to occur in P and Mg content (as shown by the negative values in the change of the nutrient), these values are based on dry weight content and when compared using fresh weight values, differences in P, Mg, Zn and Cu were negligible. One notable exception to this was the much lower Mg content in Karuparera skin after 6 months storage (5 mg cf 19 mg/100 g FW prior to storage). On the other hand, after 6 months storage 10-20% increases in K had occurred across all cultivars and 50% or greater increases of Fe occurred in Karuparera or Tūtaekuri. Both of these mineral increases are of potential nutritional value and could translate to greater than 24-29% or 17-40% of an adult AusNZ EAR for K and Fe respectively.

Variable effects on phytochemical compounds were noted following 6 months storage. Notable differences included a general decrease in TPC, FLV and ACY content in Tūtaekuri. These decreases in phenolic content likely contributed to the lower antioxidant capacity in Tūtaekuri after 6 months storage as shown by the decreased FRAP and ORAC values. In contrast, increases in TPC, FLV and ACY content as well as FRAP and ORAC values were noted in Karuparera and Moemoe flesh. Variable effects of storage on phenolic content have been found in other research. Lewis et al. (1999) found that anthocyanin content increased up to 5 months in the New Zealand coloured potato varieties Desirée, Arran Victory, Red flesh and Urenika (Urenika is a varietal name for Tūtaekuri) when stored at 4oC.

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Anthocyanin, total phenolic and flavonoid concentration also increased up to 120 days in Desirée tuber skin stored at 4oC but not when stored at 10oC, 18oC or 26oC. Although a 5% loss in weight occurred after 170 days storage, there was no change in ratios of the individual phytochemical compounds or % dry matter (Lewis et al., 1999). Kulen et al. (2013) found that the total phenolic content of tubers with pigmented flesh stored at 4oC increased after each 2, 4, 6 and 7 month time point compared to white or yellow-fleshed varieties, however, Blessington et al. (2010) found no significant difference in total phenolic content in 8 potato genotypes stored at 4oC or 20oC for 110 days. As cold storage is known to cause the production of sugars from starch, and sugars are an anthocyanin precursor, the increase in sugars may play a part in the increased synthesis of anthocyanins. It is possible that differences in the effects of storage on phenolic content may also be aligned to the inherent genetic differences in the cultivars, change in the physiochemical environment, and presence of metal ions that affect the expression of anthocyanin colour or response of the tuber to disease or injury (Lewis

et al., 1999). With regards to storage of Tūtaekuri, results from this study seem to suggest that

limiting storage of Tūtaekuri would help retain optimal antioxidant capacity, although Tūtaekuri flesh is still likely to have 5-6 times greater antioxidant capacity than Nadine flesh and 4-5 times greater antioxidant capacity than Nadine skin even after a 6 month storage period.

Although vitamins were not analysed during the 2004 harvest, research shows that reducing storage is of benefit with regards to preventing losses of Vitamin C (Kulen et al., 2013); B6, niacin and folate (Augustin et al., 1978); but may improve thiamine content (Augustin et al., 1978; Goyer and Haynes, 2011). Starch and sugar are the main components affected by post- harvest metabolism in potato tubers, which will ultimately affect potato cooking, sensory and processing characteristics. Storing potatoes at temperatures around 4oC - 7oC and in a higher CO2 to O2 atmosphere can cause an increase in reducing sugar concentration due to the breakdown of starch (Kumar et al., 2004; Halford et al., 2012). Low reducing sugar (less than 2% fresh weight of total glucose and fructose content) and low sugar content in raw potato is preferable for potential use in the potato processing industry so as to minimise the undesirable formation of acrylamide and after-cooking darkening in fried potato products (Marquez and Anon, 1986; Coffin et al., 1987; Rodriquez-Saona, 1997; Amrein et al., 2003). 6 months storage caused a slight increase in total sugars in Taewa (1.6-2.5 g cf 0.4-2.1 g/100 g FW tuber flesh prior to storage) and the ratio of reducing monosaccharide sugars doubled compared to disaccharide sugars in Huakaroro.

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Thus, although the reducing sugar content is likely to be lower than 2% after 6 months storage, it appears that limiting the storage time where possible to prevent further increases in reducing sugar content would still be of benefit.

Singh et al. (2008b) measured the effects of 6 months storage at 4°C and 80-90% humidity on raw starch physico-chemical and functional characteristics in Huakaroro, Moemoe, Karuparera, Tūtaekuri and Nadine varieties. They found that storage increased the ratio of smaller starch granule sizes, caused a decrease in solubility but increased peak viscosities (Singh et al., 2008b). Such changes may have effects on the texture of the cooked potato and relative intestinal digestibility and thus may affect sensory acceptability and ratios of rapidly digestible, slowly digestible or resistant starch components of the cooked tubers. Although the effect of storage on sensory parameters were not measured in this research, an effect of storage was noted with regards to resistant starch accumulation in cooked then cooled potatoes with potatoes that had been stored for greater than 6 months appearing to accumulate greater resistant starch compared to potatoes that had been stored for less than 2 months postharvest (Chapter 5).

During storage, glycoalkaloid levels can become elevated after exposure to light (associated with tuber greening), infection with tuber rot organisms, or due to sprouting (Stark and Love, 2003). Glycoalkaloid content in Taewa flesh and skin slightly increased after 6 months storage (3-9 mg cf 2-8 mg/100 g FW tuber flesh prior to storage and 52-72 mg cf 37-64 mg/100 g FW tuber skin prior to storage). Total glycoalkaloid levels above 20 mg/100 g fresh weight of the potato are generally considered unacceptable due to the bitterness they impart and their potential for adverse health effects (Shakya and Navarre, 2006). As such, it would be advisable that any potential product involving Taewa skins only (such as in a snack of fried potato skins), be analysed for potential glycoalkaloid toxicity. As previously discussed in Section 2.4.5.2, previous research on Taewa found that glycoalkaloid content ranged from 3.9 to 14.3 mg/100 g FW whole potato between Taewa cultivars (Savage et al., 2000) and thus whole Taewa products are likely to fall within an acceptable range. Although the effect of cooking on glycoalkaloid content was not carried out in this research, household cooking techniques (boiling, baking, frying), do not appear to affect the glycoalkaloid content of potatoes (Friedman and Lewis, 2009), therefore efforts to reduce glycoalkaloid accumulation should focus on storing potatoes away from light and removing tuber sprouts, green skin or flesh prior to cooking.

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2.5.5.2. Nutrient Changes Due to Par-boiling

Processes to convert the potato tuber into an edible form (cooking, extraction of starch, extrusion, chipping, freezing, frying) will each impact the nutrient content and thus nutritional value of the final potato product consumed.

Overall, par-boiling Taewa tubers whole incurred minimal if any nutrient losses and thus par- boiling Taewa whole appears to have excellent potential for use as a future par-boiled Taewa product that could be marketed as a convenience product with added health benefits (increased resistant starch content as well as increased antioxidant potential compared to white or yellow-flesh, white-skinned potatoes). Par-boiled Taewa could be used in potato salads sold in delicatessens, or used at home as a pre-cooked potato that would only need warming, adding to a potato salad or adding to a recipe requiring cooked potatoes (such as a cheese and potato bake, stir-fry, or stew). Of interest was the fact that Fe content slightly increased after par-boiling in the Tūtaekuri variety. Previous research by Bethke and Jansky, (2008) supports the fact that a reduction in potato tissue disruption helps to reduce mineral losses as peeling, cubing or shredding potato tubers prior to boiling incurred 50% and 75% loss in potassium and around 50% losses for P, Mg, Zn, Mn and Fe (Bethke and Jansky, 2008). As might be expected, cooking processes that included boiling in water had a major effect on Vitamin C content (45-54% loss due to par-boiling). Although not determined in this experiment, other water-soluble and heat-sensitive vitamins such as Vitamins B6, B1,and niacin are also likely to incur significant losses due to heat destruction or leaching out into water (Finglas and Faulks, 1984). Research suggests that folate may not incur as extensive losses when boiled, as the folate content of raw whole potato was similar to that after boiling for 60 minutes (125.1 and 102.8 μg/100 g FW for raw and boiled potatoes respectively), nor did folate levels decrease if the skin was removed prior to cooking (McKillop et al., 2002). It appears that keeping the skin on during the cooking process, and minimising cooking time, temperature and volume of water used during the cooking process can help minimise losses of vitamins (Burgos et al., 2007).

In comparison to processes including chipping, frying or extrusion, par-boiling is likely to be the best option with regards to retaining nutritional value. Par-boiling is likely to not only incur the least loss of valuable nutrients compared to extrusion or frying, but par-boiled potatoes do not have the additional fat and calories that are associated with fried products.

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Realistically however, potato snacks such as potato crisps or extruded snacks are potentially a more profitable market than bags of raw, whole potatoes, thus the challenge is to explore various cooking processes which minimise nutrient losses or the addition of fat and salt, and thus maximising health potential. Suggestions of possible options will be explored in Chapter 7.