Is jaggery allowed in check-in bags for flights going to The Netherlands? If so, how many kilograms one can take? Do we need to declare it at customs?
Summary: You are very likely to have no problem getting jaggery on-board aircrafts and through customs. Though if asked, you might have to prove they are not restricted products. If in doubt, ask the airline/ declare at the customs - chances are that you will be waved through.
Each airline can decide what they will/will not carry. For example, KLM restricts animal products but did not mention any plant-derived products. Your airline might impose a different requirement
The weight limit is then what your carrier allow you to bring as part of the check-in luggage and/or carry-on bag allowance.
While there are limits on animal products (meat/milk products generally prohibited, weight limit on fishery and other animal products), and restriction on certain wildlife animals or plants and parts thereof are protected by the Convention of Washington (CITES), it is unlikely jaggery, derived from sugar cane, date, and palm sap (common plant species), are restricted, and hence the general import rule for personal luggage applies:
Other goods (including perfume, coffee, tea, electronic devices etc.)
Up to a value of €430 for air and sea travellers [...]
The value on an individual item may not be split up.
The value of personal luggage (i.e. suitcases) and medicinal products for the personal needs of the traveller do not count.
Member States may reduce the above limits to €150 for travellers under 15 years.
Note the €430 limit applies to all good you are carrying with you. Anything over that amount is subject to customs duty and tax. Though if I were to do customs spot check, I will be weary on anyone who brings €430 worth of jaggery with them.
For the sake of completeness, the customs always have the right to do spot checks and enquire about items they are not familiar with - be ready to answer what jaggery are and convince them it is not a animal product, nor part of endangered plant species.
Unless we are talking about endangered species, there are usually no restrictions on imports of plant-based agricultural products for personal use to EU countries.
There is no weight limit either, but there is a monetary limit of 300€ (430€ if entering by sea or air) for all items you bring, which will remain in the EU. If you are below this limit, you don't have to declare anything. If you are above, you must declare the items and pay customs and value added tax.